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Cranial anatomy of Pakicetidae (Cetacea Mammalia)Sirpa Nummela a c d S Taseer Hussain b amp J G M Thewissen ea Department of Anatomy Northeastern Ohio Universities College of Medicine Rootstown Ohio 44272 E-mailb Department of Anatomy Howard University College of Medicine Washington DC 20059 USAc Work completed at Northeastern Ohio Universities College of Medicine 4209 State Route44 PO Box 95 Rootstown Ohio 44272 USAd Department of Biological and Environmental Sciences University of Helsinki PO Box 65FIN-00014 Helsinki Finland E-maile Department of Anatomy Northeastern Ohio Universities College of Medicine Rootstown Ohio 44272 E-mailPublished online 02 Aug 2010

To cite this article Sirpa Nummela S Taseer Hussain amp J G M Thewissen (2006) Cranial anatomy of Pakicetidae (CetaceaMammalia) Journal of Vertebrate Paleontology 263 746-759 DOI 1016710272-4634(2006)26[746CAOPCM]20CO2

To link to this article httpdxdoiorg1016710272-4634(2006)26[746CAOPCM]20CO2

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CRANIAL ANATOMY OF PAKICETIDAE (CETACEA MAMMALIA)

SIRPA NUMMELA1 S TASEER HUSSAIN2 and J G M THEWISSEN1

1Department of Anatomy Northeastern Ohio Universities College of Medicine Rootstown Ohio 44272 snummelafastmailfmthewisseneoucomedu

2Department of Anatomy Howard University College of Medicine Washington DC 20059 USA

ABSTRACTmdashThe skulls and isolated tympanics are described for the earliest whales pakicetids from the H-GSPLocality 62 in the Ganda Kas area in Northern Pakistan Currently three pakicetid genera are known PakicetusIchthyolestes and Nalacetus Ichthyolestes is smaller than the two other genera Nalacetus and Pakicetus are similar in sizebut morphologically different Pakicetids have a nasal opening at the tip of the rostrum Their palate retains an incisiveforamen This study reveals three characters of the cranial anatomy useful for systematic analyses In pakicetids the orbitsare orientated dorsally and there is no supraorbital shield The dorsal orientation of the orbits is diagnostic for the familyand the lack of supraorbital shield distinguishes pakicetids ambulocetids and remingtonocetids from the other Eocenearchaeocetes The intertemporal region of the pakicetid skull is very narrow a feature that also occurs in many otherEocene cetaceans The tympanic which is the most abundant cranial bone (more than 30 specimens) in the pakicetidcollections from H-GSP Locality 62 can be used to distinguish the species of pakicetids In Ichthyolestes the tympanicbulla is of the same absolute size as in Pakicetus hence relatively larger and the tympanic bulla of Nalacetus is larger thaneither of these Morphologically the tympanic bullae differ between the genera and on the basis of these morphologiesit is possible to recognize a fourth species of pakicetid at this locality

INTRODUCTION

Pakicetids are archaic whales or stem Cetacea that lived inthe early and middle Eocene in Indo-Pakistan The origin ofearly whales as descendants of artiodactyls has been documentedby Eocene fossils (Gingerich et al 2001 Thewissen et al 2001bThewissen and Williams 2002 Gingerich 2003) particularly pa-kicetids which document the earliest phase of the land-to-seatransition (Thewissen and Hussain 1993 1998 Williams 1998Thewissen et al 2001b Geisler et al 2005 Madar in press)

The most important region where pakicetid archaeocetes arefound is the Ganda Kas area in Kala Chitta Hills Punjab North-ern Pakistan (Thewissen et al 1998) The Kuldana Formation iswell exposed although structurally very complex here (West andLukacs 1979) The Kuldana and the overlying Kohat Forma-tions represent the fluctuating terminal phases of the TethysSea (West 1980) The Kuldana Formation contains mainly fos-siliferous continental strata but also has thin beds containingForaminifera near the top that mark fluctuations of the Tethyssea Continental deposits of the lower Kuldana Formation arethe principal land-mammal bearing unit (Aslan and Thewissen1996) and represent a regressive phase of the Tethys Sea Thelower Kuldana Formation includes H-GSP Locality where ver-tebrate fossils have been collected since 1976 as part of a re-search project between Howard University and the GeologicalSurvey of Pakistan (H-GSP) The sediments at Locality 62 arecharacterized by a high-diversity paleofauna and generally ran-dom and unsorted fossil bone distributions with evidence ofabrasion these strata were deposited in a short time probably103ndash105 years (Aslan and Thewissen 1996) The Kuldana paleo-sol formed in a generally dry oxidizing and alkaline environ-ment (Aslan and Thewissen 1996) In contrast to other localitiesthe vertebrate fauna of the H-GSP Locality 62 is diverse andconsists of both small (90 g) to large (25 kg) animals (Aslan and

Thewissen 1996 Thewissen et al 2001a) Sedimentological dataalso suggest that H-GSP Locality 62 probably represents anabandoned channel with standing water (Aslan and Thewissen1996) and is the most important freshwater locality of the GandaKas area Pakicetid cetaceans are abundant here unlike all otherGanda Kas localities (Thewissen et al 2001a) This locality hasbeen worked extensively by the H-GSP team and this paper isbased on H-GSP material

Pakicetids are only known from the Kuldana and SubathuFormations of Northern Pakistan and northwestern India (Ku-mar and Sahni 1986 Thewissen and Hussain 1998) Currentlythree pakicetid genera are known Ichthyolestes Nalacetus andPakicetus (Thewissen and Hussain 1998) In an early studyDehm and Oettingen-Spielberg (1958) described an upper jawfragment from Ganda Kas area named the species Ichthyolestespinfoldi and placed it in the family Mesonychidae Order Car-nivora Later West (1980) recognized I pinfoldi as a whaledescribed a new cetacean species Protocetus attocki from theH-GSP Locality 62 and placed it in the only basal cetaceanfamily then named the Protocetidae The holotype is a left man-dible with p3ndashm1 (West 1980pl 1 H-GSP 1649) Gingerich andRussell (1981) referred this species to a new genus Pakicetuswhich was proposed for new cetaceans from Chorlakki includingthe new species Pakicetus inachus The holotype of P inachus isa well-preserved braincase with the right tympanic bulla intactbut with no dental remains (GSP-UM 084 Gingerich and Rus-sell 1981 Gingerich et al 1983 Thewissen and Hussain 1998Luo and Gingerich 1999) Gingerich and Russell (1990) sug-gested that P inachus might be a junior synonym of Pakicetusattocki and created a new subfamily Pakicetinae within thefamily Protocetidae for the genera Pakicetus Gandakasia andIchthyolestes Thewissen and colleagues (1996) raised this sub-family to family level (Pakicetidae) and moved Gandakasia to anew family Ambulocetidae

Pakicetid cranial anatomy has been scarcely described in theliterature Gingerich and Russell (1981) and Gingerich and col-leagues (1983) described the holotype of P inachus Thewissenand Hussain (1993) described a mandible and an incus of Paki-cetus and Thewissen and Hussain (1998) reviewed cetacean fos-sils from freshwater sediments concentrating on dental andmandibular fragments and describing the holotype of Nalacetus

Work completed at Northeastern Ohio Universities College of Medi-cine 4209 State Route 44 PO Box 95 Rootstown Ohio 44272 USAPresent address Department of Biological and Environmental SciencesUniversity of Helsinki PO Box 65 FIN-00014 Helsinki Finlandsnummelafastmailfm

Journal of Vertebrate Paleontology 26(3)746ndash759 September 2006copy 2006 by the Society of Vertebrate Paleontology

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ratimitus Luo (1998) described tympanic bullae and other audi-tory structures in relation to underwater hearing of pakicetidsand included comment on some H-GSP specimens from theGanda Kas area Luo and Gingerich (1999) provided additionalinformation on basicranial parts of P inachus and new illustra-tions on its type specimen Geisler and Luo (1998) studiedthe cranial vasculature patterns in cetaceans and reported thatit is primitive in pakicetids their work included pakicetids fromH-GSP Locality 62 Nummela and colleagues (2004) discussedpakicetid cranial material from this locality in their study on theevolution of hearing in whales A phylogenetic analysis of thesystematic relationships of Eocene whales (Geisler et al 2005)also includes pakicetid material from Locality 62

Institutional AbbreviationsmdashGSP-UM Geological Survey ofPakistan University of Michigan H-GSP Geological Survey ofPakistan Howard University IITR-SB Indian Institute of Tech-nology Roorkee (previously RUSB) LUVP Lucknow Univer-sity Vertebrate Palaeontology India

Anatomical Abbreviationsmdashaforptcan anterior foramen ofthe pterygoid canal apral anterior process of the alisphenoidal alisphenoid alfac alisphenoid facet for the anterior processof the tympanic amainv anteromedial angle of the involucrumbilatovdp bilateral oval depression bilatprso bilateral pro-cess of supraoccipital bilattub bilateral tubercle bo basioccipi-tal bs basisphenoid condfor condylar foramen dcondfs dor-sal condyloid fossa diplfor diploic foramen eam external au-ditory meatus emfor emissary foramen eo exoccipital eofacexoccipital facet for tympanic (paroccipital contact) er epitym-panic recess et Eustachian tube incisure ethfor ethmoid fora-men formg foramen magnum fr frontal frlas fronto-lacrimal suture frpas fronto-parietal suture glfs glenoidfossa hglofor hypoglossal foramen iorbcr infraorbital crestiorbfor infraorbital foramen indfrlas indentation of fronto-lacrimal suture inv involucrum jugfor jugular foramen lalacrimal lafor lacrimal foramen latprbo lateral process ofbasioccipital latptlm lateral pterygoid lamina masprpe mas-toid process of the petrosal mdptfs medial pterygoid fossamdptlm medial pterygoid lamina mdptsi medial pterygoidsinus mx maxilla na nasal nacav nasal cavity nafrs naso-frontal suture namxs naso-maxillary suture nucr nuchalcrest occcond occipital condyle optfor optic foramen or or-bitosphenoid orprs orbitosphenoid-presphenoid suture orborbit ossd ossification defect between pre- and basisphenoidovdpor oval depression on the orbitosphenoid ovfor ovalforamen ovw oval window pglfor postglenoid foramenpglpr postglenoid process porbpr postorbital process ppralposterior process (wedge) of the alisphenoid ptypr post-tympanic process pa parietal pal palatine parpreo paroccipi-tal process of exoccipital pe petrosal (periotic) pirfen piri-form fenestra pmx premaxilla pr presphenoid prm promon-torium ptfs pterygoid fossa ptpalfs pterygopalatine fossaptpral pterygoid process of the alisphenoid rdw round windowsorbfor supraorbital foramen sorbfororb supraorbital foramenof the orbit sorbr supraorbital region sagcr sagittal crest sicansinus canal sphorbfi sphenorbital fissure sphpalfor spheno-palatine foramen sq squamosal sqfac squamosal facet forthe anterior process of tympanic tetyfs tensor tympani fossatmpfor temporal foramen tmpfs temporal fossa vcondfsventral condyloid fossa zyga zygomatic arch

MATERIALS AND METHODS

Materials

The pakicetid material studied here is listed in Appendix 1Additionally cranial structures of Pakicetus inachus (cast ofGSP-UM 084) and of some other archaeocete taxa were includedin this study

Methods

We used flexible silicone rubber for making casts of the crosssections of the intertemporal supraorbital and interorbital re-gions around the skull in three different planes as shown inFigure 1 In the intertemporal region a cast was made where theskull is at its narrowest point just posterior to the frontoparietalsuture This cast was made around the skull including both leftand right sides for three pakicetid skulls and was used for draw-ing outlines of their respective areas as shown in Figure 2

In the orbital region we made casts at the plane where thesupraorbital width is narrowest and where the supraorbitalwidth is broadest ie between the postorbital processes (Fig 1)If a cast was available on only one side the other side was drawnas its mirror image These casts were made for several pakicetidand other archaeocete skulls and the outlines based on them aswell as the dorsal view of the supraorbital region are shown inFigure 3

We measured linear dimensions for several cranial structuresThe minimum and maximum supraorbital width (SOW min andSOW max respectively) were measured as shown by the dorsaloutline of the skull in Figure 1 These measurements were takenat the same regions where the orbital cross-sectional casts weremade (see previous and Figure 3) The bizygomatic width(BZW) and the occipital condyle width (OCW) were also mea-sured as possible indicators for body size These measurementswere not always available from both sides however the data inTable 1 always represent the bilateral value Additionally asfurther potential indicators of body size the foramen magnumwidth (FMW) and foramen magnum height (FMH) were mea-sured The anteroposterior tympanic bullar length (TBL) wasmeasured whenever possible A standard sliding caliper was usedfor taking these linear measurements

FIGURE 1 Pakicetid skull showing the supraorbital interorbital andintertemporal regions where the cross-sectional casts were made andwhere the minimum and maximum supraorbital widths were measuredA dorsal view B lateral view (The skull shown here is Ichthyolestespinfoldi from Figs 7A and 8A)

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Data on cranial measurements are presented in Table 1 Datafor the tympanic bullar length are presented in Appendix 1 (in-dividual lengths) and in Table 2 (the range and the mean valuesfor each species)

DESCRIPTION

Subsequent descriptions are general covering all species ofpakicetids from H-GSP Loc 62 Differences between the speciesare listed

PremaxillamdashThe premaxilla of Pakicetus attocki (H-GSP18467) is long and narrow (Fig 4A) The fossa for the externalnaris is on medial side A partial alveolus for I1 complete alveolifor I2ndashC1 and the crown of I3 can be seen

MaxillamdashThe lateral wall of the maxilla of P attocki (H-GSP18470) is high and the palate is narrow (Fig 4A) The infra-orbital foramen is located over P3 and is 100 mm high and 95mm wide (Fig 4A) On the medial side the ventral and lateralwalls of the nasal cavity surround a maxillary sinus that extendsfrom the area dorsal of P3 to M2 The unilateral width of the

palate is 146 mm (as measured from the labial side of P4 to themedian plane) A suture for the nasal bones occurs dorsal to theinfraorbital foramen (Fig 4A)

The infraorbital foramen of Nalacetus ratimitus (H-GSP30306) is similar to that of P attocki (H-GSP 18470) but themaxillary sinus is smaller A diastema is present between P2 andP3 in Nalacetus

NasalmdashThe nasal is long slender and flat in P attocki andNalacetus ratimitus (H-GSP 96231 and 96386 respectivelyFigs 4A 5B) The suture with the maxilla extends in the para-sagittal plane in P attocki (H-GSP 96231 Fig 4A) There is nocontact with the lacrimal (Fig 4A) On the face the nasofrontalsuture extends anteroposteriorly but at the posterior end of thenasals the left and right nasofrontal sutures converge showinga V-shaped medial process (H-GSP 96231 96386 Figs 4B 5B)In juvenile P attocki (H-GSP 96623) and in Ichthyolestes pin-foldi (H-GSP 98134) the suture between the nasal and frontalconsists of several parallel parasagittal lamellae (Figs 6B 7A)which are visible on the frontal The ventral side of the nasalbone in P attocki (H-GSP 96231) and Nalacetus ratimitus (H-GSP 96386) forms the dorsal wall of the nasal cavity two longgrooves are impressions of the cavity for nasal sinus (Fig 5AC)On the ventral surface of the nasal there are also fossaefor paranasal sinuses Bony ridges in the nasal cavity implyingthe presence of turbinates also occur in Remingtonocetus sp(undescribed specimen in the IITR-SB collections S Bajpaipers comm)

LacrimalmdashThe lacrimal is a sharp-edged triangular bone ros-tral to the frontal seen in P attocki (H-GSP 96231 Figs 4A5A) The lacrimal foramen (Fig 4B) is on the orbital side thesuture between lacrimal and frontal extends dorsoventrally (Fig4B) This suture can also be seen in juvenile P attocki (H-GSP96623) in N ratimitus (H-GSP 96386) and in I pinfoldi (H-GSP98134 Figs 5B 6B 7A 8A) although in these specimens thelacrimal is missing The orbital rim is indented where the fronto-lacrimal suture crosses it (H-GSP 96231 96386 96623 98134Figs 4B 5B 6C 7A 8A) This indentation probably housed theangular vein of the eye

FrontalmdashThe frontal in P attocki N ratimitus and I pinfoldi(H-GSP 96231 96254 96386 96574 96623 98134 Figs 4A5AB 6B 7A 8A) is long and slender and forms most of theorbit (Figs 4B 5 6B 7A 8C 9) and the whole supraorbitalregion (Figs 4B 5B 6B 7A 8A) The supraorbital foramen isvisible on this region (Figs 4B 5B 6B 7A) The orbits are closeto each other and face more dorsal than lateral in all pakicetid

FIGURE 2 Cross sections at the intertemporal region (the narrowestpart of the skull posterior to the frontoparietal suture) A Pakicetusattocki (H-GSP 96231) B Pakicetus attocki (H-GSP 96623) C Ich-thyolestes pinfoldi (H-GSP 98134) The dotted line shows the estimateddorsal outline of the partly broken sagittal crest Scale bar equals 2 cm

FIGURE 3 Supraorbital region of several Eocene archaeocetes representing the families Pakicetidae Ambulocetidae Remingtonocetidaeand Protocetidae showing the size and orientation of the orbits These drawings are standardized to the maximum supraorbital width Scale barsequal 5 cm

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genera (Fig 3) Ethmoidal foramina are visible on the ventralside of the orbit in pakicetids (H-GSP 96231 96254 9657496623 98134 Fig 6C) and small diploic foramina dorsally (Fig4AB 7A) The supraorbital foramen of the orbit in pakicetids(H-GSP 96231 96254 96623 96574 98134 Figs 4A 6C 8C 9)the opening for the supraorbital canal is located in the interor-bital region close to the median plane inferior to the postorbitalprocess (H-GSP 96231 96386 96574 96623 98134 Figs 4B 56AB 7A 8C 9) The minimum and maximum supraorbitalwidths vary (Fig 3 Table 1) The shape of the supraorbital re-

gion varies in pakicetids (Fig 3) it is deeply incised in P attocki(H-GSP 96231 96254 96623 Figs 3H I) slightly concave in Ipinfoldi (H-GSP 96574 98134 Fig 3J) and relatively flat in Nratimitus (H-GSP 96386 Fig 3K) The extent to which the post-orbital process projects laterally varies in P attocki (H-GSP96231 96254 96623 Figs 3O P) the process is dorsoventrallythick in I pinfoldi (H-GSP 96574 98134 Fig 3Q) the process isthin in N ratimitus (H-GSP 96386 Fig 3R) the process is thickbut short and stout compared to P attocki (for numerical valuessee Table 1)

TABLE 1 Cranial measurements (in mm) of pakicetids compared with those of other archaeocetes

Specimen no TaxonSOWmin

SOWmax

SOWminmax BZW OCW FMW FMH

H-GSP 96231 Pakicetus attocki 190 484 039 127 527 178 143H-GSP 96623 Pakicetus attocki 189 366 052 924 mdash mdash mdashH-GSP 96254 Pakicetus attocki 1673 mdash mdash mdash mdash mdash mdashGSP-UM 084 Pakicetus inachus1 mdash mdash mdash 143 537 156 137H-GSP 98134 Ichthyolestes pinfoldi 229 424 054 980 436 159 96H-GSP 96386 Nalacetus ratimitus 231 3922 059 126 4713 164 129H-GSP 30432 Nalacetus ratimitus mdash mdash mdash 109 mdash mdash mdashH-GSP 18507 Ambulocetus natans 66 74 089 259 793 328 423IITR-SB 2529 Remingtonocetus sp 952 112 085 254 957 357 306LUVP 11034 Indocetus ramani 151 181 083 mdash mdash mdash mdashGSP-UM 3012 Rodhocetus kasrani mdash 1954 mdash 2604 mdash mdash mdashChM PV5401 Carolinacetus gingerichi mdash 2565 mdash mdash mdash mdash mdash

Dorudon atrox 6ndash10 3086 3416 090 3997 1168 5389 36610

SOW supraorbital width minimum and maximum BZW bizygomatic width OCW occipital condyle width FMW foramen magnum width FMHforamen magnum height For the asterisked specimens () outlines for supraorbital regions and the cross-sections at the two points of this regionare shown in Figure 31Measurements taken on a cast the holotype described by Gingerich and Russell (1981) and Gingerich et al (1983)2Minimum value full width not preserved3Estimate4From Gingerich et al (1994 Table 1)5From Geisler et al (2005 Table 1)6These values measured from Uhen (2004Fig 32 a photo in dorsal view of GSP-UM 101222) The postorbital process distance falls well inside therange reported by Uhen (2004 Appendix IVA range is 224ndash356 mm mean 279 mm n 11)7Mean value (n 3) based on data by Uhen (2004 Appendix IVB)8Mean value (n 7) based on data by Uhen (2004 Appendix IVB)9Mean value (n 6) based on data by Uhen (2004 Appendix IVB)10Mean value (n 5) based on data by Uhen (2004 Appendix IVB)

TABLE 2 Morphological characters in pakicetid tympanics and their appearance in different species

Character Pakicetus attocki Ichthyolestes pinfoldi Nalacetus ratimitus Pakicetid sp nov cf P inachus

Incisure for theeustachian tubeshape

DeepH-GSP 30386 91035

92100 96344

ShallowH-GSP 92003

96283 96431

DeepH-GSP 30459

96384 96385

Widely open medially and very broadH-GSP 30198 30424 98141

Incisure for theeustachian tubeorientation

MediolateralH-GSP 30386 92100

96192 96344

DorsoventralH-GSP 92003 96283

More mediolateralH-GSP 96431

DorsoventralH-GSP 30459

96384 96385

DorsoventralH-GSP 30198 30424 98141

Anteromedial angleof the involucrum

Sharp and highH-GSP 30386 91035

92100 96344

Blunt and lowH-GSP 92003

96283 96431

Sharp and highH-GSP 30459

96384 96385

Very low set caudal this region issmooth no process formingan angle

H-GSP 30198 30424 98141Skull size Larger than Ichthyolestes

similar to NalacetusSmaller than Pakicetus

and NalacetusSimilar to Pakicetus

larger thanIchthyolestes

mdash

Tympanic bullarsize absolute

Small Small Large Small

Range of TBL(mm)

2488ndash2694 2327ndash2737 2873ndash3180 2639ndash2803

TBL mean plusmn SE 2596 plusmn 050 (n 4) 2562 plusmn 085 (n 4) 3007 plusmn 091 (n 3) 2706 plusmn 050 (n 3)TBL meanBZW1 020 026 024 0192

TBL meanOCW1 049 059 064 0502

Tympanic bullarsize relative

Smaller compared toIchthyolestes and Nalacetus

Larger compared toPakicetus

Larger comparedto Pakicetus

As in P attocki

For a list of all tympanics see Appendix 11BZW and OCW values are from Table 1 from specimens H-GSP 96231 96386 and 981342We used BZW and OCW for Pakicetus inachus GSP-UM 084 (see Table 1)

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Caudal to the postorbital process the frontal narrows at thefrontoparietal suture in all pakicetid genera (H-GSP 9623196254 96386 96574 96623 98134 98199 Figs 4B 5B 6B 7A8A) This suture extends across the posterior side of the postor-bital process In lateral view the frontoparietal suture extendsposteriorly into the temporal fossa (Figs 4A 5B 6B 7A 8A)until it reaches the orbitosphenoid (Figs 6C 8C) where it archesanterior and ventral reaching the presphenoid (Fig 8C) Thisarea of the skull is extremely narrow as shown by cross sectionsin Figure 2 the width is 40 mm in juvenile and 55 mm in adultP attocki (H-GSP 96623 and 96231 respectively) and 47 mm inI pinfoldi (H-GSP 98134)

ParietalmdashThe parietal shares sutures with the frontal orbito-sphenoid alisphenoid squamosal and occipital in all pakicetidgenera (H-GSP 96231 96254 96386 96574 96623 98134 Figs4A 5B 6B 7A 8A) and forms most of the sagittal crest (Figs4A 5B 6B 8A) which is somewhat higher than the braincase(the full sagittal crest is not preserved in any specimen) There isno evidence of an interparietal bone At the most anterior part ofthe sagittal crest the skull is extremely narrow (Fig 2 see thesection on the frontal bone) Posteriorly the parietal only formsa minor part of the wall of the braincase (H-GSP 96231 9638696623 98134) The parietal does not contribute to the nuchalcrest (H-GSP 96231 96254 96386 96623 Figs 4B 5B 6BD)There is no evidence of a postparietal foramen

PalatinemdashThe flat area inferior to the orbit anterior to theorbitosphenoid where the infraorbital nerve and sphenopalatine

artery are located is the pterygopalatine fossa this feature oc-curs in P attocki (H-GSP 96623) and I pinfoldi (H-GSP 9657498134 Figs 6C 8C 9) No sutures are preserved on the medialwall of the pterygopalatine fossa but this wall is probably formedby the palatine (Fig 8A) maxilla and frontal bones A horizon-tal infraorbital crest separates the pterygopalatine fossa from theorbit (Figs 4A 6C 8C 9) A small sphenopalatine foramen islocated on this crest (Figs 8C 9)

In N ratimitus (H-GSP 96055) the palatine contribution to thepalate extends anteriorly to P4 In the median plane the palatineextends far ventral causing the posterior palate to be concave

OrbitosphenoidmdashThe orbitosphenoid of P attocki (H-GSP96623) and I pinfoldi (H-GSP 98134 Figs 6C 8C) shares sutureswith the frontal parietal alisphenoid presphenoid and probablypalatine (the extent of the palatine in the pterygopalatine fossacannot be determined) Dorsally the orbitosphenoid has a smalloval depression (Fig 8C) Posteroinferior to this depression thesinus canal (Figs 6C 8C 9) is located on the parieto-orbitosphenoid suture and the groove extends anterior to itcrossing the entire orbitosphenoid Ventral to this is the opticforamen (Figs 6C 8C 9) from which a groove extends anteri-orly toward the orbit The sphenorbital fissure (Figs 6C 8C 9)is located on the suture between orbitosphenoid and alisphenoidon the extreme posterior edge of the former Only a small part ofthe suture between orbitosphenoid and presphenoid (Fig 8C) isvisible because its posterior part is covered by a large anteriorlyprojecting process of the alisphenoid (Fig 8A)

FIGURE 4 Cranial anatomy of Pakicetus attocki A skull lateral view shown by the right premaxilla right maxilla and cranium (H-GSP 1846718470 and 96231 respectively) Length of missing part of rostrum is based on Nalacetus ratimitus maxilla (H-GSP 30306) B cranium dorsal view(H-GSP 96231) These specimens although found at the same locality may not pertain to one individual Scale bar equals 5 cm

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Kellogg (1936109 oph f) identified a lsquochannel for ophthalmicarteryrsquo and lsquoforamen for ophthalmic arteryrsquo in Zygorhiza kochiilocated dorsal to the channel for optic nerve and optic foramenrespectively The ophthalmic artery is a branch of the internalcarotid artery (in mammals lacking a stapedial artery) and is

thus located ventral to the optic chiasm and optic nerve in mam-mals on the floor of the braincase Usually this artery passesthrough the optic foramen The vascular structure passingthrough the foramen described by Kellogg (ophf) instead passesposterodorsally (see Kellogg 1936fig 31bc) opposite from the

FIGURE 5 Pakicetid cranial anatomy A Pakicetus attocki cranium H-GSP 96231 ventral view B Nalacetus ratimitus cranium H-GSP 96386dorsal view C ventral view Scale bar equals 5 cm

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FIGURE 6 Pakicetid cranial anatomy A Pakicetus attocki cranium H-GSP 96623 ventral view B dorsal view C right lateral view D Pakicetusattocki H-GSP 96231 caudal view E Ichthyolestes pinfoldi H-GSP 98134 caudal view Scale bar equals 5 cm

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direction that the ophthalmic artery would have taken We in-terpret Kelloggrsquos lsquoforamen for the ophthalmic arteryrsquo as the an-terior opening for the sinus canal consistent with our interpre-tation of a similarly located foramen in pakicetids

PresphenoidmdashThe presphenoid is elongate and narrow inP attocki N ratimitus and I pinfoldi (H-GSP 96231 9638696623 98134 Figs 5AC 6A 7B) The ventral surface of the

presphenoid is rounded and smooth in Ichthyolestes whereasin Pakicetus and Nalacetus there is a distinct median crest ven-trally In Ichthyolestes much of the lateral side of the pre-sphenoid is covered by an anterior process of the alisphenoid(Fig 8A) A very small foramen occurs in this part of the pre-sphenoid and a groove extends anteriorly from this foramenWe interpret this foramen as the anterior foramen of the ptery-

FIGURE 7 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A dorsal view B ventral view C ventral view ear region in detail Dventral view outline of ear region Scale bar for AndashB equals 5 cm scale bar for CndashD equals 2 cm

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goid canal (Figs 8C 9) Anteriorly the lateral side of the pre-sphenoid reaches dorsal to touch the orbitosphenoid (Fig 8C)Ventrally the presphenoid is not fused to the basisphenoid in themidline and there is an ossification defect of 45 mm in Ich-thyolestes (Fig 7B)

AlisphenoidmdashThe alisphenoid shares sutures with pre-sphenoid orbitosphenoid parietal squamosal petrosal tym-panic basisphenoid and probably with pterygoid in P attockiN ratimitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Fig 8C) The anterior part of the alisphenoid is a rectan-gular process that lies lateral to much of the presphenoid andorbitosphenoid (H-GSP 98134 Fig 8A) and the sphenorbitalfissure is located just dorsal to this (Figs 6C 8C 9) Posteriorlythe alisphenoid forms a narrow wedge-like process betweensquamosal and basisphenoid in Pakicetus Nalacetus and Ich-thyolestes (H-GSP 30432 96231 96386 96623 98134 Figs 7D

8A) Part of this narrow process forms a prominently raised jointfacet the alisphenoid facet for the anterior process of the tym-panic (Figs 5A 6A 7D) This joint facet is continuous with asimilarly sized facet of the squamosal (Fig 5A 6A 7D) Thealisphenoid surrounds the entire oval foramen (Figs 5C 6A7D) There is no round foramen

The pterygoid process of the alisphenoid in pakicetids (H-GSP30432 96231 96386 96623 98134 Fig 7D) divides posteriorlyinto medial and lateral pterygoid laminae (Fig 7D) that sur-round a low pterygoid fossa (Fig 7D) This structure is compa-rable to the pterygoid fossa of primates and many other terres-trial mammals and should not be confused with the pterygoidsinus fossa of Basilosauridae Odontoceti and Mysticeti Themedial pterygoid lamina is low The pterygoid bone is not pre-served but was probably attached to the medial pterygoidlamina

FIGURE 9 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) right ventrolateral view showing the sphenorbital region with the foraminafrom an oblique angle The zygomatic arch is out of the focal plane of this photograph Scale bar equals 5 cm

FIGURE 8 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A right lateral view B sphenorbital region in detail C outline ofsphenorbital region Scale bar for A equals 5 cm scale bar for BndashC equals 2 cm

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BasisphenoidmdashThe basisphenoid shares sutures with pre-sphenoid alisphenoid and basioccipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Figs 5AC 6A 7B) The basisphenoid is convex vent-rally in cross section and concave in the median plane antero-posteriorly in juvenile Pakicetus and in Ichthyolestes (H-GSP96623 98134 respectively Figs 6A 7B) in adult Pakicetus(H-GSP 96231) and Nalacetus (H-GSP 96386) the basisphenoidis more or less flat in both these orientations (Figs 5AC) Pos-teriorly the suture between basisphenoid and basioccipital israised as bilateral tubercles in all pakicetid genera (H-GSP30432 96231 96386 98134 Figs 5AC 7B) One of the flexors ofthe neck rectus capitis ventralis originates from these tuberclesThere is no contact between the petrosal and the basisphenoidand the medial pterygoid sinus lies between these two bones(Fig 7D)

SquamosalmdashThe squamosal contacts the parietal alisphe-noid tympanic petrosal and occipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 98199 9625496386 96407 96623 98134 Figs 4B 5AB 6A 7AB 8A)making up most of the wall of the braincase Several smalltemporal foramina pierce the area near the occipito-squamosalsuture (Fig 6C) Posteriorly the squamosal forms part of thenuchal crest in all pakicetids (H-GSP 96231 96254 9638696623 Figs 4B 5B 6BD) Laterally the squamosal forms theposterior part of the zygomatic arch (Figs 4B 5AC 6BC 7AB8A 9) and the glenoid fossa (also called mandibular fossaFigs 5AC 6A 7Dglfs) in Pakicetus Nalacetus and Ichthyo-lestes (H-GSP 30432 96231 96386 96407 96623 98134) Theanterior part of the glenoid fossa is flat and extensive medio-laterally The postglenoid process (Figs 4A 5AC 6AC 7D8A) arches posteriorly into the external auditory meatus (Figs4A 5AC 6AC 7D 8A) The squamosal makes up the entireroof of the external auditory meatus and there is a prominentposttympanic process (Figs 4B 5B 6B 7A) The postglenoidforamen (also called retroarticular foramen Figs 5AC 6A 7D)is small and located posteromedial to the postglenoid processThe glenoid fossa extends medial to the postglenoid process andends posteromedially into a shallow groove directed to themiddle ear Medial to this is the squamosal contribution to theprominently raised joint facet (H-GSP 30432 96231 9638696623 98134 Figs 5A 6A 7D) for the anterior process of thetympanic The external auditory meatus ends medially in a re-cessed area in Pakicetus Nalacetus and Ichthyolestes (H-GSP30432 96231 96386 96407 96623 98134) The sharply depressedcenter of this recessed area represents the epitympanic recess(Figs 7D)

PetrosalmdashThe petrosal (periotic) contacts the alisphenoidsquamosal tympanic and occipital in P attocki N ratimitus andI pinfoldi (H-GSP 30432 96231 96386 96407 96431 9662398134 Fig 7D) The petrosal forms the roof of the middle earcavity The promontorium (Figs 5C 6A 7D) which houses theinner ear is a triangular structure on the ventral side The facialcanal runs anterior to the promontorium Medial to the prom-ontorium is the groove for the inferior petrosal sinus with thepiriform fenestra (H-GSP 30432 96386 96623 98134 Figs 5C7D) Rostrolateral to the promontorium is the tensor tympanifossa (H-GSP 30432 96231 96386 96407 96431 96623 98134Fig 7D) There are two foramina on the promontorium the ovalwindow for the stapes (fenestra vestibuli Fig 7D) and caudal toit the round window (fenestra cochleae Fig 7D) The mastoidprocess or posterior process of the petrosal flanks through thesquamosal posteriorly (H-GSP 30432 96231 96386 9640796623 98134 Figs 5AC 6A 7D 8A) A more detailed descrip-tion of the petrosal and tympanic of pakicetid cetaceans (Luoand Gingerich 1999) was based in part on material from H-GSPLocality 62

TympanicmdashThe tympanic (synonyms tympanic bulla audi-tory bulla ectotympanic) contacts the petrosal squamosal ali-sphenoid and exoccipital (GSP-UM 084) The tympanic sur-rounds the middle ear cavity ventrally laterally and medially(see Appendix 1 for all tympanics) The anterior part providesthe incisure for the eustachian tube (Fig 10) The medial pachy-osteosclerotic massive part of the tympanic is the involucrum(Fig 10) The involucrum is not in contact with any skull boneand is only connected to the rest of the skull by the dorsolateraland caudal parts of the tympanic The morphology described byGingerich and colleagues (1983) where the involucrum contactsthe basioccipital was based on the type specimen of P inachusGSP-UM 084 (sometimes listed as UM-GSP 081 by Luo 1998) inwhich this area is deformed

The shape and height of the anteromedial angle of the invo-lucrum (as defined by Luo 1998 Luo and Gingerich 1999Fig10) as well as the shape and orientation of the incisure forthe eustachian tube is species-specific (see Table 2) The lateralpart of the tympanic is a thin plate here the sigmoid process isa narrow and small plate of bone on the dorsal side of the lateralwall at least in I pinfoldi (H-GSP 92003) There is a small con-tact between the sigmoid process and the squamosal (see Luoand Gingerich 1999) Posterior to the sigmoid process the ex-ternal auditory meatus opens to the middle ear cavity Anteriorto the sigmoid process is the anterior process of the tympanicalso called processus tubarius which articulates with a facet onthe suture of squamosal and alisphenoid The posterior processof the tympanic contacts the mastoid (posterior) process of thepetrosal and more medially a small exoccipital facet makescontact with the tympanic (Fig 7D) this facet is the proximalpart of the paroccipital process of exoccipital (Figs 4B 5AC6ADE 7D)

There are four distinct morphologies among the tympanicsfrom H-GSP Locality 62 listed in Table 2 Three of these mor-phologies can be associated with cranial and dental material andthus be identified as P attocki I pinfoldi and N ratimitus Thefourth morphology represents a currently unidentified species ofthe family Pakicetidae Thewissen and Hussain (1998) describeda single tooth from Locality 62 tentatively as pertaining to Pinachus and it is possible that the unidentified bullae are fromthis species

In P attocki tympanics (see Appendix 1 and Table 2 Figs10AndashC) the morphology of the incisure for the eustachian tubeand of the anteromedial angle of the involucrum is similar to thatin the holotype of P inachus and therefore we refer these tym-panics to P attocki ndash which based on teeth (Thewissen andHussain 1998) is our most common species of Pakicetus Theorientation of the incisure for the eustachian tube cannot beobserved in GSP-UM 084 because the tympanic in this specimenis deformed and compressed against the basicranium

In the I pinfoldi specimen H-GSP 96431 (see Appendix 1 andTable 2 Figs 10DndashF) the fragmentary basicranium indicates asize considerably smaller than P attocki This is consistent withdental evidence (Thewissen and Hussain 1998) that indicatesthat the genus Ichthyolestes is smaller than Pakicetus We there-fore refer these specimens to I pinfoldi Although the adjoin-ing tympanic (H-GSP 96431) is the smallest one of our I pinfolditympanics two other tympanics with similar morphologyfor which the tympanic bullar length could be measured areboth within the length range of P attocki (Appendix 1 and Ta-ble 2)

The N ratimitus tympanics (see Appendix 1 and Table 2 Figs10GndashI) with a morphology clearly distinct from the two firstones described in Table 2 are larger than those of P attocki andmost of I pinfoldi and this serves to identify H-GSP 30325 and98140 Articular facets of Nalacetus tympanics (H-GSP 3032530459 96384 96385) match the facets on the exoccipital and

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 755

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squamosal of a Nalacetus skull (H-GSP 96386) The Nalacetusskull differs from Pakicetus (see previous) even though theseskulls are similar in size (Table 1) The Nalacetus skull is muchlarger than the Ichthyolestes skull (Table 1) This further cor-

roborates our identification of Nalacetus as a different genusand allocation of these tympanics to it

Three other remaining pakicetid tympanics (see Appendix 1and Table 2 Figs 10JndashL) are distinct from the three common

FIGURE 10 Pakicetid tympanics vertical columns showing the anterior lateral and dorsal views respectively AndashC Pakicetus attocki (H-GSP30386 left) DndashF Ichthyolestes pinfoldi (H-GSP 30223 left) GndashI Nalacetus ratimitus (H-GSP 96384 left) JndashL pakicetid n sp (H-GSP 98141 right)Scale bar equals 2 cm

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species of pakicetids at H-GSP Locality 62 and probably repre-sent a fourth new species of pakicetid These tympanics areclosest to P attocki tympanics in their size

OccipitalmdashThe most anterior point of the basioccipital of Pattocki N ratimitus and I pinfoldi (H-GSP 96231 96386 98134Figs 5AC 7B) is its suture with the basisphenoid which israised into bilateral tubercles (H-GSP 30432 96231 9638696623 98134 Figs 5AC 7B lsquobasioccipital crestrsquo of Luo 1998lsquoincipient falcate process of basioccipitalrsquo of Luo and Gingerich1999) These tubercles for neck flexors (see basisphenoid above)extend onto the anterior side of the basioccipital Posterior tothis the basioccipital projects laterally as a flat process in Paki-cetus (H-GSP 96231) and in Ichthyolestes (H-GSP 98134 Figs5A 7B) but this process is poorly defined in Nalacetus (H-GSP96386) The lateralmost part of this process is formed by theexoccipital (Figs 5AC 7B) and this process also forms the dor-solateral wall of the jugular foramen of pakicetids (H-GSP96231 96386 96407 96431 98134 Figs 5AC 7D) The exoc-cipitals bear the occipital condyles (H-GSP 96231 96386 9643198134 98199 Figs 4A 5AC 6DE 7B 8A 9) which are com-pressed dorsoventrally do not touch in the median plane andare somewhat raised above the level of the exoccipital The dor-sal and ventral condyloid fossae are of equal size (H-GSP 9623198134 98199 Figs 5AC 6E 7D) The hypoglossal foramen inall pakicetid genera (H-GSP 96231 96386 96407 96431 98134Figs 5AC 7D) is situated approximately halfway between thejugular foramen and the occipital condyle One or two smallcondylar foramina (Figs 5AC 7D) pierce the basioccipital lat-eral to the hypoglossal foramen The exoccipital forms the me-diolaterally elongate paroccipital process in all pakicetids (H-GSP 30432 96231 96386 96431 96623 98134 Figs 4B 5AC6ADE 7D) The medial extremity of this process bears a flatfacet for the tympanic (Fig 7D lsquobullar process of the exoccipitalrsquoof Luo and Gingerich 1999) The anterior part of the paroccipi-tal process shares a long suture with the mastoid process of pe-trosal The paroccipital process projects posterolaterally and thepart adjacent to the occipital condyle is recessed

The supraoccipital makes up most of the posterior side of thenuchal crest in all pakicetids (H-GSP 96231 96254 96386 96623Figs 4B 5B 6BD) although this structure is clearly worn insome specimens eg in I pinfoldi (H-GSP 98134) A small bi-lateral process of supraoccipital visible in Pakicetus and Ich-thyolestes (H-GSP 96231 98134 Figs 6DE) for the epaxial ex-tensors of the neck projects from the dorsal edge of the foramenmagnum (H-GSP 96231 96386 98134 Figs 6DE)Variable em-issary foramina (H-GSP 96231 96254 96386 96623 98134 Figs6DE) pierce the occipital dorsal to the foramen magnum Abilateral oval depression (H-GSP 96231 96386 98134 Figs6DE) occurs just lateral to the midline well above the foramenmagnum This depression could be for musculus longissimuscapitis The precise shape of the supraoccipital cannot be deter-mined because the suture between the exoccipital and the su-praoccipital is not visible

DISCUSSION

The pakicetid crania described here help to understand varia-tion in skull morphology of the earliest cetaceans As such theycan be expected to play a significant role in systematic studiesbut a detailed analysis of character scores for these specimensfalls outside the scope of the present description Here discus-sion will focus on a few aspects of cranial morphology that arerarely involved in cetacean systematics although they displayvariation to an extent that may be relevant systematically andinteresting evolutionarily

Supraorbital Morphology

The supraorbital region of protocetids basilosaurids anddorudontids displays a supraorbital shield (Kellogg 1936 Gin-

gerich et al 1994 1995 Bajpai and Gingerich 1998 Hulbert1998 Uhen 1998 2004) However the basal families of ceta-ceans lack a supraorbital shield pakicetids (Thewissen et al2001b) ambulocetids (Thewissen et al 1996) and remingtono-cetids (Gingerich et al 1994 1995 Bajpai and Thewissen 1998)The new pakicetid specimens indicate that there is much varia-tion in supraorbital morphology within these families In paki-cetids the orbits are convergent dorsally The area between theorbits is recessed over the superior orbital rim (Figs 4ndash8) Assuch pakicetid eyes are directed dorsolaterally There are dif-ferences between the genera (Fig 3 Table 1) in Pakicetus andIchthyolestes the supraorbital region is cuplike and cradles thedorsal part of the eye and the dorsal side of the supraorbitalregion is deeply incised in the medial plane in Pakicetus Thisincisure is less strong in Nalacetus and Ichthyolestes and in Nala-cetus the contact between supraorbital region and eye is toonarrow to be cup-shaped In Ambulocetus (Fig 3 Table 1Thewissen et al 1996fig 10 for H-GSP 18490) the orbits arepositioned near the dorsal side of the skull but they face later-ally leaving the supraorbital region more or less flat (in dorsalview) The eyes here appear to be directed laterally and are nothoused in the bony orbits as deeply as in pakicetids as indicatedby the shape of the postorbital processes As a comparison inRemingtonocetus the eyes are small are set laterally and facelaterally and the supraorbital region is strongly convex (Fig 3Sahni and Mishra 1972 Kumar and Sahni 1986 Bajpai andThewissen 1998) In protocetids the orbit is large faces later-ally and is covered by a large supraorbital shield (Fig 3 Table1) Dorudontids and basilosaurids resemble protocetids in thepresence and shape of their supraorbital shield as indicated bythe ratio between the minimum and maximum supraorbitalwidth (Table 1) and in the orientation of their orbits the latterare still clearly different from modern odontocetes and mystice-tes (Kellogg 1936 Uhen 1998 2004)

Intertemporal Morphology

In early and middle Eocene cetaceans a very narrow regioncan be seen in the skull posterior to the frontoparietal suturenear the sphenorbital fissure (Figs 1 2 4ndash8) This morphology isnot seen in most land mammals but is common in early ceta-ceans such as pakicetids ambulocetids (eg AmbulocetusThewissen et al 1996) and remingtonocetids (Kumar and Sahni1986) and may represent an important diagnostic character forthe order

Tympanic Bullar Morphology

The differences in the tympanic bullar morphology seen be-tween P attocki I pinfoldi N ratimitus and a fourth unnamedspecies can be used for specific recognition However in generalall these tympanics display a similar morphology indicating thatthese early archaeocetes used their peripheral ear structures likeland mammals while hearing in air and relied on bone conduc-tion mechanism while hearing in water (Thewissen and Hussain1993 Luo 1998 Luo and Gingerich 1999 Nummela et al 2004)

Pakicetus attocki is clearly larger than I pinfoldi whereas Pa-kicetus and Nalacetus have similar body sizes (Table 1) Thewis-sen and Hussain (1998) studied the size of upper and lower mo-lars in P attocki P inachus I pinfoldi and N ratimitus Theyfound Ichthyolestes to be the smallest P attocki and Nalacetus tobe of similar size and P inachus to be the largest Our studyshows that the absolute tympanic bullar size is similar in Paki-cetus and Ichthyolestes but larger in Nalacetus (Table 2 Appen-dix 1) This implies that in relative terms Pakicetus tympanicswhen related to body size (Table 2) are smaller than those ofIchthyolestes and Nalacetus

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ACKNOWLEDGMENTS

We thank Ellen Byron and Jeanette Killius for their technicalassistance Linda Spurlock prepared the drawings of the skullswhich is gratefully acknowledged Sunil Bajpai kindly gave usaccess to Remingtonocetus specimens in the IITR-SB collectionsunder his care We thank the editor Ewan Fordyce and Zhe-XiLuo and an anonymous reviewer for constructive suggestions forimprovement of the manuscript Financial support was providedby the National Science Foundation (current grant number EAR0207370) to JGMT and by the Ella and Georg Ehrnrooth Foun-dation to SN This article is a contribution to the Howard Uni-versity-Geological Survey of Pakistan projected entitled ldquoCeno-zoic Mammals of Pakistanrdquo We thank the Geological Survey ofPakistan for their support and help in the field

LITERATURE CITED

Aslan A and J G M Thewissen 1996 Preliminary evaluation of pa-leosols and implications for interpreting vertebrate fossil assem-blages Kuldana Formation Northern Pakistan Palaeovertebrata25261ndash277

Bajpai S and P D Gingerich 1998 A new Eocene archaeocete (Mam-malia Cetacea) from India and the time of origin of whales Pro-ceedings of the National Academy of Sciences USA 9515464ndash15468

Bajpai S and J G M Thewissen 1998 Middle Eocene cetaceans fromthe Harudi and Subathu formations of India pp 213ndash233 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Dehm R and T zu Oettingen-Spielberg 1958 Palaumlontologische undgeologische Untersuchungen im Tertiaumlr von Pakistan 2 Die mitte-leocaumlnen Saumlugetiere von Ganda Kas bei Basal in Nordwest-Pakistan Bayerische Akademie der Wissenschaften 911ndash54

Geisler J H and Z-X Luo 1998 Relationships of Cetacea to terres-trial ungulates and the evolution of cranial vasculature in Cete pp163ndash212 in J G M Thewissen (ed) The Emergence of WhalesPlenum Press New York

Geisler J H A E Sanders and Z-X Luo 2005 A new protocetidwhale (Cetacea Archaeoceti) from the late Middle Eocene of SouthCarolina American Museum Novitates 34801ndash65

Gingerich P D 2003 Land-to-sea transition in early whales evolutionof Eocene Archaeoceti (Cetacea) in relation to skeletal proportionsand locomotion of living semiaquatic mammals Paleobiology 29429ndash454

Gingerich P D and D E Russell 1981 Pakicetus inachus a newarchaeocete (Mammalia Cetacea) from the early-middle EoceneKuldana Formation of Kohat (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 25235ndash246

Gingerich P D and D E Russell 1990 Dentition of early EocenePakicetus (Mammalia Cetacea) Contributions from the Museum ofPaleontology University of Michigan 281ndash20

Gingerich P D N A Wells D E Russell and S M I Shah 1983Origin of whales in epicontinental remnant seas new evidence fromthe early Eocene of Pakistan Science 220403ndash406

Gingerich P D S M Raza M Arif M Anwar and X Zhou 1994New whale from the Eocene of Pakistan and the origin of cetaceanswimming Nature 368844ndash847

Gingerich P D M Arif and W C Clyde 1995 New archaeocetes(Mammalia Cetacea) from the middle Eocene Domanda Formationof the Sulaiman Range Punjab (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 29291ndash330

Gingerich P D M ul-Haq I S Zalmout I H Khan and M S Mal-kani 2001 Origin of whales from early artiodactyls hands and feetof Eocene Protocetidae from Pakistan Science 2932239ndash2242

Hulbert R Jr 1998 Postcranial osteology of the North American

middle Eocene protocetid Georgiacetus pp 235ndash267 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Kellogg A R 1936 A review of the Archaeoceti Carnegie Institute ofWashington Publications 4821ndash366

Kumar K and A Sahni 1986 Remingtonocetus harudiensis new com-bination a middle Eocene archaeocete (Mammalia Cetacea) fromwestern Kutch India Journal of Vertebrate Paleontology 6326ndash349

Luo Z-X 1998 Homology and transformation of cetacean ectotym-panic structures pp 269ndash301 in J G M Thewissen (ed) The Emer-gence of Whales Plenum Press New York

Luo Z-X and P D Gingerich 1999 Terrestrial Mesonychia to aquaticCetacea transformation of the basicranium and evolution of hearingin whales University of Michigan Papers on Paleontology 311ndash98

Madar S I In press The postcranial skeleton of early Eocene pakicetidcetaceans Journal of Paleontology

Nummela S J G M Thewissen S Bajpai S T Hussain and K Ku-mar 2004 Eocene evolution of whale hearing Nature 430776ndash778

Sahni A and V P Mishra 1972 A new species of Protocetus (Cetacea)from the middle Eocene of Kutch Western India Palaeontology15490ndash495

Thewissen J G M and S T Hussain 1993 Origin of underwaterhearing in whales Nature 361444ndash445

Thewissen J G M and S T Hussain 1998 Systematic review of thePakicetidae early and middle Eocene Cetacea (Mammalia) fromPakistan and India Bulletin of Carnegie Museum of Natural History34220ndash238

Thewissen J G M and E M Williams 2002 The early radiations ofCetacea (Mammalia) evolutionary patterns and developmental cor-relations Annual Review of Ecology and Systematics 3373ndash90

Thewissen J G M S I Madar and S T Hussain 1996 Ambulocetusnatans an Eocene cetacean (Mammalia) from Pakistan CourierForschungsinstitut Senckenberg 191 1ndash86

Thewissen J G M E M Williams and S T Hussain 2001a Eocenemammal faunas from Northern Indo-Pakistan Journal of Verte-brate Paleontology 21347ndash366

Thewissen J G M E M Williams L J Roe and S T Hussain 2001bSkeletons of terrestrial cetaceans and the relationship of whales toartiodactyls Nature 413277ndash281

Thewissen J G M S T Hussain M Arif A Aslan S I Madar andL J Roe 1998 New localities of Eocene vertebrates in NorthernPakistan and their significance for the origin of the modern orders ofmammals pp 19ndash34 in M I Ghaznavi S M Raza and M T Hasan(eds) Siwaliks of South Asia Proceedings of the 3rd GEOSASWorkshop Islamabad Pakistan 1ndash5 March 1997 Geological Sur-vey of Pakistan Islamabad Pakistan

Uhen M D 1998 Middle to late Eocene basilosaurines and dorudon-tines pp 29ndash61 in J G M Thewissen (ed) The Emergence ofWhales Plenum Press New York

Uhen M D 2004 Form function and anatomy of Dorudon atrox(Mammalia Cetacea) an archaeocete from the middle to lateEocene of Egypt University of Michigan Papers on Paleontology341ndash222

West R M 1980 Middle Eocene large mammal assemblage withTethyan affinities Ganda Kas region Pakistan Journal of Paleon-tology 54508ndash533

West R M and J R Lukacs 1979 Geology and vertebrate-fossil lo-calities Tertiary continental rocks Kala Chitta Hills Attock Dis-trict Pakistan Milwaukee Public Museum Contributions in Biologyand Geology 261ndash20

Williams E M 1998 Synopsis of the earliest cetaceans PakicetidaeAmbulocetidae Remingtonocetidae and Protocetidae pp 1ndash28 inJ G M Thewissen (ed) The Emergence of Whales Plenum PressNew York

Submitted 7 October 2005 accepted 24 February 2006

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APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

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ratimitus Luo (1998) described tympanic bullae and other audi-tory structures in relation to underwater hearing of pakicetidsand included comment on some H-GSP specimens from theGanda Kas area Luo and Gingerich (1999) provided additionalinformation on basicranial parts of P inachus and new illustra-tions on its type specimen Geisler and Luo (1998) studiedthe cranial vasculature patterns in cetaceans and reported thatit is primitive in pakicetids their work included pakicetids fromH-GSP Locality 62 Nummela and colleagues (2004) discussedpakicetid cranial material from this locality in their study on theevolution of hearing in whales A phylogenetic analysis of thesystematic relationships of Eocene whales (Geisler et al 2005)also includes pakicetid material from Locality 62

Institutional AbbreviationsmdashGSP-UM Geological Survey ofPakistan University of Michigan H-GSP Geological Survey ofPakistan Howard University IITR-SB Indian Institute of Tech-nology Roorkee (previously RUSB) LUVP Lucknow Univer-sity Vertebrate Palaeontology India

Anatomical Abbreviationsmdashaforptcan anterior foramen ofthe pterygoid canal apral anterior process of the alisphenoidal alisphenoid alfac alisphenoid facet for the anterior processof the tympanic amainv anteromedial angle of the involucrumbilatovdp bilateral oval depression bilatprso bilateral pro-cess of supraoccipital bilattub bilateral tubercle bo basioccipi-tal bs basisphenoid condfor condylar foramen dcondfs dor-sal condyloid fossa diplfor diploic foramen eam external au-ditory meatus emfor emissary foramen eo exoccipital eofacexoccipital facet for tympanic (paroccipital contact) er epitym-panic recess et Eustachian tube incisure ethfor ethmoid fora-men formg foramen magnum fr frontal frlas fronto-lacrimal suture frpas fronto-parietal suture glfs glenoidfossa hglofor hypoglossal foramen iorbcr infraorbital crestiorbfor infraorbital foramen indfrlas indentation of fronto-lacrimal suture inv involucrum jugfor jugular foramen lalacrimal lafor lacrimal foramen latprbo lateral process ofbasioccipital latptlm lateral pterygoid lamina masprpe mas-toid process of the petrosal mdptfs medial pterygoid fossamdptlm medial pterygoid lamina mdptsi medial pterygoidsinus mx maxilla na nasal nacav nasal cavity nafrs naso-frontal suture namxs naso-maxillary suture nucr nuchalcrest occcond occipital condyle optfor optic foramen or or-bitosphenoid orprs orbitosphenoid-presphenoid suture orborbit ossd ossification defect between pre- and basisphenoidovdpor oval depression on the orbitosphenoid ovfor ovalforamen ovw oval window pglfor postglenoid foramenpglpr postglenoid process porbpr postorbital process ppralposterior process (wedge) of the alisphenoid ptypr post-tympanic process pa parietal pal palatine parpreo paroccipi-tal process of exoccipital pe petrosal (periotic) pirfen piri-form fenestra pmx premaxilla pr presphenoid prm promon-torium ptfs pterygoid fossa ptpalfs pterygopalatine fossaptpral pterygoid process of the alisphenoid rdw round windowsorbfor supraorbital foramen sorbfororb supraorbital foramenof the orbit sorbr supraorbital region sagcr sagittal crest sicansinus canal sphorbfi sphenorbital fissure sphpalfor spheno-palatine foramen sq squamosal sqfac squamosal facet forthe anterior process of tympanic tetyfs tensor tympani fossatmpfor temporal foramen tmpfs temporal fossa vcondfsventral condyloid fossa zyga zygomatic arch

MATERIALS AND METHODS

Materials

The pakicetid material studied here is listed in Appendix 1Additionally cranial structures of Pakicetus inachus (cast ofGSP-UM 084) and of some other archaeocete taxa were includedin this study

Methods

We used flexible silicone rubber for making casts of the crosssections of the intertemporal supraorbital and interorbital re-gions around the skull in three different planes as shown inFigure 1 In the intertemporal region a cast was made where theskull is at its narrowest point just posterior to the frontoparietalsuture This cast was made around the skull including both leftand right sides for three pakicetid skulls and was used for draw-ing outlines of their respective areas as shown in Figure 2

In the orbital region we made casts at the plane where thesupraorbital width is narrowest and where the supraorbitalwidth is broadest ie between the postorbital processes (Fig 1)If a cast was available on only one side the other side was drawnas its mirror image These casts were made for several pakicetidand other archaeocete skulls and the outlines based on them aswell as the dorsal view of the supraorbital region are shown inFigure 3

We measured linear dimensions for several cranial structuresThe minimum and maximum supraorbital width (SOW min andSOW max respectively) were measured as shown by the dorsaloutline of the skull in Figure 1 These measurements were takenat the same regions where the orbital cross-sectional casts weremade (see previous and Figure 3) The bizygomatic width(BZW) and the occipital condyle width (OCW) were also mea-sured as possible indicators for body size These measurementswere not always available from both sides however the data inTable 1 always represent the bilateral value Additionally asfurther potential indicators of body size the foramen magnumwidth (FMW) and foramen magnum height (FMH) were mea-sured The anteroposterior tympanic bullar length (TBL) wasmeasured whenever possible A standard sliding caliper was usedfor taking these linear measurements

FIGURE 1 Pakicetid skull showing the supraorbital interorbital andintertemporal regions where the cross-sectional casts were made andwhere the minimum and maximum supraorbital widths were measuredA dorsal view B lateral view (The skull shown here is Ichthyolestespinfoldi from Figs 7A and 8A)

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Data on cranial measurements are presented in Table 1 Datafor the tympanic bullar length are presented in Appendix 1 (in-dividual lengths) and in Table 2 (the range and the mean valuesfor each species)

DESCRIPTION

Subsequent descriptions are general covering all species ofpakicetids from H-GSP Loc 62 Differences between the speciesare listed

PremaxillamdashThe premaxilla of Pakicetus attocki (H-GSP18467) is long and narrow (Fig 4A) The fossa for the externalnaris is on medial side A partial alveolus for I1 complete alveolifor I2ndashC1 and the crown of I3 can be seen

MaxillamdashThe lateral wall of the maxilla of P attocki (H-GSP18470) is high and the palate is narrow (Fig 4A) The infra-orbital foramen is located over P3 and is 100 mm high and 95mm wide (Fig 4A) On the medial side the ventral and lateralwalls of the nasal cavity surround a maxillary sinus that extendsfrom the area dorsal of P3 to M2 The unilateral width of the

palate is 146 mm (as measured from the labial side of P4 to themedian plane) A suture for the nasal bones occurs dorsal to theinfraorbital foramen (Fig 4A)

The infraorbital foramen of Nalacetus ratimitus (H-GSP30306) is similar to that of P attocki (H-GSP 18470) but themaxillary sinus is smaller A diastema is present between P2 andP3 in Nalacetus

NasalmdashThe nasal is long slender and flat in P attocki andNalacetus ratimitus (H-GSP 96231 and 96386 respectivelyFigs 4A 5B) The suture with the maxilla extends in the para-sagittal plane in P attocki (H-GSP 96231 Fig 4A) There is nocontact with the lacrimal (Fig 4A) On the face the nasofrontalsuture extends anteroposteriorly but at the posterior end of thenasals the left and right nasofrontal sutures converge showinga V-shaped medial process (H-GSP 96231 96386 Figs 4B 5B)In juvenile P attocki (H-GSP 96623) and in Ichthyolestes pin-foldi (H-GSP 98134) the suture between the nasal and frontalconsists of several parallel parasagittal lamellae (Figs 6B 7A)which are visible on the frontal The ventral side of the nasalbone in P attocki (H-GSP 96231) and Nalacetus ratimitus (H-GSP 96386) forms the dorsal wall of the nasal cavity two longgrooves are impressions of the cavity for nasal sinus (Fig 5AC)On the ventral surface of the nasal there are also fossaefor paranasal sinuses Bony ridges in the nasal cavity implyingthe presence of turbinates also occur in Remingtonocetus sp(undescribed specimen in the IITR-SB collections S Bajpaipers comm)

LacrimalmdashThe lacrimal is a sharp-edged triangular bone ros-tral to the frontal seen in P attocki (H-GSP 96231 Figs 4A5A) The lacrimal foramen (Fig 4B) is on the orbital side thesuture between lacrimal and frontal extends dorsoventrally (Fig4B) This suture can also be seen in juvenile P attocki (H-GSP96623) in N ratimitus (H-GSP 96386) and in I pinfoldi (H-GSP98134 Figs 5B 6B 7A 8A) although in these specimens thelacrimal is missing The orbital rim is indented where the fronto-lacrimal suture crosses it (H-GSP 96231 96386 96623 98134Figs 4B 5B 6C 7A 8A) This indentation probably housed theangular vein of the eye

FrontalmdashThe frontal in P attocki N ratimitus and I pinfoldi(H-GSP 96231 96254 96386 96574 96623 98134 Figs 4A5AB 6B 7A 8A) is long and slender and forms most of theorbit (Figs 4B 5 6B 7A 8C 9) and the whole supraorbitalregion (Figs 4B 5B 6B 7A 8A) The supraorbital foramen isvisible on this region (Figs 4B 5B 6B 7A) The orbits are closeto each other and face more dorsal than lateral in all pakicetid

FIGURE 2 Cross sections at the intertemporal region (the narrowestpart of the skull posterior to the frontoparietal suture) A Pakicetusattocki (H-GSP 96231) B Pakicetus attocki (H-GSP 96623) C Ich-thyolestes pinfoldi (H-GSP 98134) The dotted line shows the estimateddorsal outline of the partly broken sagittal crest Scale bar equals 2 cm

FIGURE 3 Supraorbital region of several Eocene archaeocetes representing the families Pakicetidae Ambulocetidae Remingtonocetidaeand Protocetidae showing the size and orientation of the orbits These drawings are standardized to the maximum supraorbital width Scale barsequal 5 cm

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genera (Fig 3) Ethmoidal foramina are visible on the ventralside of the orbit in pakicetids (H-GSP 96231 96254 9657496623 98134 Fig 6C) and small diploic foramina dorsally (Fig4AB 7A) The supraorbital foramen of the orbit in pakicetids(H-GSP 96231 96254 96623 96574 98134 Figs 4A 6C 8C 9)the opening for the supraorbital canal is located in the interor-bital region close to the median plane inferior to the postorbitalprocess (H-GSP 96231 96386 96574 96623 98134 Figs 4B 56AB 7A 8C 9) The minimum and maximum supraorbitalwidths vary (Fig 3 Table 1) The shape of the supraorbital re-

gion varies in pakicetids (Fig 3) it is deeply incised in P attocki(H-GSP 96231 96254 96623 Figs 3H I) slightly concave in Ipinfoldi (H-GSP 96574 98134 Fig 3J) and relatively flat in Nratimitus (H-GSP 96386 Fig 3K) The extent to which the post-orbital process projects laterally varies in P attocki (H-GSP96231 96254 96623 Figs 3O P) the process is dorsoventrallythick in I pinfoldi (H-GSP 96574 98134 Fig 3Q) the process isthin in N ratimitus (H-GSP 96386 Fig 3R) the process is thickbut short and stout compared to P attocki (for numerical valuessee Table 1)

TABLE 1 Cranial measurements (in mm) of pakicetids compared with those of other archaeocetes

Specimen no TaxonSOWmin

SOWmax

SOWminmax BZW OCW FMW FMH

H-GSP 96231 Pakicetus attocki 190 484 039 127 527 178 143H-GSP 96623 Pakicetus attocki 189 366 052 924 mdash mdash mdashH-GSP 96254 Pakicetus attocki 1673 mdash mdash mdash mdash mdash mdashGSP-UM 084 Pakicetus inachus1 mdash mdash mdash 143 537 156 137H-GSP 98134 Ichthyolestes pinfoldi 229 424 054 980 436 159 96H-GSP 96386 Nalacetus ratimitus 231 3922 059 126 4713 164 129H-GSP 30432 Nalacetus ratimitus mdash mdash mdash 109 mdash mdash mdashH-GSP 18507 Ambulocetus natans 66 74 089 259 793 328 423IITR-SB 2529 Remingtonocetus sp 952 112 085 254 957 357 306LUVP 11034 Indocetus ramani 151 181 083 mdash mdash mdash mdashGSP-UM 3012 Rodhocetus kasrani mdash 1954 mdash 2604 mdash mdash mdashChM PV5401 Carolinacetus gingerichi mdash 2565 mdash mdash mdash mdash mdash

Dorudon atrox 6ndash10 3086 3416 090 3997 1168 5389 36610

SOW supraorbital width minimum and maximum BZW bizygomatic width OCW occipital condyle width FMW foramen magnum width FMHforamen magnum height For the asterisked specimens () outlines for supraorbital regions and the cross-sections at the two points of this regionare shown in Figure 31Measurements taken on a cast the holotype described by Gingerich and Russell (1981) and Gingerich et al (1983)2Minimum value full width not preserved3Estimate4From Gingerich et al (1994 Table 1)5From Geisler et al (2005 Table 1)6These values measured from Uhen (2004Fig 32 a photo in dorsal view of GSP-UM 101222) The postorbital process distance falls well inside therange reported by Uhen (2004 Appendix IVA range is 224ndash356 mm mean 279 mm n 11)7Mean value (n 3) based on data by Uhen (2004 Appendix IVB)8Mean value (n 7) based on data by Uhen (2004 Appendix IVB)9Mean value (n 6) based on data by Uhen (2004 Appendix IVB)10Mean value (n 5) based on data by Uhen (2004 Appendix IVB)

TABLE 2 Morphological characters in pakicetid tympanics and their appearance in different species

Character Pakicetus attocki Ichthyolestes pinfoldi Nalacetus ratimitus Pakicetid sp nov cf P inachus

Incisure for theeustachian tubeshape

DeepH-GSP 30386 91035

92100 96344

ShallowH-GSP 92003

96283 96431

DeepH-GSP 30459

96384 96385

Widely open medially and very broadH-GSP 30198 30424 98141

Incisure for theeustachian tubeorientation

MediolateralH-GSP 30386 92100

96192 96344

DorsoventralH-GSP 92003 96283

More mediolateralH-GSP 96431

DorsoventralH-GSP 30459

96384 96385

DorsoventralH-GSP 30198 30424 98141

Anteromedial angleof the involucrum

Sharp and highH-GSP 30386 91035

92100 96344

Blunt and lowH-GSP 92003

96283 96431

Sharp and highH-GSP 30459

96384 96385

Very low set caudal this region issmooth no process formingan angle

H-GSP 30198 30424 98141Skull size Larger than Ichthyolestes

similar to NalacetusSmaller than Pakicetus

and NalacetusSimilar to Pakicetus

larger thanIchthyolestes

mdash

Tympanic bullarsize absolute

Small Small Large Small

Range of TBL(mm)

2488ndash2694 2327ndash2737 2873ndash3180 2639ndash2803

TBL mean plusmn SE 2596 plusmn 050 (n 4) 2562 plusmn 085 (n 4) 3007 plusmn 091 (n 3) 2706 plusmn 050 (n 3)TBL meanBZW1 020 026 024 0192

TBL meanOCW1 049 059 064 0502

Tympanic bullarsize relative

Smaller compared toIchthyolestes and Nalacetus

Larger compared toPakicetus

Larger comparedto Pakicetus

As in P attocki

For a list of all tympanics see Appendix 11BZW and OCW values are from Table 1 from specimens H-GSP 96231 96386 and 981342We used BZW and OCW for Pakicetus inachus GSP-UM 084 (see Table 1)

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Caudal to the postorbital process the frontal narrows at thefrontoparietal suture in all pakicetid genera (H-GSP 9623196254 96386 96574 96623 98134 98199 Figs 4B 5B 6B 7A8A) This suture extends across the posterior side of the postor-bital process In lateral view the frontoparietal suture extendsposteriorly into the temporal fossa (Figs 4A 5B 6B 7A 8A)until it reaches the orbitosphenoid (Figs 6C 8C) where it archesanterior and ventral reaching the presphenoid (Fig 8C) Thisarea of the skull is extremely narrow as shown by cross sectionsin Figure 2 the width is 40 mm in juvenile and 55 mm in adultP attocki (H-GSP 96623 and 96231 respectively) and 47 mm inI pinfoldi (H-GSP 98134)

ParietalmdashThe parietal shares sutures with the frontal orbito-sphenoid alisphenoid squamosal and occipital in all pakicetidgenera (H-GSP 96231 96254 96386 96574 96623 98134 Figs4A 5B 6B 7A 8A) and forms most of the sagittal crest (Figs4A 5B 6B 8A) which is somewhat higher than the braincase(the full sagittal crest is not preserved in any specimen) There isno evidence of an interparietal bone At the most anterior part ofthe sagittal crest the skull is extremely narrow (Fig 2 see thesection on the frontal bone) Posteriorly the parietal only formsa minor part of the wall of the braincase (H-GSP 96231 9638696623 98134) The parietal does not contribute to the nuchalcrest (H-GSP 96231 96254 96386 96623 Figs 4B 5B 6BD)There is no evidence of a postparietal foramen

PalatinemdashThe flat area inferior to the orbit anterior to theorbitosphenoid where the infraorbital nerve and sphenopalatine

artery are located is the pterygopalatine fossa this feature oc-curs in P attocki (H-GSP 96623) and I pinfoldi (H-GSP 9657498134 Figs 6C 8C 9) No sutures are preserved on the medialwall of the pterygopalatine fossa but this wall is probably formedby the palatine (Fig 8A) maxilla and frontal bones A horizon-tal infraorbital crest separates the pterygopalatine fossa from theorbit (Figs 4A 6C 8C 9) A small sphenopalatine foramen islocated on this crest (Figs 8C 9)

In N ratimitus (H-GSP 96055) the palatine contribution to thepalate extends anteriorly to P4 In the median plane the palatineextends far ventral causing the posterior palate to be concave

OrbitosphenoidmdashThe orbitosphenoid of P attocki (H-GSP96623) and I pinfoldi (H-GSP 98134 Figs 6C 8C) shares sutureswith the frontal parietal alisphenoid presphenoid and probablypalatine (the extent of the palatine in the pterygopalatine fossacannot be determined) Dorsally the orbitosphenoid has a smalloval depression (Fig 8C) Posteroinferior to this depression thesinus canal (Figs 6C 8C 9) is located on the parieto-orbitosphenoid suture and the groove extends anterior to itcrossing the entire orbitosphenoid Ventral to this is the opticforamen (Figs 6C 8C 9) from which a groove extends anteri-orly toward the orbit The sphenorbital fissure (Figs 6C 8C 9)is located on the suture between orbitosphenoid and alisphenoidon the extreme posterior edge of the former Only a small part ofthe suture between orbitosphenoid and presphenoid (Fig 8C) isvisible because its posterior part is covered by a large anteriorlyprojecting process of the alisphenoid (Fig 8A)

FIGURE 4 Cranial anatomy of Pakicetus attocki A skull lateral view shown by the right premaxilla right maxilla and cranium (H-GSP 1846718470 and 96231 respectively) Length of missing part of rostrum is based on Nalacetus ratimitus maxilla (H-GSP 30306) B cranium dorsal view(H-GSP 96231) These specimens although found at the same locality may not pertain to one individual Scale bar equals 5 cm

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Kellogg (1936109 oph f) identified a lsquochannel for ophthalmicarteryrsquo and lsquoforamen for ophthalmic arteryrsquo in Zygorhiza kochiilocated dorsal to the channel for optic nerve and optic foramenrespectively The ophthalmic artery is a branch of the internalcarotid artery (in mammals lacking a stapedial artery) and is

thus located ventral to the optic chiasm and optic nerve in mam-mals on the floor of the braincase Usually this artery passesthrough the optic foramen The vascular structure passingthrough the foramen described by Kellogg (ophf) instead passesposterodorsally (see Kellogg 1936fig 31bc) opposite from the

FIGURE 5 Pakicetid cranial anatomy A Pakicetus attocki cranium H-GSP 96231 ventral view B Nalacetus ratimitus cranium H-GSP 96386dorsal view C ventral view Scale bar equals 5 cm

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FIGURE 6 Pakicetid cranial anatomy A Pakicetus attocki cranium H-GSP 96623 ventral view B dorsal view C right lateral view D Pakicetusattocki H-GSP 96231 caudal view E Ichthyolestes pinfoldi H-GSP 98134 caudal view Scale bar equals 5 cm

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direction that the ophthalmic artery would have taken We in-terpret Kelloggrsquos lsquoforamen for the ophthalmic arteryrsquo as the an-terior opening for the sinus canal consistent with our interpre-tation of a similarly located foramen in pakicetids

PresphenoidmdashThe presphenoid is elongate and narrow inP attocki N ratimitus and I pinfoldi (H-GSP 96231 9638696623 98134 Figs 5AC 6A 7B) The ventral surface of the

presphenoid is rounded and smooth in Ichthyolestes whereasin Pakicetus and Nalacetus there is a distinct median crest ven-trally In Ichthyolestes much of the lateral side of the pre-sphenoid is covered by an anterior process of the alisphenoid(Fig 8A) A very small foramen occurs in this part of the pre-sphenoid and a groove extends anteriorly from this foramenWe interpret this foramen as the anterior foramen of the ptery-

FIGURE 7 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A dorsal view B ventral view C ventral view ear region in detail Dventral view outline of ear region Scale bar for AndashB equals 5 cm scale bar for CndashD equals 2 cm

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goid canal (Figs 8C 9) Anteriorly the lateral side of the pre-sphenoid reaches dorsal to touch the orbitosphenoid (Fig 8C)Ventrally the presphenoid is not fused to the basisphenoid in themidline and there is an ossification defect of 45 mm in Ich-thyolestes (Fig 7B)

AlisphenoidmdashThe alisphenoid shares sutures with pre-sphenoid orbitosphenoid parietal squamosal petrosal tym-panic basisphenoid and probably with pterygoid in P attockiN ratimitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Fig 8C) The anterior part of the alisphenoid is a rectan-gular process that lies lateral to much of the presphenoid andorbitosphenoid (H-GSP 98134 Fig 8A) and the sphenorbitalfissure is located just dorsal to this (Figs 6C 8C 9) Posteriorlythe alisphenoid forms a narrow wedge-like process betweensquamosal and basisphenoid in Pakicetus Nalacetus and Ich-thyolestes (H-GSP 30432 96231 96386 96623 98134 Figs 7D

8A) Part of this narrow process forms a prominently raised jointfacet the alisphenoid facet for the anterior process of the tym-panic (Figs 5A 6A 7D) This joint facet is continuous with asimilarly sized facet of the squamosal (Fig 5A 6A 7D) Thealisphenoid surrounds the entire oval foramen (Figs 5C 6A7D) There is no round foramen

The pterygoid process of the alisphenoid in pakicetids (H-GSP30432 96231 96386 96623 98134 Fig 7D) divides posteriorlyinto medial and lateral pterygoid laminae (Fig 7D) that sur-round a low pterygoid fossa (Fig 7D) This structure is compa-rable to the pterygoid fossa of primates and many other terres-trial mammals and should not be confused with the pterygoidsinus fossa of Basilosauridae Odontoceti and Mysticeti Themedial pterygoid lamina is low The pterygoid bone is not pre-served but was probably attached to the medial pterygoidlamina

FIGURE 9 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) right ventrolateral view showing the sphenorbital region with the foraminafrom an oblique angle The zygomatic arch is out of the focal plane of this photograph Scale bar equals 5 cm

FIGURE 8 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A right lateral view B sphenorbital region in detail C outline ofsphenorbital region Scale bar for A equals 5 cm scale bar for BndashC equals 2 cm

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BasisphenoidmdashThe basisphenoid shares sutures with pre-sphenoid alisphenoid and basioccipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Figs 5AC 6A 7B) The basisphenoid is convex vent-rally in cross section and concave in the median plane antero-posteriorly in juvenile Pakicetus and in Ichthyolestes (H-GSP96623 98134 respectively Figs 6A 7B) in adult Pakicetus(H-GSP 96231) and Nalacetus (H-GSP 96386) the basisphenoidis more or less flat in both these orientations (Figs 5AC) Pos-teriorly the suture between basisphenoid and basioccipital israised as bilateral tubercles in all pakicetid genera (H-GSP30432 96231 96386 98134 Figs 5AC 7B) One of the flexors ofthe neck rectus capitis ventralis originates from these tuberclesThere is no contact between the petrosal and the basisphenoidand the medial pterygoid sinus lies between these two bones(Fig 7D)

SquamosalmdashThe squamosal contacts the parietal alisphe-noid tympanic petrosal and occipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 98199 9625496386 96407 96623 98134 Figs 4B 5AB 6A 7AB 8A)making up most of the wall of the braincase Several smalltemporal foramina pierce the area near the occipito-squamosalsuture (Fig 6C) Posteriorly the squamosal forms part of thenuchal crest in all pakicetids (H-GSP 96231 96254 9638696623 Figs 4B 5B 6BD) Laterally the squamosal forms theposterior part of the zygomatic arch (Figs 4B 5AC 6BC 7AB8A 9) and the glenoid fossa (also called mandibular fossaFigs 5AC 6A 7Dglfs) in Pakicetus Nalacetus and Ichthyo-lestes (H-GSP 30432 96231 96386 96407 96623 98134) Theanterior part of the glenoid fossa is flat and extensive medio-laterally The postglenoid process (Figs 4A 5AC 6AC 7D8A) arches posteriorly into the external auditory meatus (Figs4A 5AC 6AC 7D 8A) The squamosal makes up the entireroof of the external auditory meatus and there is a prominentposttympanic process (Figs 4B 5B 6B 7A) The postglenoidforamen (also called retroarticular foramen Figs 5AC 6A 7D)is small and located posteromedial to the postglenoid processThe glenoid fossa extends medial to the postglenoid process andends posteromedially into a shallow groove directed to themiddle ear Medial to this is the squamosal contribution to theprominently raised joint facet (H-GSP 30432 96231 9638696623 98134 Figs 5A 6A 7D) for the anterior process of thetympanic The external auditory meatus ends medially in a re-cessed area in Pakicetus Nalacetus and Ichthyolestes (H-GSP30432 96231 96386 96407 96623 98134) The sharply depressedcenter of this recessed area represents the epitympanic recess(Figs 7D)

PetrosalmdashThe petrosal (periotic) contacts the alisphenoidsquamosal tympanic and occipital in P attocki N ratimitus andI pinfoldi (H-GSP 30432 96231 96386 96407 96431 9662398134 Fig 7D) The petrosal forms the roof of the middle earcavity The promontorium (Figs 5C 6A 7D) which houses theinner ear is a triangular structure on the ventral side The facialcanal runs anterior to the promontorium Medial to the prom-ontorium is the groove for the inferior petrosal sinus with thepiriform fenestra (H-GSP 30432 96386 96623 98134 Figs 5C7D) Rostrolateral to the promontorium is the tensor tympanifossa (H-GSP 30432 96231 96386 96407 96431 96623 98134Fig 7D) There are two foramina on the promontorium the ovalwindow for the stapes (fenestra vestibuli Fig 7D) and caudal toit the round window (fenestra cochleae Fig 7D) The mastoidprocess or posterior process of the petrosal flanks through thesquamosal posteriorly (H-GSP 30432 96231 96386 9640796623 98134 Figs 5AC 6A 7D 8A) A more detailed descrip-tion of the petrosal and tympanic of pakicetid cetaceans (Luoand Gingerich 1999) was based in part on material from H-GSPLocality 62

TympanicmdashThe tympanic (synonyms tympanic bulla audi-tory bulla ectotympanic) contacts the petrosal squamosal ali-sphenoid and exoccipital (GSP-UM 084) The tympanic sur-rounds the middle ear cavity ventrally laterally and medially(see Appendix 1 for all tympanics) The anterior part providesthe incisure for the eustachian tube (Fig 10) The medial pachy-osteosclerotic massive part of the tympanic is the involucrum(Fig 10) The involucrum is not in contact with any skull boneand is only connected to the rest of the skull by the dorsolateraland caudal parts of the tympanic The morphology described byGingerich and colleagues (1983) where the involucrum contactsthe basioccipital was based on the type specimen of P inachusGSP-UM 084 (sometimes listed as UM-GSP 081 by Luo 1998) inwhich this area is deformed

The shape and height of the anteromedial angle of the invo-lucrum (as defined by Luo 1998 Luo and Gingerich 1999Fig10) as well as the shape and orientation of the incisure forthe eustachian tube is species-specific (see Table 2) The lateralpart of the tympanic is a thin plate here the sigmoid process isa narrow and small plate of bone on the dorsal side of the lateralwall at least in I pinfoldi (H-GSP 92003) There is a small con-tact between the sigmoid process and the squamosal (see Luoand Gingerich 1999) Posterior to the sigmoid process the ex-ternal auditory meatus opens to the middle ear cavity Anteriorto the sigmoid process is the anterior process of the tympanicalso called processus tubarius which articulates with a facet onthe suture of squamosal and alisphenoid The posterior processof the tympanic contacts the mastoid (posterior) process of thepetrosal and more medially a small exoccipital facet makescontact with the tympanic (Fig 7D) this facet is the proximalpart of the paroccipital process of exoccipital (Figs 4B 5AC6ADE 7D)

There are four distinct morphologies among the tympanicsfrom H-GSP Locality 62 listed in Table 2 Three of these mor-phologies can be associated with cranial and dental material andthus be identified as P attocki I pinfoldi and N ratimitus Thefourth morphology represents a currently unidentified species ofthe family Pakicetidae Thewissen and Hussain (1998) describeda single tooth from Locality 62 tentatively as pertaining to Pinachus and it is possible that the unidentified bullae are fromthis species

In P attocki tympanics (see Appendix 1 and Table 2 Figs10AndashC) the morphology of the incisure for the eustachian tubeand of the anteromedial angle of the involucrum is similar to thatin the holotype of P inachus and therefore we refer these tym-panics to P attocki ndash which based on teeth (Thewissen andHussain 1998) is our most common species of Pakicetus Theorientation of the incisure for the eustachian tube cannot beobserved in GSP-UM 084 because the tympanic in this specimenis deformed and compressed against the basicranium

In the I pinfoldi specimen H-GSP 96431 (see Appendix 1 andTable 2 Figs 10DndashF) the fragmentary basicranium indicates asize considerably smaller than P attocki This is consistent withdental evidence (Thewissen and Hussain 1998) that indicatesthat the genus Ichthyolestes is smaller than Pakicetus We there-fore refer these specimens to I pinfoldi Although the adjoin-ing tympanic (H-GSP 96431) is the smallest one of our I pinfolditympanics two other tympanics with similar morphologyfor which the tympanic bullar length could be measured areboth within the length range of P attocki (Appendix 1 and Ta-ble 2)

The N ratimitus tympanics (see Appendix 1 and Table 2 Figs10GndashI) with a morphology clearly distinct from the two firstones described in Table 2 are larger than those of P attocki andmost of I pinfoldi and this serves to identify H-GSP 30325 and98140 Articular facets of Nalacetus tympanics (H-GSP 3032530459 96384 96385) match the facets on the exoccipital and

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 755

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squamosal of a Nalacetus skull (H-GSP 96386) The Nalacetusskull differs from Pakicetus (see previous) even though theseskulls are similar in size (Table 1) The Nalacetus skull is muchlarger than the Ichthyolestes skull (Table 1) This further cor-

roborates our identification of Nalacetus as a different genusand allocation of these tympanics to it

Three other remaining pakicetid tympanics (see Appendix 1and Table 2 Figs 10JndashL) are distinct from the three common

FIGURE 10 Pakicetid tympanics vertical columns showing the anterior lateral and dorsal views respectively AndashC Pakicetus attocki (H-GSP30386 left) DndashF Ichthyolestes pinfoldi (H-GSP 30223 left) GndashI Nalacetus ratimitus (H-GSP 96384 left) JndashL pakicetid n sp (H-GSP 98141 right)Scale bar equals 2 cm

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species of pakicetids at H-GSP Locality 62 and probably repre-sent a fourth new species of pakicetid These tympanics areclosest to P attocki tympanics in their size

OccipitalmdashThe most anterior point of the basioccipital of Pattocki N ratimitus and I pinfoldi (H-GSP 96231 96386 98134Figs 5AC 7B) is its suture with the basisphenoid which israised into bilateral tubercles (H-GSP 30432 96231 9638696623 98134 Figs 5AC 7B lsquobasioccipital crestrsquo of Luo 1998lsquoincipient falcate process of basioccipitalrsquo of Luo and Gingerich1999) These tubercles for neck flexors (see basisphenoid above)extend onto the anterior side of the basioccipital Posterior tothis the basioccipital projects laterally as a flat process in Paki-cetus (H-GSP 96231) and in Ichthyolestes (H-GSP 98134 Figs5A 7B) but this process is poorly defined in Nalacetus (H-GSP96386) The lateralmost part of this process is formed by theexoccipital (Figs 5AC 7B) and this process also forms the dor-solateral wall of the jugular foramen of pakicetids (H-GSP96231 96386 96407 96431 98134 Figs 5AC 7D) The exoc-cipitals bear the occipital condyles (H-GSP 96231 96386 9643198134 98199 Figs 4A 5AC 6DE 7B 8A 9) which are com-pressed dorsoventrally do not touch in the median plane andare somewhat raised above the level of the exoccipital The dor-sal and ventral condyloid fossae are of equal size (H-GSP 9623198134 98199 Figs 5AC 6E 7D) The hypoglossal foramen inall pakicetid genera (H-GSP 96231 96386 96407 96431 98134Figs 5AC 7D) is situated approximately halfway between thejugular foramen and the occipital condyle One or two smallcondylar foramina (Figs 5AC 7D) pierce the basioccipital lat-eral to the hypoglossal foramen The exoccipital forms the me-diolaterally elongate paroccipital process in all pakicetids (H-GSP 30432 96231 96386 96431 96623 98134 Figs 4B 5AC6ADE 7D) The medial extremity of this process bears a flatfacet for the tympanic (Fig 7D lsquobullar process of the exoccipitalrsquoof Luo and Gingerich 1999) The anterior part of the paroccipi-tal process shares a long suture with the mastoid process of pe-trosal The paroccipital process projects posterolaterally and thepart adjacent to the occipital condyle is recessed

The supraoccipital makes up most of the posterior side of thenuchal crest in all pakicetids (H-GSP 96231 96254 96386 96623Figs 4B 5B 6BD) although this structure is clearly worn insome specimens eg in I pinfoldi (H-GSP 98134) A small bi-lateral process of supraoccipital visible in Pakicetus and Ich-thyolestes (H-GSP 96231 98134 Figs 6DE) for the epaxial ex-tensors of the neck projects from the dorsal edge of the foramenmagnum (H-GSP 96231 96386 98134 Figs 6DE)Variable em-issary foramina (H-GSP 96231 96254 96386 96623 98134 Figs6DE) pierce the occipital dorsal to the foramen magnum Abilateral oval depression (H-GSP 96231 96386 98134 Figs6DE) occurs just lateral to the midline well above the foramenmagnum This depression could be for musculus longissimuscapitis The precise shape of the supraoccipital cannot be deter-mined because the suture between the exoccipital and the su-praoccipital is not visible

DISCUSSION

The pakicetid crania described here help to understand varia-tion in skull morphology of the earliest cetaceans As such theycan be expected to play a significant role in systematic studiesbut a detailed analysis of character scores for these specimensfalls outside the scope of the present description Here discus-sion will focus on a few aspects of cranial morphology that arerarely involved in cetacean systematics although they displayvariation to an extent that may be relevant systematically andinteresting evolutionarily

Supraorbital Morphology

The supraorbital region of protocetids basilosaurids anddorudontids displays a supraorbital shield (Kellogg 1936 Gin-

gerich et al 1994 1995 Bajpai and Gingerich 1998 Hulbert1998 Uhen 1998 2004) However the basal families of ceta-ceans lack a supraorbital shield pakicetids (Thewissen et al2001b) ambulocetids (Thewissen et al 1996) and remingtono-cetids (Gingerich et al 1994 1995 Bajpai and Thewissen 1998)The new pakicetid specimens indicate that there is much varia-tion in supraorbital morphology within these families In paki-cetids the orbits are convergent dorsally The area between theorbits is recessed over the superior orbital rim (Figs 4ndash8) Assuch pakicetid eyes are directed dorsolaterally There are dif-ferences between the genera (Fig 3 Table 1) in Pakicetus andIchthyolestes the supraorbital region is cuplike and cradles thedorsal part of the eye and the dorsal side of the supraorbitalregion is deeply incised in the medial plane in Pakicetus Thisincisure is less strong in Nalacetus and Ichthyolestes and in Nala-cetus the contact between supraorbital region and eye is toonarrow to be cup-shaped In Ambulocetus (Fig 3 Table 1Thewissen et al 1996fig 10 for H-GSP 18490) the orbits arepositioned near the dorsal side of the skull but they face later-ally leaving the supraorbital region more or less flat (in dorsalview) The eyes here appear to be directed laterally and are nothoused in the bony orbits as deeply as in pakicetids as indicatedby the shape of the postorbital processes As a comparison inRemingtonocetus the eyes are small are set laterally and facelaterally and the supraorbital region is strongly convex (Fig 3Sahni and Mishra 1972 Kumar and Sahni 1986 Bajpai andThewissen 1998) In protocetids the orbit is large faces later-ally and is covered by a large supraorbital shield (Fig 3 Table1) Dorudontids and basilosaurids resemble protocetids in thepresence and shape of their supraorbital shield as indicated bythe ratio between the minimum and maximum supraorbitalwidth (Table 1) and in the orientation of their orbits the latterare still clearly different from modern odontocetes and mystice-tes (Kellogg 1936 Uhen 1998 2004)

Intertemporal Morphology

In early and middle Eocene cetaceans a very narrow regioncan be seen in the skull posterior to the frontoparietal suturenear the sphenorbital fissure (Figs 1 2 4ndash8) This morphology isnot seen in most land mammals but is common in early ceta-ceans such as pakicetids ambulocetids (eg AmbulocetusThewissen et al 1996) and remingtonocetids (Kumar and Sahni1986) and may represent an important diagnostic character forthe order

Tympanic Bullar Morphology

The differences in the tympanic bullar morphology seen be-tween P attocki I pinfoldi N ratimitus and a fourth unnamedspecies can be used for specific recognition However in generalall these tympanics display a similar morphology indicating thatthese early archaeocetes used their peripheral ear structures likeland mammals while hearing in air and relied on bone conduc-tion mechanism while hearing in water (Thewissen and Hussain1993 Luo 1998 Luo and Gingerich 1999 Nummela et al 2004)

Pakicetus attocki is clearly larger than I pinfoldi whereas Pa-kicetus and Nalacetus have similar body sizes (Table 1) Thewis-sen and Hussain (1998) studied the size of upper and lower mo-lars in P attocki P inachus I pinfoldi and N ratimitus Theyfound Ichthyolestes to be the smallest P attocki and Nalacetus tobe of similar size and P inachus to be the largest Our studyshows that the absolute tympanic bullar size is similar in Paki-cetus and Ichthyolestes but larger in Nalacetus (Table 2 Appen-dix 1) This implies that in relative terms Pakicetus tympanicswhen related to body size (Table 2) are smaller than those ofIchthyolestes and Nalacetus

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ACKNOWLEDGMENTS

We thank Ellen Byron and Jeanette Killius for their technicalassistance Linda Spurlock prepared the drawings of the skullswhich is gratefully acknowledged Sunil Bajpai kindly gave usaccess to Remingtonocetus specimens in the IITR-SB collectionsunder his care We thank the editor Ewan Fordyce and Zhe-XiLuo and an anonymous reviewer for constructive suggestions forimprovement of the manuscript Financial support was providedby the National Science Foundation (current grant number EAR0207370) to JGMT and by the Ella and Georg Ehrnrooth Foun-dation to SN This article is a contribution to the Howard Uni-versity-Geological Survey of Pakistan projected entitled ldquoCeno-zoic Mammals of Pakistanrdquo We thank the Geological Survey ofPakistan for their support and help in the field

LITERATURE CITED

Aslan A and J G M Thewissen 1996 Preliminary evaluation of pa-leosols and implications for interpreting vertebrate fossil assem-blages Kuldana Formation Northern Pakistan Palaeovertebrata25261ndash277

Bajpai S and P D Gingerich 1998 A new Eocene archaeocete (Mam-malia Cetacea) from India and the time of origin of whales Pro-ceedings of the National Academy of Sciences USA 9515464ndash15468

Bajpai S and J G M Thewissen 1998 Middle Eocene cetaceans fromthe Harudi and Subathu formations of India pp 213ndash233 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Dehm R and T zu Oettingen-Spielberg 1958 Palaumlontologische undgeologische Untersuchungen im Tertiaumlr von Pakistan 2 Die mitte-leocaumlnen Saumlugetiere von Ganda Kas bei Basal in Nordwest-Pakistan Bayerische Akademie der Wissenschaften 911ndash54

Geisler J H and Z-X Luo 1998 Relationships of Cetacea to terres-trial ungulates and the evolution of cranial vasculature in Cete pp163ndash212 in J G M Thewissen (ed) The Emergence of WhalesPlenum Press New York

Geisler J H A E Sanders and Z-X Luo 2005 A new protocetidwhale (Cetacea Archaeoceti) from the late Middle Eocene of SouthCarolina American Museum Novitates 34801ndash65

Gingerich P D 2003 Land-to-sea transition in early whales evolutionof Eocene Archaeoceti (Cetacea) in relation to skeletal proportionsand locomotion of living semiaquatic mammals Paleobiology 29429ndash454

Gingerich P D and D E Russell 1981 Pakicetus inachus a newarchaeocete (Mammalia Cetacea) from the early-middle EoceneKuldana Formation of Kohat (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 25235ndash246

Gingerich P D and D E Russell 1990 Dentition of early EocenePakicetus (Mammalia Cetacea) Contributions from the Museum ofPaleontology University of Michigan 281ndash20

Gingerich P D N A Wells D E Russell and S M I Shah 1983Origin of whales in epicontinental remnant seas new evidence fromthe early Eocene of Pakistan Science 220403ndash406

Gingerich P D S M Raza M Arif M Anwar and X Zhou 1994New whale from the Eocene of Pakistan and the origin of cetaceanswimming Nature 368844ndash847

Gingerich P D M Arif and W C Clyde 1995 New archaeocetes(Mammalia Cetacea) from the middle Eocene Domanda Formationof the Sulaiman Range Punjab (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 29291ndash330

Gingerich P D M ul-Haq I S Zalmout I H Khan and M S Mal-kani 2001 Origin of whales from early artiodactyls hands and feetof Eocene Protocetidae from Pakistan Science 2932239ndash2242

Hulbert R Jr 1998 Postcranial osteology of the North American

middle Eocene protocetid Georgiacetus pp 235ndash267 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Kellogg A R 1936 A review of the Archaeoceti Carnegie Institute ofWashington Publications 4821ndash366

Kumar K and A Sahni 1986 Remingtonocetus harudiensis new com-bination a middle Eocene archaeocete (Mammalia Cetacea) fromwestern Kutch India Journal of Vertebrate Paleontology 6326ndash349

Luo Z-X 1998 Homology and transformation of cetacean ectotym-panic structures pp 269ndash301 in J G M Thewissen (ed) The Emer-gence of Whales Plenum Press New York

Luo Z-X and P D Gingerich 1999 Terrestrial Mesonychia to aquaticCetacea transformation of the basicranium and evolution of hearingin whales University of Michigan Papers on Paleontology 311ndash98

Madar S I In press The postcranial skeleton of early Eocene pakicetidcetaceans Journal of Paleontology

Nummela S J G M Thewissen S Bajpai S T Hussain and K Ku-mar 2004 Eocene evolution of whale hearing Nature 430776ndash778

Sahni A and V P Mishra 1972 A new species of Protocetus (Cetacea)from the middle Eocene of Kutch Western India Palaeontology15490ndash495

Thewissen J G M and S T Hussain 1993 Origin of underwaterhearing in whales Nature 361444ndash445

Thewissen J G M and S T Hussain 1998 Systematic review of thePakicetidae early and middle Eocene Cetacea (Mammalia) fromPakistan and India Bulletin of Carnegie Museum of Natural History34220ndash238

Thewissen J G M and E M Williams 2002 The early radiations ofCetacea (Mammalia) evolutionary patterns and developmental cor-relations Annual Review of Ecology and Systematics 3373ndash90

Thewissen J G M S I Madar and S T Hussain 1996 Ambulocetusnatans an Eocene cetacean (Mammalia) from Pakistan CourierForschungsinstitut Senckenberg 191 1ndash86

Thewissen J G M E M Williams and S T Hussain 2001a Eocenemammal faunas from Northern Indo-Pakistan Journal of Verte-brate Paleontology 21347ndash366

Thewissen J G M E M Williams L J Roe and S T Hussain 2001bSkeletons of terrestrial cetaceans and the relationship of whales toartiodactyls Nature 413277ndash281

Thewissen J G M S T Hussain M Arif A Aslan S I Madar andL J Roe 1998 New localities of Eocene vertebrates in NorthernPakistan and their significance for the origin of the modern orders ofmammals pp 19ndash34 in M I Ghaznavi S M Raza and M T Hasan(eds) Siwaliks of South Asia Proceedings of the 3rd GEOSASWorkshop Islamabad Pakistan 1ndash5 March 1997 Geological Sur-vey of Pakistan Islamabad Pakistan

Uhen M D 1998 Middle to late Eocene basilosaurines and dorudon-tines pp 29ndash61 in J G M Thewissen (ed) The Emergence ofWhales Plenum Press New York

Uhen M D 2004 Form function and anatomy of Dorudon atrox(Mammalia Cetacea) an archaeocete from the middle to lateEocene of Egypt University of Michigan Papers on Paleontology341ndash222

West R M 1980 Middle Eocene large mammal assemblage withTethyan affinities Ganda Kas region Pakistan Journal of Paleon-tology 54508ndash533

West R M and J R Lukacs 1979 Geology and vertebrate-fossil lo-calities Tertiary continental rocks Kala Chitta Hills Attock Dis-trict Pakistan Milwaukee Public Museum Contributions in Biologyand Geology 261ndash20

Williams E M 1998 Synopsis of the earliest cetaceans PakicetidaeAmbulocetidae Remingtonocetidae and Protocetidae pp 1ndash28 inJ G M Thewissen (ed) The Emergence of Whales Plenum PressNew York

Submitted 7 October 2005 accepted 24 February 2006

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APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

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Data on cranial measurements are presented in Table 1 Datafor the tympanic bullar length are presented in Appendix 1 (in-dividual lengths) and in Table 2 (the range and the mean valuesfor each species)

DESCRIPTION

Subsequent descriptions are general covering all species ofpakicetids from H-GSP Loc 62 Differences between the speciesare listed

PremaxillamdashThe premaxilla of Pakicetus attocki (H-GSP18467) is long and narrow (Fig 4A) The fossa for the externalnaris is on medial side A partial alveolus for I1 complete alveolifor I2ndashC1 and the crown of I3 can be seen

MaxillamdashThe lateral wall of the maxilla of P attocki (H-GSP18470) is high and the palate is narrow (Fig 4A) The infra-orbital foramen is located over P3 and is 100 mm high and 95mm wide (Fig 4A) On the medial side the ventral and lateralwalls of the nasal cavity surround a maxillary sinus that extendsfrom the area dorsal of P3 to M2 The unilateral width of the

palate is 146 mm (as measured from the labial side of P4 to themedian plane) A suture for the nasal bones occurs dorsal to theinfraorbital foramen (Fig 4A)

The infraorbital foramen of Nalacetus ratimitus (H-GSP30306) is similar to that of P attocki (H-GSP 18470) but themaxillary sinus is smaller A diastema is present between P2 andP3 in Nalacetus

NasalmdashThe nasal is long slender and flat in P attocki andNalacetus ratimitus (H-GSP 96231 and 96386 respectivelyFigs 4A 5B) The suture with the maxilla extends in the para-sagittal plane in P attocki (H-GSP 96231 Fig 4A) There is nocontact with the lacrimal (Fig 4A) On the face the nasofrontalsuture extends anteroposteriorly but at the posterior end of thenasals the left and right nasofrontal sutures converge showinga V-shaped medial process (H-GSP 96231 96386 Figs 4B 5B)In juvenile P attocki (H-GSP 96623) and in Ichthyolestes pin-foldi (H-GSP 98134) the suture between the nasal and frontalconsists of several parallel parasagittal lamellae (Figs 6B 7A)which are visible on the frontal The ventral side of the nasalbone in P attocki (H-GSP 96231) and Nalacetus ratimitus (H-GSP 96386) forms the dorsal wall of the nasal cavity two longgrooves are impressions of the cavity for nasal sinus (Fig 5AC)On the ventral surface of the nasal there are also fossaefor paranasal sinuses Bony ridges in the nasal cavity implyingthe presence of turbinates also occur in Remingtonocetus sp(undescribed specimen in the IITR-SB collections S Bajpaipers comm)

LacrimalmdashThe lacrimal is a sharp-edged triangular bone ros-tral to the frontal seen in P attocki (H-GSP 96231 Figs 4A5A) The lacrimal foramen (Fig 4B) is on the orbital side thesuture between lacrimal and frontal extends dorsoventrally (Fig4B) This suture can also be seen in juvenile P attocki (H-GSP96623) in N ratimitus (H-GSP 96386) and in I pinfoldi (H-GSP98134 Figs 5B 6B 7A 8A) although in these specimens thelacrimal is missing The orbital rim is indented where the fronto-lacrimal suture crosses it (H-GSP 96231 96386 96623 98134Figs 4B 5B 6C 7A 8A) This indentation probably housed theangular vein of the eye

FrontalmdashThe frontal in P attocki N ratimitus and I pinfoldi(H-GSP 96231 96254 96386 96574 96623 98134 Figs 4A5AB 6B 7A 8A) is long and slender and forms most of theorbit (Figs 4B 5 6B 7A 8C 9) and the whole supraorbitalregion (Figs 4B 5B 6B 7A 8A) The supraorbital foramen isvisible on this region (Figs 4B 5B 6B 7A) The orbits are closeto each other and face more dorsal than lateral in all pakicetid

FIGURE 2 Cross sections at the intertemporal region (the narrowestpart of the skull posterior to the frontoparietal suture) A Pakicetusattocki (H-GSP 96231) B Pakicetus attocki (H-GSP 96623) C Ich-thyolestes pinfoldi (H-GSP 98134) The dotted line shows the estimateddorsal outline of the partly broken sagittal crest Scale bar equals 2 cm

FIGURE 3 Supraorbital region of several Eocene archaeocetes representing the families Pakicetidae Ambulocetidae Remingtonocetidaeand Protocetidae showing the size and orientation of the orbits These drawings are standardized to the maximum supraorbital width Scale barsequal 5 cm

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genera (Fig 3) Ethmoidal foramina are visible on the ventralside of the orbit in pakicetids (H-GSP 96231 96254 9657496623 98134 Fig 6C) and small diploic foramina dorsally (Fig4AB 7A) The supraorbital foramen of the orbit in pakicetids(H-GSP 96231 96254 96623 96574 98134 Figs 4A 6C 8C 9)the opening for the supraorbital canal is located in the interor-bital region close to the median plane inferior to the postorbitalprocess (H-GSP 96231 96386 96574 96623 98134 Figs 4B 56AB 7A 8C 9) The minimum and maximum supraorbitalwidths vary (Fig 3 Table 1) The shape of the supraorbital re-

gion varies in pakicetids (Fig 3) it is deeply incised in P attocki(H-GSP 96231 96254 96623 Figs 3H I) slightly concave in Ipinfoldi (H-GSP 96574 98134 Fig 3J) and relatively flat in Nratimitus (H-GSP 96386 Fig 3K) The extent to which the post-orbital process projects laterally varies in P attocki (H-GSP96231 96254 96623 Figs 3O P) the process is dorsoventrallythick in I pinfoldi (H-GSP 96574 98134 Fig 3Q) the process isthin in N ratimitus (H-GSP 96386 Fig 3R) the process is thickbut short and stout compared to P attocki (for numerical valuessee Table 1)

TABLE 1 Cranial measurements (in mm) of pakicetids compared with those of other archaeocetes

Specimen no TaxonSOWmin

SOWmax

SOWminmax BZW OCW FMW FMH

H-GSP 96231 Pakicetus attocki 190 484 039 127 527 178 143H-GSP 96623 Pakicetus attocki 189 366 052 924 mdash mdash mdashH-GSP 96254 Pakicetus attocki 1673 mdash mdash mdash mdash mdash mdashGSP-UM 084 Pakicetus inachus1 mdash mdash mdash 143 537 156 137H-GSP 98134 Ichthyolestes pinfoldi 229 424 054 980 436 159 96H-GSP 96386 Nalacetus ratimitus 231 3922 059 126 4713 164 129H-GSP 30432 Nalacetus ratimitus mdash mdash mdash 109 mdash mdash mdashH-GSP 18507 Ambulocetus natans 66 74 089 259 793 328 423IITR-SB 2529 Remingtonocetus sp 952 112 085 254 957 357 306LUVP 11034 Indocetus ramani 151 181 083 mdash mdash mdash mdashGSP-UM 3012 Rodhocetus kasrani mdash 1954 mdash 2604 mdash mdash mdashChM PV5401 Carolinacetus gingerichi mdash 2565 mdash mdash mdash mdash mdash

Dorudon atrox 6ndash10 3086 3416 090 3997 1168 5389 36610

SOW supraorbital width minimum and maximum BZW bizygomatic width OCW occipital condyle width FMW foramen magnum width FMHforamen magnum height For the asterisked specimens () outlines for supraorbital regions and the cross-sections at the two points of this regionare shown in Figure 31Measurements taken on a cast the holotype described by Gingerich and Russell (1981) and Gingerich et al (1983)2Minimum value full width not preserved3Estimate4From Gingerich et al (1994 Table 1)5From Geisler et al (2005 Table 1)6These values measured from Uhen (2004Fig 32 a photo in dorsal view of GSP-UM 101222) The postorbital process distance falls well inside therange reported by Uhen (2004 Appendix IVA range is 224ndash356 mm mean 279 mm n 11)7Mean value (n 3) based on data by Uhen (2004 Appendix IVB)8Mean value (n 7) based on data by Uhen (2004 Appendix IVB)9Mean value (n 6) based on data by Uhen (2004 Appendix IVB)10Mean value (n 5) based on data by Uhen (2004 Appendix IVB)

TABLE 2 Morphological characters in pakicetid tympanics and their appearance in different species

Character Pakicetus attocki Ichthyolestes pinfoldi Nalacetus ratimitus Pakicetid sp nov cf P inachus

Incisure for theeustachian tubeshape

DeepH-GSP 30386 91035

92100 96344

ShallowH-GSP 92003

96283 96431

DeepH-GSP 30459

96384 96385

Widely open medially and very broadH-GSP 30198 30424 98141

Incisure for theeustachian tubeorientation

MediolateralH-GSP 30386 92100

96192 96344

DorsoventralH-GSP 92003 96283

More mediolateralH-GSP 96431

DorsoventralH-GSP 30459

96384 96385

DorsoventralH-GSP 30198 30424 98141

Anteromedial angleof the involucrum

Sharp and highH-GSP 30386 91035

92100 96344

Blunt and lowH-GSP 92003

96283 96431

Sharp and highH-GSP 30459

96384 96385

Very low set caudal this region issmooth no process formingan angle

H-GSP 30198 30424 98141Skull size Larger than Ichthyolestes

similar to NalacetusSmaller than Pakicetus

and NalacetusSimilar to Pakicetus

larger thanIchthyolestes

mdash

Tympanic bullarsize absolute

Small Small Large Small

Range of TBL(mm)

2488ndash2694 2327ndash2737 2873ndash3180 2639ndash2803

TBL mean plusmn SE 2596 plusmn 050 (n 4) 2562 plusmn 085 (n 4) 3007 plusmn 091 (n 3) 2706 plusmn 050 (n 3)TBL meanBZW1 020 026 024 0192

TBL meanOCW1 049 059 064 0502

Tympanic bullarsize relative

Smaller compared toIchthyolestes and Nalacetus

Larger compared toPakicetus

Larger comparedto Pakicetus

As in P attocki

For a list of all tympanics see Appendix 11BZW and OCW values are from Table 1 from specimens H-GSP 96231 96386 and 981342We used BZW and OCW for Pakicetus inachus GSP-UM 084 (see Table 1)

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Caudal to the postorbital process the frontal narrows at thefrontoparietal suture in all pakicetid genera (H-GSP 9623196254 96386 96574 96623 98134 98199 Figs 4B 5B 6B 7A8A) This suture extends across the posterior side of the postor-bital process In lateral view the frontoparietal suture extendsposteriorly into the temporal fossa (Figs 4A 5B 6B 7A 8A)until it reaches the orbitosphenoid (Figs 6C 8C) where it archesanterior and ventral reaching the presphenoid (Fig 8C) Thisarea of the skull is extremely narrow as shown by cross sectionsin Figure 2 the width is 40 mm in juvenile and 55 mm in adultP attocki (H-GSP 96623 and 96231 respectively) and 47 mm inI pinfoldi (H-GSP 98134)

ParietalmdashThe parietal shares sutures with the frontal orbito-sphenoid alisphenoid squamosal and occipital in all pakicetidgenera (H-GSP 96231 96254 96386 96574 96623 98134 Figs4A 5B 6B 7A 8A) and forms most of the sagittal crest (Figs4A 5B 6B 8A) which is somewhat higher than the braincase(the full sagittal crest is not preserved in any specimen) There isno evidence of an interparietal bone At the most anterior part ofthe sagittal crest the skull is extremely narrow (Fig 2 see thesection on the frontal bone) Posteriorly the parietal only formsa minor part of the wall of the braincase (H-GSP 96231 9638696623 98134) The parietal does not contribute to the nuchalcrest (H-GSP 96231 96254 96386 96623 Figs 4B 5B 6BD)There is no evidence of a postparietal foramen

PalatinemdashThe flat area inferior to the orbit anterior to theorbitosphenoid where the infraorbital nerve and sphenopalatine

artery are located is the pterygopalatine fossa this feature oc-curs in P attocki (H-GSP 96623) and I pinfoldi (H-GSP 9657498134 Figs 6C 8C 9) No sutures are preserved on the medialwall of the pterygopalatine fossa but this wall is probably formedby the palatine (Fig 8A) maxilla and frontal bones A horizon-tal infraorbital crest separates the pterygopalatine fossa from theorbit (Figs 4A 6C 8C 9) A small sphenopalatine foramen islocated on this crest (Figs 8C 9)

In N ratimitus (H-GSP 96055) the palatine contribution to thepalate extends anteriorly to P4 In the median plane the palatineextends far ventral causing the posterior palate to be concave

OrbitosphenoidmdashThe orbitosphenoid of P attocki (H-GSP96623) and I pinfoldi (H-GSP 98134 Figs 6C 8C) shares sutureswith the frontal parietal alisphenoid presphenoid and probablypalatine (the extent of the palatine in the pterygopalatine fossacannot be determined) Dorsally the orbitosphenoid has a smalloval depression (Fig 8C) Posteroinferior to this depression thesinus canal (Figs 6C 8C 9) is located on the parieto-orbitosphenoid suture and the groove extends anterior to itcrossing the entire orbitosphenoid Ventral to this is the opticforamen (Figs 6C 8C 9) from which a groove extends anteri-orly toward the orbit The sphenorbital fissure (Figs 6C 8C 9)is located on the suture between orbitosphenoid and alisphenoidon the extreme posterior edge of the former Only a small part ofthe suture between orbitosphenoid and presphenoid (Fig 8C) isvisible because its posterior part is covered by a large anteriorlyprojecting process of the alisphenoid (Fig 8A)

FIGURE 4 Cranial anatomy of Pakicetus attocki A skull lateral view shown by the right premaxilla right maxilla and cranium (H-GSP 1846718470 and 96231 respectively) Length of missing part of rostrum is based on Nalacetus ratimitus maxilla (H-GSP 30306) B cranium dorsal view(H-GSP 96231) These specimens although found at the same locality may not pertain to one individual Scale bar equals 5 cm

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Kellogg (1936109 oph f) identified a lsquochannel for ophthalmicarteryrsquo and lsquoforamen for ophthalmic arteryrsquo in Zygorhiza kochiilocated dorsal to the channel for optic nerve and optic foramenrespectively The ophthalmic artery is a branch of the internalcarotid artery (in mammals lacking a stapedial artery) and is

thus located ventral to the optic chiasm and optic nerve in mam-mals on the floor of the braincase Usually this artery passesthrough the optic foramen The vascular structure passingthrough the foramen described by Kellogg (ophf) instead passesposterodorsally (see Kellogg 1936fig 31bc) opposite from the

FIGURE 5 Pakicetid cranial anatomy A Pakicetus attocki cranium H-GSP 96231 ventral view B Nalacetus ratimitus cranium H-GSP 96386dorsal view C ventral view Scale bar equals 5 cm

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FIGURE 6 Pakicetid cranial anatomy A Pakicetus attocki cranium H-GSP 96623 ventral view B dorsal view C right lateral view D Pakicetusattocki H-GSP 96231 caudal view E Ichthyolestes pinfoldi H-GSP 98134 caudal view Scale bar equals 5 cm

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direction that the ophthalmic artery would have taken We in-terpret Kelloggrsquos lsquoforamen for the ophthalmic arteryrsquo as the an-terior opening for the sinus canal consistent with our interpre-tation of a similarly located foramen in pakicetids

PresphenoidmdashThe presphenoid is elongate and narrow inP attocki N ratimitus and I pinfoldi (H-GSP 96231 9638696623 98134 Figs 5AC 6A 7B) The ventral surface of the

presphenoid is rounded and smooth in Ichthyolestes whereasin Pakicetus and Nalacetus there is a distinct median crest ven-trally In Ichthyolestes much of the lateral side of the pre-sphenoid is covered by an anterior process of the alisphenoid(Fig 8A) A very small foramen occurs in this part of the pre-sphenoid and a groove extends anteriorly from this foramenWe interpret this foramen as the anterior foramen of the ptery-

FIGURE 7 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A dorsal view B ventral view C ventral view ear region in detail Dventral view outline of ear region Scale bar for AndashB equals 5 cm scale bar for CndashD equals 2 cm

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goid canal (Figs 8C 9) Anteriorly the lateral side of the pre-sphenoid reaches dorsal to touch the orbitosphenoid (Fig 8C)Ventrally the presphenoid is not fused to the basisphenoid in themidline and there is an ossification defect of 45 mm in Ich-thyolestes (Fig 7B)

AlisphenoidmdashThe alisphenoid shares sutures with pre-sphenoid orbitosphenoid parietal squamosal petrosal tym-panic basisphenoid and probably with pterygoid in P attockiN ratimitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Fig 8C) The anterior part of the alisphenoid is a rectan-gular process that lies lateral to much of the presphenoid andorbitosphenoid (H-GSP 98134 Fig 8A) and the sphenorbitalfissure is located just dorsal to this (Figs 6C 8C 9) Posteriorlythe alisphenoid forms a narrow wedge-like process betweensquamosal and basisphenoid in Pakicetus Nalacetus and Ich-thyolestes (H-GSP 30432 96231 96386 96623 98134 Figs 7D

8A) Part of this narrow process forms a prominently raised jointfacet the alisphenoid facet for the anterior process of the tym-panic (Figs 5A 6A 7D) This joint facet is continuous with asimilarly sized facet of the squamosal (Fig 5A 6A 7D) Thealisphenoid surrounds the entire oval foramen (Figs 5C 6A7D) There is no round foramen

The pterygoid process of the alisphenoid in pakicetids (H-GSP30432 96231 96386 96623 98134 Fig 7D) divides posteriorlyinto medial and lateral pterygoid laminae (Fig 7D) that sur-round a low pterygoid fossa (Fig 7D) This structure is compa-rable to the pterygoid fossa of primates and many other terres-trial mammals and should not be confused with the pterygoidsinus fossa of Basilosauridae Odontoceti and Mysticeti Themedial pterygoid lamina is low The pterygoid bone is not pre-served but was probably attached to the medial pterygoidlamina

FIGURE 9 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) right ventrolateral view showing the sphenorbital region with the foraminafrom an oblique angle The zygomatic arch is out of the focal plane of this photograph Scale bar equals 5 cm

FIGURE 8 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A right lateral view B sphenorbital region in detail C outline ofsphenorbital region Scale bar for A equals 5 cm scale bar for BndashC equals 2 cm

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BasisphenoidmdashThe basisphenoid shares sutures with pre-sphenoid alisphenoid and basioccipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Figs 5AC 6A 7B) The basisphenoid is convex vent-rally in cross section and concave in the median plane antero-posteriorly in juvenile Pakicetus and in Ichthyolestes (H-GSP96623 98134 respectively Figs 6A 7B) in adult Pakicetus(H-GSP 96231) and Nalacetus (H-GSP 96386) the basisphenoidis more or less flat in both these orientations (Figs 5AC) Pos-teriorly the suture between basisphenoid and basioccipital israised as bilateral tubercles in all pakicetid genera (H-GSP30432 96231 96386 98134 Figs 5AC 7B) One of the flexors ofthe neck rectus capitis ventralis originates from these tuberclesThere is no contact between the petrosal and the basisphenoidand the medial pterygoid sinus lies between these two bones(Fig 7D)

SquamosalmdashThe squamosal contacts the parietal alisphe-noid tympanic petrosal and occipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 98199 9625496386 96407 96623 98134 Figs 4B 5AB 6A 7AB 8A)making up most of the wall of the braincase Several smalltemporal foramina pierce the area near the occipito-squamosalsuture (Fig 6C) Posteriorly the squamosal forms part of thenuchal crest in all pakicetids (H-GSP 96231 96254 9638696623 Figs 4B 5B 6BD) Laterally the squamosal forms theposterior part of the zygomatic arch (Figs 4B 5AC 6BC 7AB8A 9) and the glenoid fossa (also called mandibular fossaFigs 5AC 6A 7Dglfs) in Pakicetus Nalacetus and Ichthyo-lestes (H-GSP 30432 96231 96386 96407 96623 98134) Theanterior part of the glenoid fossa is flat and extensive medio-laterally The postglenoid process (Figs 4A 5AC 6AC 7D8A) arches posteriorly into the external auditory meatus (Figs4A 5AC 6AC 7D 8A) The squamosal makes up the entireroof of the external auditory meatus and there is a prominentposttympanic process (Figs 4B 5B 6B 7A) The postglenoidforamen (also called retroarticular foramen Figs 5AC 6A 7D)is small and located posteromedial to the postglenoid processThe glenoid fossa extends medial to the postglenoid process andends posteromedially into a shallow groove directed to themiddle ear Medial to this is the squamosal contribution to theprominently raised joint facet (H-GSP 30432 96231 9638696623 98134 Figs 5A 6A 7D) for the anterior process of thetympanic The external auditory meatus ends medially in a re-cessed area in Pakicetus Nalacetus and Ichthyolestes (H-GSP30432 96231 96386 96407 96623 98134) The sharply depressedcenter of this recessed area represents the epitympanic recess(Figs 7D)

PetrosalmdashThe petrosal (periotic) contacts the alisphenoidsquamosal tympanic and occipital in P attocki N ratimitus andI pinfoldi (H-GSP 30432 96231 96386 96407 96431 9662398134 Fig 7D) The petrosal forms the roof of the middle earcavity The promontorium (Figs 5C 6A 7D) which houses theinner ear is a triangular structure on the ventral side The facialcanal runs anterior to the promontorium Medial to the prom-ontorium is the groove for the inferior petrosal sinus with thepiriform fenestra (H-GSP 30432 96386 96623 98134 Figs 5C7D) Rostrolateral to the promontorium is the tensor tympanifossa (H-GSP 30432 96231 96386 96407 96431 96623 98134Fig 7D) There are two foramina on the promontorium the ovalwindow for the stapes (fenestra vestibuli Fig 7D) and caudal toit the round window (fenestra cochleae Fig 7D) The mastoidprocess or posterior process of the petrosal flanks through thesquamosal posteriorly (H-GSP 30432 96231 96386 9640796623 98134 Figs 5AC 6A 7D 8A) A more detailed descrip-tion of the petrosal and tympanic of pakicetid cetaceans (Luoand Gingerich 1999) was based in part on material from H-GSPLocality 62

TympanicmdashThe tympanic (synonyms tympanic bulla audi-tory bulla ectotympanic) contacts the petrosal squamosal ali-sphenoid and exoccipital (GSP-UM 084) The tympanic sur-rounds the middle ear cavity ventrally laterally and medially(see Appendix 1 for all tympanics) The anterior part providesthe incisure for the eustachian tube (Fig 10) The medial pachy-osteosclerotic massive part of the tympanic is the involucrum(Fig 10) The involucrum is not in contact with any skull boneand is only connected to the rest of the skull by the dorsolateraland caudal parts of the tympanic The morphology described byGingerich and colleagues (1983) where the involucrum contactsthe basioccipital was based on the type specimen of P inachusGSP-UM 084 (sometimes listed as UM-GSP 081 by Luo 1998) inwhich this area is deformed

The shape and height of the anteromedial angle of the invo-lucrum (as defined by Luo 1998 Luo and Gingerich 1999Fig10) as well as the shape and orientation of the incisure forthe eustachian tube is species-specific (see Table 2) The lateralpart of the tympanic is a thin plate here the sigmoid process isa narrow and small plate of bone on the dorsal side of the lateralwall at least in I pinfoldi (H-GSP 92003) There is a small con-tact between the sigmoid process and the squamosal (see Luoand Gingerich 1999) Posterior to the sigmoid process the ex-ternal auditory meatus opens to the middle ear cavity Anteriorto the sigmoid process is the anterior process of the tympanicalso called processus tubarius which articulates with a facet onthe suture of squamosal and alisphenoid The posterior processof the tympanic contacts the mastoid (posterior) process of thepetrosal and more medially a small exoccipital facet makescontact with the tympanic (Fig 7D) this facet is the proximalpart of the paroccipital process of exoccipital (Figs 4B 5AC6ADE 7D)

There are four distinct morphologies among the tympanicsfrom H-GSP Locality 62 listed in Table 2 Three of these mor-phologies can be associated with cranial and dental material andthus be identified as P attocki I pinfoldi and N ratimitus Thefourth morphology represents a currently unidentified species ofthe family Pakicetidae Thewissen and Hussain (1998) describeda single tooth from Locality 62 tentatively as pertaining to Pinachus and it is possible that the unidentified bullae are fromthis species

In P attocki tympanics (see Appendix 1 and Table 2 Figs10AndashC) the morphology of the incisure for the eustachian tubeand of the anteromedial angle of the involucrum is similar to thatin the holotype of P inachus and therefore we refer these tym-panics to P attocki ndash which based on teeth (Thewissen andHussain 1998) is our most common species of Pakicetus Theorientation of the incisure for the eustachian tube cannot beobserved in GSP-UM 084 because the tympanic in this specimenis deformed and compressed against the basicranium

In the I pinfoldi specimen H-GSP 96431 (see Appendix 1 andTable 2 Figs 10DndashF) the fragmentary basicranium indicates asize considerably smaller than P attocki This is consistent withdental evidence (Thewissen and Hussain 1998) that indicatesthat the genus Ichthyolestes is smaller than Pakicetus We there-fore refer these specimens to I pinfoldi Although the adjoin-ing tympanic (H-GSP 96431) is the smallest one of our I pinfolditympanics two other tympanics with similar morphologyfor which the tympanic bullar length could be measured areboth within the length range of P attocki (Appendix 1 and Ta-ble 2)

The N ratimitus tympanics (see Appendix 1 and Table 2 Figs10GndashI) with a morphology clearly distinct from the two firstones described in Table 2 are larger than those of P attocki andmost of I pinfoldi and this serves to identify H-GSP 30325 and98140 Articular facets of Nalacetus tympanics (H-GSP 3032530459 96384 96385) match the facets on the exoccipital and

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 755

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squamosal of a Nalacetus skull (H-GSP 96386) The Nalacetusskull differs from Pakicetus (see previous) even though theseskulls are similar in size (Table 1) The Nalacetus skull is muchlarger than the Ichthyolestes skull (Table 1) This further cor-

roborates our identification of Nalacetus as a different genusand allocation of these tympanics to it

Three other remaining pakicetid tympanics (see Appendix 1and Table 2 Figs 10JndashL) are distinct from the three common

FIGURE 10 Pakicetid tympanics vertical columns showing the anterior lateral and dorsal views respectively AndashC Pakicetus attocki (H-GSP30386 left) DndashF Ichthyolestes pinfoldi (H-GSP 30223 left) GndashI Nalacetus ratimitus (H-GSP 96384 left) JndashL pakicetid n sp (H-GSP 98141 right)Scale bar equals 2 cm

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species of pakicetids at H-GSP Locality 62 and probably repre-sent a fourth new species of pakicetid These tympanics areclosest to P attocki tympanics in their size

OccipitalmdashThe most anterior point of the basioccipital of Pattocki N ratimitus and I pinfoldi (H-GSP 96231 96386 98134Figs 5AC 7B) is its suture with the basisphenoid which israised into bilateral tubercles (H-GSP 30432 96231 9638696623 98134 Figs 5AC 7B lsquobasioccipital crestrsquo of Luo 1998lsquoincipient falcate process of basioccipitalrsquo of Luo and Gingerich1999) These tubercles for neck flexors (see basisphenoid above)extend onto the anterior side of the basioccipital Posterior tothis the basioccipital projects laterally as a flat process in Paki-cetus (H-GSP 96231) and in Ichthyolestes (H-GSP 98134 Figs5A 7B) but this process is poorly defined in Nalacetus (H-GSP96386) The lateralmost part of this process is formed by theexoccipital (Figs 5AC 7B) and this process also forms the dor-solateral wall of the jugular foramen of pakicetids (H-GSP96231 96386 96407 96431 98134 Figs 5AC 7D) The exoc-cipitals bear the occipital condyles (H-GSP 96231 96386 9643198134 98199 Figs 4A 5AC 6DE 7B 8A 9) which are com-pressed dorsoventrally do not touch in the median plane andare somewhat raised above the level of the exoccipital The dor-sal and ventral condyloid fossae are of equal size (H-GSP 9623198134 98199 Figs 5AC 6E 7D) The hypoglossal foramen inall pakicetid genera (H-GSP 96231 96386 96407 96431 98134Figs 5AC 7D) is situated approximately halfway between thejugular foramen and the occipital condyle One or two smallcondylar foramina (Figs 5AC 7D) pierce the basioccipital lat-eral to the hypoglossal foramen The exoccipital forms the me-diolaterally elongate paroccipital process in all pakicetids (H-GSP 30432 96231 96386 96431 96623 98134 Figs 4B 5AC6ADE 7D) The medial extremity of this process bears a flatfacet for the tympanic (Fig 7D lsquobullar process of the exoccipitalrsquoof Luo and Gingerich 1999) The anterior part of the paroccipi-tal process shares a long suture with the mastoid process of pe-trosal The paroccipital process projects posterolaterally and thepart adjacent to the occipital condyle is recessed

The supraoccipital makes up most of the posterior side of thenuchal crest in all pakicetids (H-GSP 96231 96254 96386 96623Figs 4B 5B 6BD) although this structure is clearly worn insome specimens eg in I pinfoldi (H-GSP 98134) A small bi-lateral process of supraoccipital visible in Pakicetus and Ich-thyolestes (H-GSP 96231 98134 Figs 6DE) for the epaxial ex-tensors of the neck projects from the dorsal edge of the foramenmagnum (H-GSP 96231 96386 98134 Figs 6DE)Variable em-issary foramina (H-GSP 96231 96254 96386 96623 98134 Figs6DE) pierce the occipital dorsal to the foramen magnum Abilateral oval depression (H-GSP 96231 96386 98134 Figs6DE) occurs just lateral to the midline well above the foramenmagnum This depression could be for musculus longissimuscapitis The precise shape of the supraoccipital cannot be deter-mined because the suture between the exoccipital and the su-praoccipital is not visible

DISCUSSION

The pakicetid crania described here help to understand varia-tion in skull morphology of the earliest cetaceans As such theycan be expected to play a significant role in systematic studiesbut a detailed analysis of character scores for these specimensfalls outside the scope of the present description Here discus-sion will focus on a few aspects of cranial morphology that arerarely involved in cetacean systematics although they displayvariation to an extent that may be relevant systematically andinteresting evolutionarily

Supraorbital Morphology

The supraorbital region of protocetids basilosaurids anddorudontids displays a supraorbital shield (Kellogg 1936 Gin-

gerich et al 1994 1995 Bajpai and Gingerich 1998 Hulbert1998 Uhen 1998 2004) However the basal families of ceta-ceans lack a supraorbital shield pakicetids (Thewissen et al2001b) ambulocetids (Thewissen et al 1996) and remingtono-cetids (Gingerich et al 1994 1995 Bajpai and Thewissen 1998)The new pakicetid specimens indicate that there is much varia-tion in supraorbital morphology within these families In paki-cetids the orbits are convergent dorsally The area between theorbits is recessed over the superior orbital rim (Figs 4ndash8) Assuch pakicetid eyes are directed dorsolaterally There are dif-ferences between the genera (Fig 3 Table 1) in Pakicetus andIchthyolestes the supraorbital region is cuplike and cradles thedorsal part of the eye and the dorsal side of the supraorbitalregion is deeply incised in the medial plane in Pakicetus Thisincisure is less strong in Nalacetus and Ichthyolestes and in Nala-cetus the contact between supraorbital region and eye is toonarrow to be cup-shaped In Ambulocetus (Fig 3 Table 1Thewissen et al 1996fig 10 for H-GSP 18490) the orbits arepositioned near the dorsal side of the skull but they face later-ally leaving the supraorbital region more or less flat (in dorsalview) The eyes here appear to be directed laterally and are nothoused in the bony orbits as deeply as in pakicetids as indicatedby the shape of the postorbital processes As a comparison inRemingtonocetus the eyes are small are set laterally and facelaterally and the supraorbital region is strongly convex (Fig 3Sahni and Mishra 1972 Kumar and Sahni 1986 Bajpai andThewissen 1998) In protocetids the orbit is large faces later-ally and is covered by a large supraorbital shield (Fig 3 Table1) Dorudontids and basilosaurids resemble protocetids in thepresence and shape of their supraorbital shield as indicated bythe ratio between the minimum and maximum supraorbitalwidth (Table 1) and in the orientation of their orbits the latterare still clearly different from modern odontocetes and mystice-tes (Kellogg 1936 Uhen 1998 2004)

Intertemporal Morphology

In early and middle Eocene cetaceans a very narrow regioncan be seen in the skull posterior to the frontoparietal suturenear the sphenorbital fissure (Figs 1 2 4ndash8) This morphology isnot seen in most land mammals but is common in early ceta-ceans such as pakicetids ambulocetids (eg AmbulocetusThewissen et al 1996) and remingtonocetids (Kumar and Sahni1986) and may represent an important diagnostic character forthe order

Tympanic Bullar Morphology

The differences in the tympanic bullar morphology seen be-tween P attocki I pinfoldi N ratimitus and a fourth unnamedspecies can be used for specific recognition However in generalall these tympanics display a similar morphology indicating thatthese early archaeocetes used their peripheral ear structures likeland mammals while hearing in air and relied on bone conduc-tion mechanism while hearing in water (Thewissen and Hussain1993 Luo 1998 Luo and Gingerich 1999 Nummela et al 2004)

Pakicetus attocki is clearly larger than I pinfoldi whereas Pa-kicetus and Nalacetus have similar body sizes (Table 1) Thewis-sen and Hussain (1998) studied the size of upper and lower mo-lars in P attocki P inachus I pinfoldi and N ratimitus Theyfound Ichthyolestes to be the smallest P attocki and Nalacetus tobe of similar size and P inachus to be the largest Our studyshows that the absolute tympanic bullar size is similar in Paki-cetus and Ichthyolestes but larger in Nalacetus (Table 2 Appen-dix 1) This implies that in relative terms Pakicetus tympanicswhen related to body size (Table 2) are smaller than those ofIchthyolestes and Nalacetus

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 757

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ACKNOWLEDGMENTS

We thank Ellen Byron and Jeanette Killius for their technicalassistance Linda Spurlock prepared the drawings of the skullswhich is gratefully acknowledged Sunil Bajpai kindly gave usaccess to Remingtonocetus specimens in the IITR-SB collectionsunder his care We thank the editor Ewan Fordyce and Zhe-XiLuo and an anonymous reviewer for constructive suggestions forimprovement of the manuscript Financial support was providedby the National Science Foundation (current grant number EAR0207370) to JGMT and by the Ella and Georg Ehrnrooth Foun-dation to SN This article is a contribution to the Howard Uni-versity-Geological Survey of Pakistan projected entitled ldquoCeno-zoic Mammals of Pakistanrdquo We thank the Geological Survey ofPakistan for their support and help in the field

LITERATURE CITED

Aslan A and J G M Thewissen 1996 Preliminary evaluation of pa-leosols and implications for interpreting vertebrate fossil assem-blages Kuldana Formation Northern Pakistan Palaeovertebrata25261ndash277

Bajpai S and P D Gingerich 1998 A new Eocene archaeocete (Mam-malia Cetacea) from India and the time of origin of whales Pro-ceedings of the National Academy of Sciences USA 9515464ndash15468

Bajpai S and J G M Thewissen 1998 Middle Eocene cetaceans fromthe Harudi and Subathu formations of India pp 213ndash233 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Dehm R and T zu Oettingen-Spielberg 1958 Palaumlontologische undgeologische Untersuchungen im Tertiaumlr von Pakistan 2 Die mitte-leocaumlnen Saumlugetiere von Ganda Kas bei Basal in Nordwest-Pakistan Bayerische Akademie der Wissenschaften 911ndash54

Geisler J H and Z-X Luo 1998 Relationships of Cetacea to terres-trial ungulates and the evolution of cranial vasculature in Cete pp163ndash212 in J G M Thewissen (ed) The Emergence of WhalesPlenum Press New York

Geisler J H A E Sanders and Z-X Luo 2005 A new protocetidwhale (Cetacea Archaeoceti) from the late Middle Eocene of SouthCarolina American Museum Novitates 34801ndash65

Gingerich P D 2003 Land-to-sea transition in early whales evolutionof Eocene Archaeoceti (Cetacea) in relation to skeletal proportionsand locomotion of living semiaquatic mammals Paleobiology 29429ndash454

Gingerich P D and D E Russell 1981 Pakicetus inachus a newarchaeocete (Mammalia Cetacea) from the early-middle EoceneKuldana Formation of Kohat (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 25235ndash246

Gingerich P D and D E Russell 1990 Dentition of early EocenePakicetus (Mammalia Cetacea) Contributions from the Museum ofPaleontology University of Michigan 281ndash20

Gingerich P D N A Wells D E Russell and S M I Shah 1983Origin of whales in epicontinental remnant seas new evidence fromthe early Eocene of Pakistan Science 220403ndash406

Gingerich P D S M Raza M Arif M Anwar and X Zhou 1994New whale from the Eocene of Pakistan and the origin of cetaceanswimming Nature 368844ndash847

Gingerich P D M Arif and W C Clyde 1995 New archaeocetes(Mammalia Cetacea) from the middle Eocene Domanda Formationof the Sulaiman Range Punjab (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 29291ndash330

Gingerich P D M ul-Haq I S Zalmout I H Khan and M S Mal-kani 2001 Origin of whales from early artiodactyls hands and feetof Eocene Protocetidae from Pakistan Science 2932239ndash2242

Hulbert R Jr 1998 Postcranial osteology of the North American

middle Eocene protocetid Georgiacetus pp 235ndash267 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Kellogg A R 1936 A review of the Archaeoceti Carnegie Institute ofWashington Publications 4821ndash366

Kumar K and A Sahni 1986 Remingtonocetus harudiensis new com-bination a middle Eocene archaeocete (Mammalia Cetacea) fromwestern Kutch India Journal of Vertebrate Paleontology 6326ndash349

Luo Z-X 1998 Homology and transformation of cetacean ectotym-panic structures pp 269ndash301 in J G M Thewissen (ed) The Emer-gence of Whales Plenum Press New York

Luo Z-X and P D Gingerich 1999 Terrestrial Mesonychia to aquaticCetacea transformation of the basicranium and evolution of hearingin whales University of Michigan Papers on Paleontology 311ndash98

Madar S I In press The postcranial skeleton of early Eocene pakicetidcetaceans Journal of Paleontology

Nummela S J G M Thewissen S Bajpai S T Hussain and K Ku-mar 2004 Eocene evolution of whale hearing Nature 430776ndash778

Sahni A and V P Mishra 1972 A new species of Protocetus (Cetacea)from the middle Eocene of Kutch Western India Palaeontology15490ndash495

Thewissen J G M and S T Hussain 1993 Origin of underwaterhearing in whales Nature 361444ndash445

Thewissen J G M and S T Hussain 1998 Systematic review of thePakicetidae early and middle Eocene Cetacea (Mammalia) fromPakistan and India Bulletin of Carnegie Museum of Natural History34220ndash238

Thewissen J G M and E M Williams 2002 The early radiations ofCetacea (Mammalia) evolutionary patterns and developmental cor-relations Annual Review of Ecology and Systematics 3373ndash90

Thewissen J G M S I Madar and S T Hussain 1996 Ambulocetusnatans an Eocene cetacean (Mammalia) from Pakistan CourierForschungsinstitut Senckenberg 191 1ndash86

Thewissen J G M E M Williams and S T Hussain 2001a Eocenemammal faunas from Northern Indo-Pakistan Journal of Verte-brate Paleontology 21347ndash366

Thewissen J G M E M Williams L J Roe and S T Hussain 2001bSkeletons of terrestrial cetaceans and the relationship of whales toartiodactyls Nature 413277ndash281

Thewissen J G M S T Hussain M Arif A Aslan S I Madar andL J Roe 1998 New localities of Eocene vertebrates in NorthernPakistan and their significance for the origin of the modern orders ofmammals pp 19ndash34 in M I Ghaznavi S M Raza and M T Hasan(eds) Siwaliks of South Asia Proceedings of the 3rd GEOSASWorkshop Islamabad Pakistan 1ndash5 March 1997 Geological Sur-vey of Pakistan Islamabad Pakistan

Uhen M D 1998 Middle to late Eocene basilosaurines and dorudon-tines pp 29ndash61 in J G M Thewissen (ed) The Emergence ofWhales Plenum Press New York

Uhen M D 2004 Form function and anatomy of Dorudon atrox(Mammalia Cetacea) an archaeocete from the middle to lateEocene of Egypt University of Michigan Papers on Paleontology341ndash222

West R M 1980 Middle Eocene large mammal assemblage withTethyan affinities Ganda Kas region Pakistan Journal of Paleon-tology 54508ndash533

West R M and J R Lukacs 1979 Geology and vertebrate-fossil lo-calities Tertiary continental rocks Kala Chitta Hills Attock Dis-trict Pakistan Milwaukee Public Museum Contributions in Biologyand Geology 261ndash20

Williams E M 1998 Synopsis of the earliest cetaceans PakicetidaeAmbulocetidae Remingtonocetidae and Protocetidae pp 1ndash28 inJ G M Thewissen (ed) The Emergence of Whales Plenum PressNew York

Submitted 7 October 2005 accepted 24 February 2006

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APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

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genera (Fig 3) Ethmoidal foramina are visible on the ventralside of the orbit in pakicetids (H-GSP 96231 96254 9657496623 98134 Fig 6C) and small diploic foramina dorsally (Fig4AB 7A) The supraorbital foramen of the orbit in pakicetids(H-GSP 96231 96254 96623 96574 98134 Figs 4A 6C 8C 9)the opening for the supraorbital canal is located in the interor-bital region close to the median plane inferior to the postorbitalprocess (H-GSP 96231 96386 96574 96623 98134 Figs 4B 56AB 7A 8C 9) The minimum and maximum supraorbitalwidths vary (Fig 3 Table 1) The shape of the supraorbital re-

gion varies in pakicetids (Fig 3) it is deeply incised in P attocki(H-GSP 96231 96254 96623 Figs 3H I) slightly concave in Ipinfoldi (H-GSP 96574 98134 Fig 3J) and relatively flat in Nratimitus (H-GSP 96386 Fig 3K) The extent to which the post-orbital process projects laterally varies in P attocki (H-GSP96231 96254 96623 Figs 3O P) the process is dorsoventrallythick in I pinfoldi (H-GSP 96574 98134 Fig 3Q) the process isthin in N ratimitus (H-GSP 96386 Fig 3R) the process is thickbut short and stout compared to P attocki (for numerical valuessee Table 1)

TABLE 1 Cranial measurements (in mm) of pakicetids compared with those of other archaeocetes

Specimen no TaxonSOWmin

SOWmax

SOWminmax BZW OCW FMW FMH

H-GSP 96231 Pakicetus attocki 190 484 039 127 527 178 143H-GSP 96623 Pakicetus attocki 189 366 052 924 mdash mdash mdashH-GSP 96254 Pakicetus attocki 1673 mdash mdash mdash mdash mdash mdashGSP-UM 084 Pakicetus inachus1 mdash mdash mdash 143 537 156 137H-GSP 98134 Ichthyolestes pinfoldi 229 424 054 980 436 159 96H-GSP 96386 Nalacetus ratimitus 231 3922 059 126 4713 164 129H-GSP 30432 Nalacetus ratimitus mdash mdash mdash 109 mdash mdash mdashH-GSP 18507 Ambulocetus natans 66 74 089 259 793 328 423IITR-SB 2529 Remingtonocetus sp 952 112 085 254 957 357 306LUVP 11034 Indocetus ramani 151 181 083 mdash mdash mdash mdashGSP-UM 3012 Rodhocetus kasrani mdash 1954 mdash 2604 mdash mdash mdashChM PV5401 Carolinacetus gingerichi mdash 2565 mdash mdash mdash mdash mdash

Dorudon atrox 6ndash10 3086 3416 090 3997 1168 5389 36610

SOW supraorbital width minimum and maximum BZW bizygomatic width OCW occipital condyle width FMW foramen magnum width FMHforamen magnum height For the asterisked specimens () outlines for supraorbital regions and the cross-sections at the two points of this regionare shown in Figure 31Measurements taken on a cast the holotype described by Gingerich and Russell (1981) and Gingerich et al (1983)2Minimum value full width not preserved3Estimate4From Gingerich et al (1994 Table 1)5From Geisler et al (2005 Table 1)6These values measured from Uhen (2004Fig 32 a photo in dorsal view of GSP-UM 101222) The postorbital process distance falls well inside therange reported by Uhen (2004 Appendix IVA range is 224ndash356 mm mean 279 mm n 11)7Mean value (n 3) based on data by Uhen (2004 Appendix IVB)8Mean value (n 7) based on data by Uhen (2004 Appendix IVB)9Mean value (n 6) based on data by Uhen (2004 Appendix IVB)10Mean value (n 5) based on data by Uhen (2004 Appendix IVB)

TABLE 2 Morphological characters in pakicetid tympanics and their appearance in different species

Character Pakicetus attocki Ichthyolestes pinfoldi Nalacetus ratimitus Pakicetid sp nov cf P inachus

Incisure for theeustachian tubeshape

DeepH-GSP 30386 91035

92100 96344

ShallowH-GSP 92003

96283 96431

DeepH-GSP 30459

96384 96385

Widely open medially and very broadH-GSP 30198 30424 98141

Incisure for theeustachian tubeorientation

MediolateralH-GSP 30386 92100

96192 96344

DorsoventralH-GSP 92003 96283

More mediolateralH-GSP 96431

DorsoventralH-GSP 30459

96384 96385

DorsoventralH-GSP 30198 30424 98141

Anteromedial angleof the involucrum

Sharp and highH-GSP 30386 91035

92100 96344

Blunt and lowH-GSP 92003

96283 96431

Sharp and highH-GSP 30459

96384 96385

Very low set caudal this region issmooth no process formingan angle

H-GSP 30198 30424 98141Skull size Larger than Ichthyolestes

similar to NalacetusSmaller than Pakicetus

and NalacetusSimilar to Pakicetus

larger thanIchthyolestes

mdash

Tympanic bullarsize absolute

Small Small Large Small

Range of TBL(mm)

2488ndash2694 2327ndash2737 2873ndash3180 2639ndash2803

TBL mean plusmn SE 2596 plusmn 050 (n 4) 2562 plusmn 085 (n 4) 3007 plusmn 091 (n 3) 2706 plusmn 050 (n 3)TBL meanBZW1 020 026 024 0192

TBL meanOCW1 049 059 064 0502

Tympanic bullarsize relative

Smaller compared toIchthyolestes and Nalacetus

Larger compared toPakicetus

Larger comparedto Pakicetus

As in P attocki

For a list of all tympanics see Appendix 11BZW and OCW values are from Table 1 from specimens H-GSP 96231 96386 and 981342We used BZW and OCW for Pakicetus inachus GSP-UM 084 (see Table 1)

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Caudal to the postorbital process the frontal narrows at thefrontoparietal suture in all pakicetid genera (H-GSP 9623196254 96386 96574 96623 98134 98199 Figs 4B 5B 6B 7A8A) This suture extends across the posterior side of the postor-bital process In lateral view the frontoparietal suture extendsposteriorly into the temporal fossa (Figs 4A 5B 6B 7A 8A)until it reaches the orbitosphenoid (Figs 6C 8C) where it archesanterior and ventral reaching the presphenoid (Fig 8C) Thisarea of the skull is extremely narrow as shown by cross sectionsin Figure 2 the width is 40 mm in juvenile and 55 mm in adultP attocki (H-GSP 96623 and 96231 respectively) and 47 mm inI pinfoldi (H-GSP 98134)

ParietalmdashThe parietal shares sutures with the frontal orbito-sphenoid alisphenoid squamosal and occipital in all pakicetidgenera (H-GSP 96231 96254 96386 96574 96623 98134 Figs4A 5B 6B 7A 8A) and forms most of the sagittal crest (Figs4A 5B 6B 8A) which is somewhat higher than the braincase(the full sagittal crest is not preserved in any specimen) There isno evidence of an interparietal bone At the most anterior part ofthe sagittal crest the skull is extremely narrow (Fig 2 see thesection on the frontal bone) Posteriorly the parietal only formsa minor part of the wall of the braincase (H-GSP 96231 9638696623 98134) The parietal does not contribute to the nuchalcrest (H-GSP 96231 96254 96386 96623 Figs 4B 5B 6BD)There is no evidence of a postparietal foramen

PalatinemdashThe flat area inferior to the orbit anterior to theorbitosphenoid where the infraorbital nerve and sphenopalatine

artery are located is the pterygopalatine fossa this feature oc-curs in P attocki (H-GSP 96623) and I pinfoldi (H-GSP 9657498134 Figs 6C 8C 9) No sutures are preserved on the medialwall of the pterygopalatine fossa but this wall is probably formedby the palatine (Fig 8A) maxilla and frontal bones A horizon-tal infraorbital crest separates the pterygopalatine fossa from theorbit (Figs 4A 6C 8C 9) A small sphenopalatine foramen islocated on this crest (Figs 8C 9)

In N ratimitus (H-GSP 96055) the palatine contribution to thepalate extends anteriorly to P4 In the median plane the palatineextends far ventral causing the posterior palate to be concave

OrbitosphenoidmdashThe orbitosphenoid of P attocki (H-GSP96623) and I pinfoldi (H-GSP 98134 Figs 6C 8C) shares sutureswith the frontal parietal alisphenoid presphenoid and probablypalatine (the extent of the palatine in the pterygopalatine fossacannot be determined) Dorsally the orbitosphenoid has a smalloval depression (Fig 8C) Posteroinferior to this depression thesinus canal (Figs 6C 8C 9) is located on the parieto-orbitosphenoid suture and the groove extends anterior to itcrossing the entire orbitosphenoid Ventral to this is the opticforamen (Figs 6C 8C 9) from which a groove extends anteri-orly toward the orbit The sphenorbital fissure (Figs 6C 8C 9)is located on the suture between orbitosphenoid and alisphenoidon the extreme posterior edge of the former Only a small part ofthe suture between orbitosphenoid and presphenoid (Fig 8C) isvisible because its posterior part is covered by a large anteriorlyprojecting process of the alisphenoid (Fig 8A)

FIGURE 4 Cranial anatomy of Pakicetus attocki A skull lateral view shown by the right premaxilla right maxilla and cranium (H-GSP 1846718470 and 96231 respectively) Length of missing part of rostrum is based on Nalacetus ratimitus maxilla (H-GSP 30306) B cranium dorsal view(H-GSP 96231) These specimens although found at the same locality may not pertain to one individual Scale bar equals 5 cm

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Kellogg (1936109 oph f) identified a lsquochannel for ophthalmicarteryrsquo and lsquoforamen for ophthalmic arteryrsquo in Zygorhiza kochiilocated dorsal to the channel for optic nerve and optic foramenrespectively The ophthalmic artery is a branch of the internalcarotid artery (in mammals lacking a stapedial artery) and is

thus located ventral to the optic chiasm and optic nerve in mam-mals on the floor of the braincase Usually this artery passesthrough the optic foramen The vascular structure passingthrough the foramen described by Kellogg (ophf) instead passesposterodorsally (see Kellogg 1936fig 31bc) opposite from the

FIGURE 5 Pakicetid cranial anatomy A Pakicetus attocki cranium H-GSP 96231 ventral view B Nalacetus ratimitus cranium H-GSP 96386dorsal view C ventral view Scale bar equals 5 cm

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FIGURE 6 Pakicetid cranial anatomy A Pakicetus attocki cranium H-GSP 96623 ventral view B dorsal view C right lateral view D Pakicetusattocki H-GSP 96231 caudal view E Ichthyolestes pinfoldi H-GSP 98134 caudal view Scale bar equals 5 cm

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direction that the ophthalmic artery would have taken We in-terpret Kelloggrsquos lsquoforamen for the ophthalmic arteryrsquo as the an-terior opening for the sinus canal consistent with our interpre-tation of a similarly located foramen in pakicetids

PresphenoidmdashThe presphenoid is elongate and narrow inP attocki N ratimitus and I pinfoldi (H-GSP 96231 9638696623 98134 Figs 5AC 6A 7B) The ventral surface of the

presphenoid is rounded and smooth in Ichthyolestes whereasin Pakicetus and Nalacetus there is a distinct median crest ven-trally In Ichthyolestes much of the lateral side of the pre-sphenoid is covered by an anterior process of the alisphenoid(Fig 8A) A very small foramen occurs in this part of the pre-sphenoid and a groove extends anteriorly from this foramenWe interpret this foramen as the anterior foramen of the ptery-

FIGURE 7 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A dorsal view B ventral view C ventral view ear region in detail Dventral view outline of ear region Scale bar for AndashB equals 5 cm scale bar for CndashD equals 2 cm

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goid canal (Figs 8C 9) Anteriorly the lateral side of the pre-sphenoid reaches dorsal to touch the orbitosphenoid (Fig 8C)Ventrally the presphenoid is not fused to the basisphenoid in themidline and there is an ossification defect of 45 mm in Ich-thyolestes (Fig 7B)

AlisphenoidmdashThe alisphenoid shares sutures with pre-sphenoid orbitosphenoid parietal squamosal petrosal tym-panic basisphenoid and probably with pterygoid in P attockiN ratimitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Fig 8C) The anterior part of the alisphenoid is a rectan-gular process that lies lateral to much of the presphenoid andorbitosphenoid (H-GSP 98134 Fig 8A) and the sphenorbitalfissure is located just dorsal to this (Figs 6C 8C 9) Posteriorlythe alisphenoid forms a narrow wedge-like process betweensquamosal and basisphenoid in Pakicetus Nalacetus and Ich-thyolestes (H-GSP 30432 96231 96386 96623 98134 Figs 7D

8A) Part of this narrow process forms a prominently raised jointfacet the alisphenoid facet for the anterior process of the tym-panic (Figs 5A 6A 7D) This joint facet is continuous with asimilarly sized facet of the squamosal (Fig 5A 6A 7D) Thealisphenoid surrounds the entire oval foramen (Figs 5C 6A7D) There is no round foramen

The pterygoid process of the alisphenoid in pakicetids (H-GSP30432 96231 96386 96623 98134 Fig 7D) divides posteriorlyinto medial and lateral pterygoid laminae (Fig 7D) that sur-round a low pterygoid fossa (Fig 7D) This structure is compa-rable to the pterygoid fossa of primates and many other terres-trial mammals and should not be confused with the pterygoidsinus fossa of Basilosauridae Odontoceti and Mysticeti Themedial pterygoid lamina is low The pterygoid bone is not pre-served but was probably attached to the medial pterygoidlamina

FIGURE 9 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) right ventrolateral view showing the sphenorbital region with the foraminafrom an oblique angle The zygomatic arch is out of the focal plane of this photograph Scale bar equals 5 cm

FIGURE 8 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A right lateral view B sphenorbital region in detail C outline ofsphenorbital region Scale bar for A equals 5 cm scale bar for BndashC equals 2 cm

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BasisphenoidmdashThe basisphenoid shares sutures with pre-sphenoid alisphenoid and basioccipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Figs 5AC 6A 7B) The basisphenoid is convex vent-rally in cross section and concave in the median plane antero-posteriorly in juvenile Pakicetus and in Ichthyolestes (H-GSP96623 98134 respectively Figs 6A 7B) in adult Pakicetus(H-GSP 96231) and Nalacetus (H-GSP 96386) the basisphenoidis more or less flat in both these orientations (Figs 5AC) Pos-teriorly the suture between basisphenoid and basioccipital israised as bilateral tubercles in all pakicetid genera (H-GSP30432 96231 96386 98134 Figs 5AC 7B) One of the flexors ofthe neck rectus capitis ventralis originates from these tuberclesThere is no contact between the petrosal and the basisphenoidand the medial pterygoid sinus lies between these two bones(Fig 7D)

SquamosalmdashThe squamosal contacts the parietal alisphe-noid tympanic petrosal and occipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 98199 9625496386 96407 96623 98134 Figs 4B 5AB 6A 7AB 8A)making up most of the wall of the braincase Several smalltemporal foramina pierce the area near the occipito-squamosalsuture (Fig 6C) Posteriorly the squamosal forms part of thenuchal crest in all pakicetids (H-GSP 96231 96254 9638696623 Figs 4B 5B 6BD) Laterally the squamosal forms theposterior part of the zygomatic arch (Figs 4B 5AC 6BC 7AB8A 9) and the glenoid fossa (also called mandibular fossaFigs 5AC 6A 7Dglfs) in Pakicetus Nalacetus and Ichthyo-lestes (H-GSP 30432 96231 96386 96407 96623 98134) Theanterior part of the glenoid fossa is flat and extensive medio-laterally The postglenoid process (Figs 4A 5AC 6AC 7D8A) arches posteriorly into the external auditory meatus (Figs4A 5AC 6AC 7D 8A) The squamosal makes up the entireroof of the external auditory meatus and there is a prominentposttympanic process (Figs 4B 5B 6B 7A) The postglenoidforamen (also called retroarticular foramen Figs 5AC 6A 7D)is small and located posteromedial to the postglenoid processThe glenoid fossa extends medial to the postglenoid process andends posteromedially into a shallow groove directed to themiddle ear Medial to this is the squamosal contribution to theprominently raised joint facet (H-GSP 30432 96231 9638696623 98134 Figs 5A 6A 7D) for the anterior process of thetympanic The external auditory meatus ends medially in a re-cessed area in Pakicetus Nalacetus and Ichthyolestes (H-GSP30432 96231 96386 96407 96623 98134) The sharply depressedcenter of this recessed area represents the epitympanic recess(Figs 7D)

PetrosalmdashThe petrosal (periotic) contacts the alisphenoidsquamosal tympanic and occipital in P attocki N ratimitus andI pinfoldi (H-GSP 30432 96231 96386 96407 96431 9662398134 Fig 7D) The petrosal forms the roof of the middle earcavity The promontorium (Figs 5C 6A 7D) which houses theinner ear is a triangular structure on the ventral side The facialcanal runs anterior to the promontorium Medial to the prom-ontorium is the groove for the inferior petrosal sinus with thepiriform fenestra (H-GSP 30432 96386 96623 98134 Figs 5C7D) Rostrolateral to the promontorium is the tensor tympanifossa (H-GSP 30432 96231 96386 96407 96431 96623 98134Fig 7D) There are two foramina on the promontorium the ovalwindow for the stapes (fenestra vestibuli Fig 7D) and caudal toit the round window (fenestra cochleae Fig 7D) The mastoidprocess or posterior process of the petrosal flanks through thesquamosal posteriorly (H-GSP 30432 96231 96386 9640796623 98134 Figs 5AC 6A 7D 8A) A more detailed descrip-tion of the petrosal and tympanic of pakicetid cetaceans (Luoand Gingerich 1999) was based in part on material from H-GSPLocality 62

TympanicmdashThe tympanic (synonyms tympanic bulla audi-tory bulla ectotympanic) contacts the petrosal squamosal ali-sphenoid and exoccipital (GSP-UM 084) The tympanic sur-rounds the middle ear cavity ventrally laterally and medially(see Appendix 1 for all tympanics) The anterior part providesthe incisure for the eustachian tube (Fig 10) The medial pachy-osteosclerotic massive part of the tympanic is the involucrum(Fig 10) The involucrum is not in contact with any skull boneand is only connected to the rest of the skull by the dorsolateraland caudal parts of the tympanic The morphology described byGingerich and colleagues (1983) where the involucrum contactsthe basioccipital was based on the type specimen of P inachusGSP-UM 084 (sometimes listed as UM-GSP 081 by Luo 1998) inwhich this area is deformed

The shape and height of the anteromedial angle of the invo-lucrum (as defined by Luo 1998 Luo and Gingerich 1999Fig10) as well as the shape and orientation of the incisure forthe eustachian tube is species-specific (see Table 2) The lateralpart of the tympanic is a thin plate here the sigmoid process isa narrow and small plate of bone on the dorsal side of the lateralwall at least in I pinfoldi (H-GSP 92003) There is a small con-tact between the sigmoid process and the squamosal (see Luoand Gingerich 1999) Posterior to the sigmoid process the ex-ternal auditory meatus opens to the middle ear cavity Anteriorto the sigmoid process is the anterior process of the tympanicalso called processus tubarius which articulates with a facet onthe suture of squamosal and alisphenoid The posterior processof the tympanic contacts the mastoid (posterior) process of thepetrosal and more medially a small exoccipital facet makescontact with the tympanic (Fig 7D) this facet is the proximalpart of the paroccipital process of exoccipital (Figs 4B 5AC6ADE 7D)

There are four distinct morphologies among the tympanicsfrom H-GSP Locality 62 listed in Table 2 Three of these mor-phologies can be associated with cranial and dental material andthus be identified as P attocki I pinfoldi and N ratimitus Thefourth morphology represents a currently unidentified species ofthe family Pakicetidae Thewissen and Hussain (1998) describeda single tooth from Locality 62 tentatively as pertaining to Pinachus and it is possible that the unidentified bullae are fromthis species

In P attocki tympanics (see Appendix 1 and Table 2 Figs10AndashC) the morphology of the incisure for the eustachian tubeand of the anteromedial angle of the involucrum is similar to thatin the holotype of P inachus and therefore we refer these tym-panics to P attocki ndash which based on teeth (Thewissen andHussain 1998) is our most common species of Pakicetus Theorientation of the incisure for the eustachian tube cannot beobserved in GSP-UM 084 because the tympanic in this specimenis deformed and compressed against the basicranium

In the I pinfoldi specimen H-GSP 96431 (see Appendix 1 andTable 2 Figs 10DndashF) the fragmentary basicranium indicates asize considerably smaller than P attocki This is consistent withdental evidence (Thewissen and Hussain 1998) that indicatesthat the genus Ichthyolestes is smaller than Pakicetus We there-fore refer these specimens to I pinfoldi Although the adjoin-ing tympanic (H-GSP 96431) is the smallest one of our I pinfolditympanics two other tympanics with similar morphologyfor which the tympanic bullar length could be measured areboth within the length range of P attocki (Appendix 1 and Ta-ble 2)

The N ratimitus tympanics (see Appendix 1 and Table 2 Figs10GndashI) with a morphology clearly distinct from the two firstones described in Table 2 are larger than those of P attocki andmost of I pinfoldi and this serves to identify H-GSP 30325 and98140 Articular facets of Nalacetus tympanics (H-GSP 3032530459 96384 96385) match the facets on the exoccipital and

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squamosal of a Nalacetus skull (H-GSP 96386) The Nalacetusskull differs from Pakicetus (see previous) even though theseskulls are similar in size (Table 1) The Nalacetus skull is muchlarger than the Ichthyolestes skull (Table 1) This further cor-

roborates our identification of Nalacetus as a different genusand allocation of these tympanics to it

Three other remaining pakicetid tympanics (see Appendix 1and Table 2 Figs 10JndashL) are distinct from the three common

FIGURE 10 Pakicetid tympanics vertical columns showing the anterior lateral and dorsal views respectively AndashC Pakicetus attocki (H-GSP30386 left) DndashF Ichthyolestes pinfoldi (H-GSP 30223 left) GndashI Nalacetus ratimitus (H-GSP 96384 left) JndashL pakicetid n sp (H-GSP 98141 right)Scale bar equals 2 cm

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species of pakicetids at H-GSP Locality 62 and probably repre-sent a fourth new species of pakicetid These tympanics areclosest to P attocki tympanics in their size

OccipitalmdashThe most anterior point of the basioccipital of Pattocki N ratimitus and I pinfoldi (H-GSP 96231 96386 98134Figs 5AC 7B) is its suture with the basisphenoid which israised into bilateral tubercles (H-GSP 30432 96231 9638696623 98134 Figs 5AC 7B lsquobasioccipital crestrsquo of Luo 1998lsquoincipient falcate process of basioccipitalrsquo of Luo and Gingerich1999) These tubercles for neck flexors (see basisphenoid above)extend onto the anterior side of the basioccipital Posterior tothis the basioccipital projects laterally as a flat process in Paki-cetus (H-GSP 96231) and in Ichthyolestes (H-GSP 98134 Figs5A 7B) but this process is poorly defined in Nalacetus (H-GSP96386) The lateralmost part of this process is formed by theexoccipital (Figs 5AC 7B) and this process also forms the dor-solateral wall of the jugular foramen of pakicetids (H-GSP96231 96386 96407 96431 98134 Figs 5AC 7D) The exoc-cipitals bear the occipital condyles (H-GSP 96231 96386 9643198134 98199 Figs 4A 5AC 6DE 7B 8A 9) which are com-pressed dorsoventrally do not touch in the median plane andare somewhat raised above the level of the exoccipital The dor-sal and ventral condyloid fossae are of equal size (H-GSP 9623198134 98199 Figs 5AC 6E 7D) The hypoglossal foramen inall pakicetid genera (H-GSP 96231 96386 96407 96431 98134Figs 5AC 7D) is situated approximately halfway between thejugular foramen and the occipital condyle One or two smallcondylar foramina (Figs 5AC 7D) pierce the basioccipital lat-eral to the hypoglossal foramen The exoccipital forms the me-diolaterally elongate paroccipital process in all pakicetids (H-GSP 30432 96231 96386 96431 96623 98134 Figs 4B 5AC6ADE 7D) The medial extremity of this process bears a flatfacet for the tympanic (Fig 7D lsquobullar process of the exoccipitalrsquoof Luo and Gingerich 1999) The anterior part of the paroccipi-tal process shares a long suture with the mastoid process of pe-trosal The paroccipital process projects posterolaterally and thepart adjacent to the occipital condyle is recessed

The supraoccipital makes up most of the posterior side of thenuchal crest in all pakicetids (H-GSP 96231 96254 96386 96623Figs 4B 5B 6BD) although this structure is clearly worn insome specimens eg in I pinfoldi (H-GSP 98134) A small bi-lateral process of supraoccipital visible in Pakicetus and Ich-thyolestes (H-GSP 96231 98134 Figs 6DE) for the epaxial ex-tensors of the neck projects from the dorsal edge of the foramenmagnum (H-GSP 96231 96386 98134 Figs 6DE)Variable em-issary foramina (H-GSP 96231 96254 96386 96623 98134 Figs6DE) pierce the occipital dorsal to the foramen magnum Abilateral oval depression (H-GSP 96231 96386 98134 Figs6DE) occurs just lateral to the midline well above the foramenmagnum This depression could be for musculus longissimuscapitis The precise shape of the supraoccipital cannot be deter-mined because the suture between the exoccipital and the su-praoccipital is not visible

DISCUSSION

The pakicetid crania described here help to understand varia-tion in skull morphology of the earliest cetaceans As such theycan be expected to play a significant role in systematic studiesbut a detailed analysis of character scores for these specimensfalls outside the scope of the present description Here discus-sion will focus on a few aspects of cranial morphology that arerarely involved in cetacean systematics although they displayvariation to an extent that may be relevant systematically andinteresting evolutionarily

Supraorbital Morphology

The supraorbital region of protocetids basilosaurids anddorudontids displays a supraorbital shield (Kellogg 1936 Gin-

gerich et al 1994 1995 Bajpai and Gingerich 1998 Hulbert1998 Uhen 1998 2004) However the basal families of ceta-ceans lack a supraorbital shield pakicetids (Thewissen et al2001b) ambulocetids (Thewissen et al 1996) and remingtono-cetids (Gingerich et al 1994 1995 Bajpai and Thewissen 1998)The new pakicetid specimens indicate that there is much varia-tion in supraorbital morphology within these families In paki-cetids the orbits are convergent dorsally The area between theorbits is recessed over the superior orbital rim (Figs 4ndash8) Assuch pakicetid eyes are directed dorsolaterally There are dif-ferences between the genera (Fig 3 Table 1) in Pakicetus andIchthyolestes the supraorbital region is cuplike and cradles thedorsal part of the eye and the dorsal side of the supraorbitalregion is deeply incised in the medial plane in Pakicetus Thisincisure is less strong in Nalacetus and Ichthyolestes and in Nala-cetus the contact between supraorbital region and eye is toonarrow to be cup-shaped In Ambulocetus (Fig 3 Table 1Thewissen et al 1996fig 10 for H-GSP 18490) the orbits arepositioned near the dorsal side of the skull but they face later-ally leaving the supraorbital region more or less flat (in dorsalview) The eyes here appear to be directed laterally and are nothoused in the bony orbits as deeply as in pakicetids as indicatedby the shape of the postorbital processes As a comparison inRemingtonocetus the eyes are small are set laterally and facelaterally and the supraorbital region is strongly convex (Fig 3Sahni and Mishra 1972 Kumar and Sahni 1986 Bajpai andThewissen 1998) In protocetids the orbit is large faces later-ally and is covered by a large supraorbital shield (Fig 3 Table1) Dorudontids and basilosaurids resemble protocetids in thepresence and shape of their supraorbital shield as indicated bythe ratio between the minimum and maximum supraorbitalwidth (Table 1) and in the orientation of their orbits the latterare still clearly different from modern odontocetes and mystice-tes (Kellogg 1936 Uhen 1998 2004)

Intertemporal Morphology

In early and middle Eocene cetaceans a very narrow regioncan be seen in the skull posterior to the frontoparietal suturenear the sphenorbital fissure (Figs 1 2 4ndash8) This morphology isnot seen in most land mammals but is common in early ceta-ceans such as pakicetids ambulocetids (eg AmbulocetusThewissen et al 1996) and remingtonocetids (Kumar and Sahni1986) and may represent an important diagnostic character forthe order

Tympanic Bullar Morphology

The differences in the tympanic bullar morphology seen be-tween P attocki I pinfoldi N ratimitus and a fourth unnamedspecies can be used for specific recognition However in generalall these tympanics display a similar morphology indicating thatthese early archaeocetes used their peripheral ear structures likeland mammals while hearing in air and relied on bone conduc-tion mechanism while hearing in water (Thewissen and Hussain1993 Luo 1998 Luo and Gingerich 1999 Nummela et al 2004)

Pakicetus attocki is clearly larger than I pinfoldi whereas Pa-kicetus and Nalacetus have similar body sizes (Table 1) Thewis-sen and Hussain (1998) studied the size of upper and lower mo-lars in P attocki P inachus I pinfoldi and N ratimitus Theyfound Ichthyolestes to be the smallest P attocki and Nalacetus tobe of similar size and P inachus to be the largest Our studyshows that the absolute tympanic bullar size is similar in Paki-cetus and Ichthyolestes but larger in Nalacetus (Table 2 Appen-dix 1) This implies that in relative terms Pakicetus tympanicswhen related to body size (Table 2) are smaller than those ofIchthyolestes and Nalacetus

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 757

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ACKNOWLEDGMENTS

We thank Ellen Byron and Jeanette Killius for their technicalassistance Linda Spurlock prepared the drawings of the skullswhich is gratefully acknowledged Sunil Bajpai kindly gave usaccess to Remingtonocetus specimens in the IITR-SB collectionsunder his care We thank the editor Ewan Fordyce and Zhe-XiLuo and an anonymous reviewer for constructive suggestions forimprovement of the manuscript Financial support was providedby the National Science Foundation (current grant number EAR0207370) to JGMT and by the Ella and Georg Ehrnrooth Foun-dation to SN This article is a contribution to the Howard Uni-versity-Geological Survey of Pakistan projected entitled ldquoCeno-zoic Mammals of Pakistanrdquo We thank the Geological Survey ofPakistan for their support and help in the field

LITERATURE CITED

Aslan A and J G M Thewissen 1996 Preliminary evaluation of pa-leosols and implications for interpreting vertebrate fossil assem-blages Kuldana Formation Northern Pakistan Palaeovertebrata25261ndash277

Bajpai S and P D Gingerich 1998 A new Eocene archaeocete (Mam-malia Cetacea) from India and the time of origin of whales Pro-ceedings of the National Academy of Sciences USA 9515464ndash15468

Bajpai S and J G M Thewissen 1998 Middle Eocene cetaceans fromthe Harudi and Subathu formations of India pp 213ndash233 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Dehm R and T zu Oettingen-Spielberg 1958 Palaumlontologische undgeologische Untersuchungen im Tertiaumlr von Pakistan 2 Die mitte-leocaumlnen Saumlugetiere von Ganda Kas bei Basal in Nordwest-Pakistan Bayerische Akademie der Wissenschaften 911ndash54

Geisler J H and Z-X Luo 1998 Relationships of Cetacea to terres-trial ungulates and the evolution of cranial vasculature in Cete pp163ndash212 in J G M Thewissen (ed) The Emergence of WhalesPlenum Press New York

Geisler J H A E Sanders and Z-X Luo 2005 A new protocetidwhale (Cetacea Archaeoceti) from the late Middle Eocene of SouthCarolina American Museum Novitates 34801ndash65

Gingerich P D 2003 Land-to-sea transition in early whales evolutionof Eocene Archaeoceti (Cetacea) in relation to skeletal proportionsand locomotion of living semiaquatic mammals Paleobiology 29429ndash454

Gingerich P D and D E Russell 1981 Pakicetus inachus a newarchaeocete (Mammalia Cetacea) from the early-middle EoceneKuldana Formation of Kohat (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 25235ndash246

Gingerich P D and D E Russell 1990 Dentition of early EocenePakicetus (Mammalia Cetacea) Contributions from the Museum ofPaleontology University of Michigan 281ndash20

Gingerich P D N A Wells D E Russell and S M I Shah 1983Origin of whales in epicontinental remnant seas new evidence fromthe early Eocene of Pakistan Science 220403ndash406

Gingerich P D S M Raza M Arif M Anwar and X Zhou 1994New whale from the Eocene of Pakistan and the origin of cetaceanswimming Nature 368844ndash847

Gingerich P D M Arif and W C Clyde 1995 New archaeocetes(Mammalia Cetacea) from the middle Eocene Domanda Formationof the Sulaiman Range Punjab (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 29291ndash330

Gingerich P D M ul-Haq I S Zalmout I H Khan and M S Mal-kani 2001 Origin of whales from early artiodactyls hands and feetof Eocene Protocetidae from Pakistan Science 2932239ndash2242

Hulbert R Jr 1998 Postcranial osteology of the North American

middle Eocene protocetid Georgiacetus pp 235ndash267 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Kellogg A R 1936 A review of the Archaeoceti Carnegie Institute ofWashington Publications 4821ndash366

Kumar K and A Sahni 1986 Remingtonocetus harudiensis new com-bination a middle Eocene archaeocete (Mammalia Cetacea) fromwestern Kutch India Journal of Vertebrate Paleontology 6326ndash349

Luo Z-X 1998 Homology and transformation of cetacean ectotym-panic structures pp 269ndash301 in J G M Thewissen (ed) The Emer-gence of Whales Plenum Press New York

Luo Z-X and P D Gingerich 1999 Terrestrial Mesonychia to aquaticCetacea transformation of the basicranium and evolution of hearingin whales University of Michigan Papers on Paleontology 311ndash98

Madar S I In press The postcranial skeleton of early Eocene pakicetidcetaceans Journal of Paleontology

Nummela S J G M Thewissen S Bajpai S T Hussain and K Ku-mar 2004 Eocene evolution of whale hearing Nature 430776ndash778

Sahni A and V P Mishra 1972 A new species of Protocetus (Cetacea)from the middle Eocene of Kutch Western India Palaeontology15490ndash495

Thewissen J G M and S T Hussain 1993 Origin of underwaterhearing in whales Nature 361444ndash445

Thewissen J G M and S T Hussain 1998 Systematic review of thePakicetidae early and middle Eocene Cetacea (Mammalia) fromPakistan and India Bulletin of Carnegie Museum of Natural History34220ndash238

Thewissen J G M and E M Williams 2002 The early radiations ofCetacea (Mammalia) evolutionary patterns and developmental cor-relations Annual Review of Ecology and Systematics 3373ndash90

Thewissen J G M S I Madar and S T Hussain 1996 Ambulocetusnatans an Eocene cetacean (Mammalia) from Pakistan CourierForschungsinstitut Senckenberg 191 1ndash86

Thewissen J G M E M Williams and S T Hussain 2001a Eocenemammal faunas from Northern Indo-Pakistan Journal of Verte-brate Paleontology 21347ndash366

Thewissen J G M E M Williams L J Roe and S T Hussain 2001bSkeletons of terrestrial cetaceans and the relationship of whales toartiodactyls Nature 413277ndash281

Thewissen J G M S T Hussain M Arif A Aslan S I Madar andL J Roe 1998 New localities of Eocene vertebrates in NorthernPakistan and their significance for the origin of the modern orders ofmammals pp 19ndash34 in M I Ghaznavi S M Raza and M T Hasan(eds) Siwaliks of South Asia Proceedings of the 3rd GEOSASWorkshop Islamabad Pakistan 1ndash5 March 1997 Geological Sur-vey of Pakistan Islamabad Pakistan

Uhen M D 1998 Middle to late Eocene basilosaurines and dorudon-tines pp 29ndash61 in J G M Thewissen (ed) The Emergence ofWhales Plenum Press New York

Uhen M D 2004 Form function and anatomy of Dorudon atrox(Mammalia Cetacea) an archaeocete from the middle to lateEocene of Egypt University of Michigan Papers on Paleontology341ndash222

West R M 1980 Middle Eocene large mammal assemblage withTethyan affinities Ganda Kas region Pakistan Journal of Paleon-tology 54508ndash533

West R M and J R Lukacs 1979 Geology and vertebrate-fossil lo-calities Tertiary continental rocks Kala Chitta Hills Attock Dis-trict Pakistan Milwaukee Public Museum Contributions in Biologyand Geology 261ndash20

Williams E M 1998 Synopsis of the earliest cetaceans PakicetidaeAmbulocetidae Remingtonocetidae and Protocetidae pp 1ndash28 inJ G M Thewissen (ed) The Emergence of Whales Plenum PressNew York

Submitted 7 October 2005 accepted 24 February 2006

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APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

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Caudal to the postorbital process the frontal narrows at thefrontoparietal suture in all pakicetid genera (H-GSP 9623196254 96386 96574 96623 98134 98199 Figs 4B 5B 6B 7A8A) This suture extends across the posterior side of the postor-bital process In lateral view the frontoparietal suture extendsposteriorly into the temporal fossa (Figs 4A 5B 6B 7A 8A)until it reaches the orbitosphenoid (Figs 6C 8C) where it archesanterior and ventral reaching the presphenoid (Fig 8C) Thisarea of the skull is extremely narrow as shown by cross sectionsin Figure 2 the width is 40 mm in juvenile and 55 mm in adultP attocki (H-GSP 96623 and 96231 respectively) and 47 mm inI pinfoldi (H-GSP 98134)

ParietalmdashThe parietal shares sutures with the frontal orbito-sphenoid alisphenoid squamosal and occipital in all pakicetidgenera (H-GSP 96231 96254 96386 96574 96623 98134 Figs4A 5B 6B 7A 8A) and forms most of the sagittal crest (Figs4A 5B 6B 8A) which is somewhat higher than the braincase(the full sagittal crest is not preserved in any specimen) There isno evidence of an interparietal bone At the most anterior part ofthe sagittal crest the skull is extremely narrow (Fig 2 see thesection on the frontal bone) Posteriorly the parietal only formsa minor part of the wall of the braincase (H-GSP 96231 9638696623 98134) The parietal does not contribute to the nuchalcrest (H-GSP 96231 96254 96386 96623 Figs 4B 5B 6BD)There is no evidence of a postparietal foramen

PalatinemdashThe flat area inferior to the orbit anterior to theorbitosphenoid where the infraorbital nerve and sphenopalatine

artery are located is the pterygopalatine fossa this feature oc-curs in P attocki (H-GSP 96623) and I pinfoldi (H-GSP 9657498134 Figs 6C 8C 9) No sutures are preserved on the medialwall of the pterygopalatine fossa but this wall is probably formedby the palatine (Fig 8A) maxilla and frontal bones A horizon-tal infraorbital crest separates the pterygopalatine fossa from theorbit (Figs 4A 6C 8C 9) A small sphenopalatine foramen islocated on this crest (Figs 8C 9)

In N ratimitus (H-GSP 96055) the palatine contribution to thepalate extends anteriorly to P4 In the median plane the palatineextends far ventral causing the posterior palate to be concave

OrbitosphenoidmdashThe orbitosphenoid of P attocki (H-GSP96623) and I pinfoldi (H-GSP 98134 Figs 6C 8C) shares sutureswith the frontal parietal alisphenoid presphenoid and probablypalatine (the extent of the palatine in the pterygopalatine fossacannot be determined) Dorsally the orbitosphenoid has a smalloval depression (Fig 8C) Posteroinferior to this depression thesinus canal (Figs 6C 8C 9) is located on the parieto-orbitosphenoid suture and the groove extends anterior to itcrossing the entire orbitosphenoid Ventral to this is the opticforamen (Figs 6C 8C 9) from which a groove extends anteri-orly toward the orbit The sphenorbital fissure (Figs 6C 8C 9)is located on the suture between orbitosphenoid and alisphenoidon the extreme posterior edge of the former Only a small part ofthe suture between orbitosphenoid and presphenoid (Fig 8C) isvisible because its posterior part is covered by a large anteriorlyprojecting process of the alisphenoid (Fig 8A)

FIGURE 4 Cranial anatomy of Pakicetus attocki A skull lateral view shown by the right premaxilla right maxilla and cranium (H-GSP 1846718470 and 96231 respectively) Length of missing part of rostrum is based on Nalacetus ratimitus maxilla (H-GSP 30306) B cranium dorsal view(H-GSP 96231) These specimens although found at the same locality may not pertain to one individual Scale bar equals 5 cm

JOURNAL OF VERTEBRATE PALEONTOLOGY VOL 26 NO 3 2006750

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Kellogg (1936109 oph f) identified a lsquochannel for ophthalmicarteryrsquo and lsquoforamen for ophthalmic arteryrsquo in Zygorhiza kochiilocated dorsal to the channel for optic nerve and optic foramenrespectively The ophthalmic artery is a branch of the internalcarotid artery (in mammals lacking a stapedial artery) and is

thus located ventral to the optic chiasm and optic nerve in mam-mals on the floor of the braincase Usually this artery passesthrough the optic foramen The vascular structure passingthrough the foramen described by Kellogg (ophf) instead passesposterodorsally (see Kellogg 1936fig 31bc) opposite from the

FIGURE 5 Pakicetid cranial anatomy A Pakicetus attocki cranium H-GSP 96231 ventral view B Nalacetus ratimitus cranium H-GSP 96386dorsal view C ventral view Scale bar equals 5 cm

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 751

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FIGURE 6 Pakicetid cranial anatomy A Pakicetus attocki cranium H-GSP 96623 ventral view B dorsal view C right lateral view D Pakicetusattocki H-GSP 96231 caudal view E Ichthyolestes pinfoldi H-GSP 98134 caudal view Scale bar equals 5 cm

JOURNAL OF VERTEBRATE PALEONTOLOGY VOL 26 NO 3 2006752

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direction that the ophthalmic artery would have taken We in-terpret Kelloggrsquos lsquoforamen for the ophthalmic arteryrsquo as the an-terior opening for the sinus canal consistent with our interpre-tation of a similarly located foramen in pakicetids

PresphenoidmdashThe presphenoid is elongate and narrow inP attocki N ratimitus and I pinfoldi (H-GSP 96231 9638696623 98134 Figs 5AC 6A 7B) The ventral surface of the

presphenoid is rounded and smooth in Ichthyolestes whereasin Pakicetus and Nalacetus there is a distinct median crest ven-trally In Ichthyolestes much of the lateral side of the pre-sphenoid is covered by an anterior process of the alisphenoid(Fig 8A) A very small foramen occurs in this part of the pre-sphenoid and a groove extends anteriorly from this foramenWe interpret this foramen as the anterior foramen of the ptery-

FIGURE 7 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A dorsal view B ventral view C ventral view ear region in detail Dventral view outline of ear region Scale bar for AndashB equals 5 cm scale bar for CndashD equals 2 cm

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 753

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goid canal (Figs 8C 9) Anteriorly the lateral side of the pre-sphenoid reaches dorsal to touch the orbitosphenoid (Fig 8C)Ventrally the presphenoid is not fused to the basisphenoid in themidline and there is an ossification defect of 45 mm in Ich-thyolestes (Fig 7B)

AlisphenoidmdashThe alisphenoid shares sutures with pre-sphenoid orbitosphenoid parietal squamosal petrosal tym-panic basisphenoid and probably with pterygoid in P attockiN ratimitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Fig 8C) The anterior part of the alisphenoid is a rectan-gular process that lies lateral to much of the presphenoid andorbitosphenoid (H-GSP 98134 Fig 8A) and the sphenorbitalfissure is located just dorsal to this (Figs 6C 8C 9) Posteriorlythe alisphenoid forms a narrow wedge-like process betweensquamosal and basisphenoid in Pakicetus Nalacetus and Ich-thyolestes (H-GSP 30432 96231 96386 96623 98134 Figs 7D

8A) Part of this narrow process forms a prominently raised jointfacet the alisphenoid facet for the anterior process of the tym-panic (Figs 5A 6A 7D) This joint facet is continuous with asimilarly sized facet of the squamosal (Fig 5A 6A 7D) Thealisphenoid surrounds the entire oval foramen (Figs 5C 6A7D) There is no round foramen

The pterygoid process of the alisphenoid in pakicetids (H-GSP30432 96231 96386 96623 98134 Fig 7D) divides posteriorlyinto medial and lateral pterygoid laminae (Fig 7D) that sur-round a low pterygoid fossa (Fig 7D) This structure is compa-rable to the pterygoid fossa of primates and many other terres-trial mammals and should not be confused with the pterygoidsinus fossa of Basilosauridae Odontoceti and Mysticeti Themedial pterygoid lamina is low The pterygoid bone is not pre-served but was probably attached to the medial pterygoidlamina

FIGURE 9 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) right ventrolateral view showing the sphenorbital region with the foraminafrom an oblique angle The zygomatic arch is out of the focal plane of this photograph Scale bar equals 5 cm

FIGURE 8 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A right lateral view B sphenorbital region in detail C outline ofsphenorbital region Scale bar for A equals 5 cm scale bar for BndashC equals 2 cm

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BasisphenoidmdashThe basisphenoid shares sutures with pre-sphenoid alisphenoid and basioccipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Figs 5AC 6A 7B) The basisphenoid is convex vent-rally in cross section and concave in the median plane antero-posteriorly in juvenile Pakicetus and in Ichthyolestes (H-GSP96623 98134 respectively Figs 6A 7B) in adult Pakicetus(H-GSP 96231) and Nalacetus (H-GSP 96386) the basisphenoidis more or less flat in both these orientations (Figs 5AC) Pos-teriorly the suture between basisphenoid and basioccipital israised as bilateral tubercles in all pakicetid genera (H-GSP30432 96231 96386 98134 Figs 5AC 7B) One of the flexors ofthe neck rectus capitis ventralis originates from these tuberclesThere is no contact between the petrosal and the basisphenoidand the medial pterygoid sinus lies between these two bones(Fig 7D)

SquamosalmdashThe squamosal contacts the parietal alisphe-noid tympanic petrosal and occipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 98199 9625496386 96407 96623 98134 Figs 4B 5AB 6A 7AB 8A)making up most of the wall of the braincase Several smalltemporal foramina pierce the area near the occipito-squamosalsuture (Fig 6C) Posteriorly the squamosal forms part of thenuchal crest in all pakicetids (H-GSP 96231 96254 9638696623 Figs 4B 5B 6BD) Laterally the squamosal forms theposterior part of the zygomatic arch (Figs 4B 5AC 6BC 7AB8A 9) and the glenoid fossa (also called mandibular fossaFigs 5AC 6A 7Dglfs) in Pakicetus Nalacetus and Ichthyo-lestes (H-GSP 30432 96231 96386 96407 96623 98134) Theanterior part of the glenoid fossa is flat and extensive medio-laterally The postglenoid process (Figs 4A 5AC 6AC 7D8A) arches posteriorly into the external auditory meatus (Figs4A 5AC 6AC 7D 8A) The squamosal makes up the entireroof of the external auditory meatus and there is a prominentposttympanic process (Figs 4B 5B 6B 7A) The postglenoidforamen (also called retroarticular foramen Figs 5AC 6A 7D)is small and located posteromedial to the postglenoid processThe glenoid fossa extends medial to the postglenoid process andends posteromedially into a shallow groove directed to themiddle ear Medial to this is the squamosal contribution to theprominently raised joint facet (H-GSP 30432 96231 9638696623 98134 Figs 5A 6A 7D) for the anterior process of thetympanic The external auditory meatus ends medially in a re-cessed area in Pakicetus Nalacetus and Ichthyolestes (H-GSP30432 96231 96386 96407 96623 98134) The sharply depressedcenter of this recessed area represents the epitympanic recess(Figs 7D)

PetrosalmdashThe petrosal (periotic) contacts the alisphenoidsquamosal tympanic and occipital in P attocki N ratimitus andI pinfoldi (H-GSP 30432 96231 96386 96407 96431 9662398134 Fig 7D) The petrosal forms the roof of the middle earcavity The promontorium (Figs 5C 6A 7D) which houses theinner ear is a triangular structure on the ventral side The facialcanal runs anterior to the promontorium Medial to the prom-ontorium is the groove for the inferior petrosal sinus with thepiriform fenestra (H-GSP 30432 96386 96623 98134 Figs 5C7D) Rostrolateral to the promontorium is the tensor tympanifossa (H-GSP 30432 96231 96386 96407 96431 96623 98134Fig 7D) There are two foramina on the promontorium the ovalwindow for the stapes (fenestra vestibuli Fig 7D) and caudal toit the round window (fenestra cochleae Fig 7D) The mastoidprocess or posterior process of the petrosal flanks through thesquamosal posteriorly (H-GSP 30432 96231 96386 9640796623 98134 Figs 5AC 6A 7D 8A) A more detailed descrip-tion of the petrosal and tympanic of pakicetid cetaceans (Luoand Gingerich 1999) was based in part on material from H-GSPLocality 62

TympanicmdashThe tympanic (synonyms tympanic bulla audi-tory bulla ectotympanic) contacts the petrosal squamosal ali-sphenoid and exoccipital (GSP-UM 084) The tympanic sur-rounds the middle ear cavity ventrally laterally and medially(see Appendix 1 for all tympanics) The anterior part providesthe incisure for the eustachian tube (Fig 10) The medial pachy-osteosclerotic massive part of the tympanic is the involucrum(Fig 10) The involucrum is not in contact with any skull boneand is only connected to the rest of the skull by the dorsolateraland caudal parts of the tympanic The morphology described byGingerich and colleagues (1983) where the involucrum contactsthe basioccipital was based on the type specimen of P inachusGSP-UM 084 (sometimes listed as UM-GSP 081 by Luo 1998) inwhich this area is deformed

The shape and height of the anteromedial angle of the invo-lucrum (as defined by Luo 1998 Luo and Gingerich 1999Fig10) as well as the shape and orientation of the incisure forthe eustachian tube is species-specific (see Table 2) The lateralpart of the tympanic is a thin plate here the sigmoid process isa narrow and small plate of bone on the dorsal side of the lateralwall at least in I pinfoldi (H-GSP 92003) There is a small con-tact between the sigmoid process and the squamosal (see Luoand Gingerich 1999) Posterior to the sigmoid process the ex-ternal auditory meatus opens to the middle ear cavity Anteriorto the sigmoid process is the anterior process of the tympanicalso called processus tubarius which articulates with a facet onthe suture of squamosal and alisphenoid The posterior processof the tympanic contacts the mastoid (posterior) process of thepetrosal and more medially a small exoccipital facet makescontact with the tympanic (Fig 7D) this facet is the proximalpart of the paroccipital process of exoccipital (Figs 4B 5AC6ADE 7D)

There are four distinct morphologies among the tympanicsfrom H-GSP Locality 62 listed in Table 2 Three of these mor-phologies can be associated with cranial and dental material andthus be identified as P attocki I pinfoldi and N ratimitus Thefourth morphology represents a currently unidentified species ofthe family Pakicetidae Thewissen and Hussain (1998) describeda single tooth from Locality 62 tentatively as pertaining to Pinachus and it is possible that the unidentified bullae are fromthis species

In P attocki tympanics (see Appendix 1 and Table 2 Figs10AndashC) the morphology of the incisure for the eustachian tubeand of the anteromedial angle of the involucrum is similar to thatin the holotype of P inachus and therefore we refer these tym-panics to P attocki ndash which based on teeth (Thewissen andHussain 1998) is our most common species of Pakicetus Theorientation of the incisure for the eustachian tube cannot beobserved in GSP-UM 084 because the tympanic in this specimenis deformed and compressed against the basicranium

In the I pinfoldi specimen H-GSP 96431 (see Appendix 1 andTable 2 Figs 10DndashF) the fragmentary basicranium indicates asize considerably smaller than P attocki This is consistent withdental evidence (Thewissen and Hussain 1998) that indicatesthat the genus Ichthyolestes is smaller than Pakicetus We there-fore refer these specimens to I pinfoldi Although the adjoin-ing tympanic (H-GSP 96431) is the smallest one of our I pinfolditympanics two other tympanics with similar morphologyfor which the tympanic bullar length could be measured areboth within the length range of P attocki (Appendix 1 and Ta-ble 2)

The N ratimitus tympanics (see Appendix 1 and Table 2 Figs10GndashI) with a morphology clearly distinct from the two firstones described in Table 2 are larger than those of P attocki andmost of I pinfoldi and this serves to identify H-GSP 30325 and98140 Articular facets of Nalacetus tympanics (H-GSP 3032530459 96384 96385) match the facets on the exoccipital and

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 755

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squamosal of a Nalacetus skull (H-GSP 96386) The Nalacetusskull differs from Pakicetus (see previous) even though theseskulls are similar in size (Table 1) The Nalacetus skull is muchlarger than the Ichthyolestes skull (Table 1) This further cor-

roborates our identification of Nalacetus as a different genusand allocation of these tympanics to it

Three other remaining pakicetid tympanics (see Appendix 1and Table 2 Figs 10JndashL) are distinct from the three common

FIGURE 10 Pakicetid tympanics vertical columns showing the anterior lateral and dorsal views respectively AndashC Pakicetus attocki (H-GSP30386 left) DndashF Ichthyolestes pinfoldi (H-GSP 30223 left) GndashI Nalacetus ratimitus (H-GSP 96384 left) JndashL pakicetid n sp (H-GSP 98141 right)Scale bar equals 2 cm

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species of pakicetids at H-GSP Locality 62 and probably repre-sent a fourth new species of pakicetid These tympanics areclosest to P attocki tympanics in their size

OccipitalmdashThe most anterior point of the basioccipital of Pattocki N ratimitus and I pinfoldi (H-GSP 96231 96386 98134Figs 5AC 7B) is its suture with the basisphenoid which israised into bilateral tubercles (H-GSP 30432 96231 9638696623 98134 Figs 5AC 7B lsquobasioccipital crestrsquo of Luo 1998lsquoincipient falcate process of basioccipitalrsquo of Luo and Gingerich1999) These tubercles for neck flexors (see basisphenoid above)extend onto the anterior side of the basioccipital Posterior tothis the basioccipital projects laterally as a flat process in Paki-cetus (H-GSP 96231) and in Ichthyolestes (H-GSP 98134 Figs5A 7B) but this process is poorly defined in Nalacetus (H-GSP96386) The lateralmost part of this process is formed by theexoccipital (Figs 5AC 7B) and this process also forms the dor-solateral wall of the jugular foramen of pakicetids (H-GSP96231 96386 96407 96431 98134 Figs 5AC 7D) The exoc-cipitals bear the occipital condyles (H-GSP 96231 96386 9643198134 98199 Figs 4A 5AC 6DE 7B 8A 9) which are com-pressed dorsoventrally do not touch in the median plane andare somewhat raised above the level of the exoccipital The dor-sal and ventral condyloid fossae are of equal size (H-GSP 9623198134 98199 Figs 5AC 6E 7D) The hypoglossal foramen inall pakicetid genera (H-GSP 96231 96386 96407 96431 98134Figs 5AC 7D) is situated approximately halfway between thejugular foramen and the occipital condyle One or two smallcondylar foramina (Figs 5AC 7D) pierce the basioccipital lat-eral to the hypoglossal foramen The exoccipital forms the me-diolaterally elongate paroccipital process in all pakicetids (H-GSP 30432 96231 96386 96431 96623 98134 Figs 4B 5AC6ADE 7D) The medial extremity of this process bears a flatfacet for the tympanic (Fig 7D lsquobullar process of the exoccipitalrsquoof Luo and Gingerich 1999) The anterior part of the paroccipi-tal process shares a long suture with the mastoid process of pe-trosal The paroccipital process projects posterolaterally and thepart adjacent to the occipital condyle is recessed

The supraoccipital makes up most of the posterior side of thenuchal crest in all pakicetids (H-GSP 96231 96254 96386 96623Figs 4B 5B 6BD) although this structure is clearly worn insome specimens eg in I pinfoldi (H-GSP 98134) A small bi-lateral process of supraoccipital visible in Pakicetus and Ich-thyolestes (H-GSP 96231 98134 Figs 6DE) for the epaxial ex-tensors of the neck projects from the dorsal edge of the foramenmagnum (H-GSP 96231 96386 98134 Figs 6DE)Variable em-issary foramina (H-GSP 96231 96254 96386 96623 98134 Figs6DE) pierce the occipital dorsal to the foramen magnum Abilateral oval depression (H-GSP 96231 96386 98134 Figs6DE) occurs just lateral to the midline well above the foramenmagnum This depression could be for musculus longissimuscapitis The precise shape of the supraoccipital cannot be deter-mined because the suture between the exoccipital and the su-praoccipital is not visible

DISCUSSION

The pakicetid crania described here help to understand varia-tion in skull morphology of the earliest cetaceans As such theycan be expected to play a significant role in systematic studiesbut a detailed analysis of character scores for these specimensfalls outside the scope of the present description Here discus-sion will focus on a few aspects of cranial morphology that arerarely involved in cetacean systematics although they displayvariation to an extent that may be relevant systematically andinteresting evolutionarily

Supraorbital Morphology

The supraorbital region of protocetids basilosaurids anddorudontids displays a supraorbital shield (Kellogg 1936 Gin-

gerich et al 1994 1995 Bajpai and Gingerich 1998 Hulbert1998 Uhen 1998 2004) However the basal families of ceta-ceans lack a supraorbital shield pakicetids (Thewissen et al2001b) ambulocetids (Thewissen et al 1996) and remingtono-cetids (Gingerich et al 1994 1995 Bajpai and Thewissen 1998)The new pakicetid specimens indicate that there is much varia-tion in supraorbital morphology within these families In paki-cetids the orbits are convergent dorsally The area between theorbits is recessed over the superior orbital rim (Figs 4ndash8) Assuch pakicetid eyes are directed dorsolaterally There are dif-ferences between the genera (Fig 3 Table 1) in Pakicetus andIchthyolestes the supraorbital region is cuplike and cradles thedorsal part of the eye and the dorsal side of the supraorbitalregion is deeply incised in the medial plane in Pakicetus Thisincisure is less strong in Nalacetus and Ichthyolestes and in Nala-cetus the contact between supraorbital region and eye is toonarrow to be cup-shaped In Ambulocetus (Fig 3 Table 1Thewissen et al 1996fig 10 for H-GSP 18490) the orbits arepositioned near the dorsal side of the skull but they face later-ally leaving the supraorbital region more or less flat (in dorsalview) The eyes here appear to be directed laterally and are nothoused in the bony orbits as deeply as in pakicetids as indicatedby the shape of the postorbital processes As a comparison inRemingtonocetus the eyes are small are set laterally and facelaterally and the supraorbital region is strongly convex (Fig 3Sahni and Mishra 1972 Kumar and Sahni 1986 Bajpai andThewissen 1998) In protocetids the orbit is large faces later-ally and is covered by a large supraorbital shield (Fig 3 Table1) Dorudontids and basilosaurids resemble protocetids in thepresence and shape of their supraorbital shield as indicated bythe ratio between the minimum and maximum supraorbitalwidth (Table 1) and in the orientation of their orbits the latterare still clearly different from modern odontocetes and mystice-tes (Kellogg 1936 Uhen 1998 2004)

Intertemporal Morphology

In early and middle Eocene cetaceans a very narrow regioncan be seen in the skull posterior to the frontoparietal suturenear the sphenorbital fissure (Figs 1 2 4ndash8) This morphology isnot seen in most land mammals but is common in early ceta-ceans such as pakicetids ambulocetids (eg AmbulocetusThewissen et al 1996) and remingtonocetids (Kumar and Sahni1986) and may represent an important diagnostic character forthe order

Tympanic Bullar Morphology

The differences in the tympanic bullar morphology seen be-tween P attocki I pinfoldi N ratimitus and a fourth unnamedspecies can be used for specific recognition However in generalall these tympanics display a similar morphology indicating thatthese early archaeocetes used their peripheral ear structures likeland mammals while hearing in air and relied on bone conduc-tion mechanism while hearing in water (Thewissen and Hussain1993 Luo 1998 Luo and Gingerich 1999 Nummela et al 2004)

Pakicetus attocki is clearly larger than I pinfoldi whereas Pa-kicetus and Nalacetus have similar body sizes (Table 1) Thewis-sen and Hussain (1998) studied the size of upper and lower mo-lars in P attocki P inachus I pinfoldi and N ratimitus Theyfound Ichthyolestes to be the smallest P attocki and Nalacetus tobe of similar size and P inachus to be the largest Our studyshows that the absolute tympanic bullar size is similar in Paki-cetus and Ichthyolestes but larger in Nalacetus (Table 2 Appen-dix 1) This implies that in relative terms Pakicetus tympanicswhen related to body size (Table 2) are smaller than those ofIchthyolestes and Nalacetus

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 757

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ACKNOWLEDGMENTS

We thank Ellen Byron and Jeanette Killius for their technicalassistance Linda Spurlock prepared the drawings of the skullswhich is gratefully acknowledged Sunil Bajpai kindly gave usaccess to Remingtonocetus specimens in the IITR-SB collectionsunder his care We thank the editor Ewan Fordyce and Zhe-XiLuo and an anonymous reviewer for constructive suggestions forimprovement of the manuscript Financial support was providedby the National Science Foundation (current grant number EAR0207370) to JGMT and by the Ella and Georg Ehrnrooth Foun-dation to SN This article is a contribution to the Howard Uni-versity-Geological Survey of Pakistan projected entitled ldquoCeno-zoic Mammals of Pakistanrdquo We thank the Geological Survey ofPakistan for their support and help in the field

LITERATURE CITED

Aslan A and J G M Thewissen 1996 Preliminary evaluation of pa-leosols and implications for interpreting vertebrate fossil assem-blages Kuldana Formation Northern Pakistan Palaeovertebrata25261ndash277

Bajpai S and P D Gingerich 1998 A new Eocene archaeocete (Mam-malia Cetacea) from India and the time of origin of whales Pro-ceedings of the National Academy of Sciences USA 9515464ndash15468

Bajpai S and J G M Thewissen 1998 Middle Eocene cetaceans fromthe Harudi and Subathu formations of India pp 213ndash233 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Dehm R and T zu Oettingen-Spielberg 1958 Palaumlontologische undgeologische Untersuchungen im Tertiaumlr von Pakistan 2 Die mitte-leocaumlnen Saumlugetiere von Ganda Kas bei Basal in Nordwest-Pakistan Bayerische Akademie der Wissenschaften 911ndash54

Geisler J H and Z-X Luo 1998 Relationships of Cetacea to terres-trial ungulates and the evolution of cranial vasculature in Cete pp163ndash212 in J G M Thewissen (ed) The Emergence of WhalesPlenum Press New York

Geisler J H A E Sanders and Z-X Luo 2005 A new protocetidwhale (Cetacea Archaeoceti) from the late Middle Eocene of SouthCarolina American Museum Novitates 34801ndash65

Gingerich P D 2003 Land-to-sea transition in early whales evolutionof Eocene Archaeoceti (Cetacea) in relation to skeletal proportionsand locomotion of living semiaquatic mammals Paleobiology 29429ndash454

Gingerich P D and D E Russell 1981 Pakicetus inachus a newarchaeocete (Mammalia Cetacea) from the early-middle EoceneKuldana Formation of Kohat (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 25235ndash246

Gingerich P D and D E Russell 1990 Dentition of early EocenePakicetus (Mammalia Cetacea) Contributions from the Museum ofPaleontology University of Michigan 281ndash20

Gingerich P D N A Wells D E Russell and S M I Shah 1983Origin of whales in epicontinental remnant seas new evidence fromthe early Eocene of Pakistan Science 220403ndash406

Gingerich P D S M Raza M Arif M Anwar and X Zhou 1994New whale from the Eocene of Pakistan and the origin of cetaceanswimming Nature 368844ndash847

Gingerich P D M Arif and W C Clyde 1995 New archaeocetes(Mammalia Cetacea) from the middle Eocene Domanda Formationof the Sulaiman Range Punjab (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 29291ndash330

Gingerich P D M ul-Haq I S Zalmout I H Khan and M S Mal-kani 2001 Origin of whales from early artiodactyls hands and feetof Eocene Protocetidae from Pakistan Science 2932239ndash2242

Hulbert R Jr 1998 Postcranial osteology of the North American

middle Eocene protocetid Georgiacetus pp 235ndash267 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Kellogg A R 1936 A review of the Archaeoceti Carnegie Institute ofWashington Publications 4821ndash366

Kumar K and A Sahni 1986 Remingtonocetus harudiensis new com-bination a middle Eocene archaeocete (Mammalia Cetacea) fromwestern Kutch India Journal of Vertebrate Paleontology 6326ndash349

Luo Z-X 1998 Homology and transformation of cetacean ectotym-panic structures pp 269ndash301 in J G M Thewissen (ed) The Emer-gence of Whales Plenum Press New York

Luo Z-X and P D Gingerich 1999 Terrestrial Mesonychia to aquaticCetacea transformation of the basicranium and evolution of hearingin whales University of Michigan Papers on Paleontology 311ndash98

Madar S I In press The postcranial skeleton of early Eocene pakicetidcetaceans Journal of Paleontology

Nummela S J G M Thewissen S Bajpai S T Hussain and K Ku-mar 2004 Eocene evolution of whale hearing Nature 430776ndash778

Sahni A and V P Mishra 1972 A new species of Protocetus (Cetacea)from the middle Eocene of Kutch Western India Palaeontology15490ndash495

Thewissen J G M and S T Hussain 1993 Origin of underwaterhearing in whales Nature 361444ndash445

Thewissen J G M and S T Hussain 1998 Systematic review of thePakicetidae early and middle Eocene Cetacea (Mammalia) fromPakistan and India Bulletin of Carnegie Museum of Natural History34220ndash238

Thewissen J G M and E M Williams 2002 The early radiations ofCetacea (Mammalia) evolutionary patterns and developmental cor-relations Annual Review of Ecology and Systematics 3373ndash90

Thewissen J G M S I Madar and S T Hussain 1996 Ambulocetusnatans an Eocene cetacean (Mammalia) from Pakistan CourierForschungsinstitut Senckenberg 191 1ndash86

Thewissen J G M E M Williams and S T Hussain 2001a Eocenemammal faunas from Northern Indo-Pakistan Journal of Verte-brate Paleontology 21347ndash366

Thewissen J G M E M Williams L J Roe and S T Hussain 2001bSkeletons of terrestrial cetaceans and the relationship of whales toartiodactyls Nature 413277ndash281

Thewissen J G M S T Hussain M Arif A Aslan S I Madar andL J Roe 1998 New localities of Eocene vertebrates in NorthernPakistan and their significance for the origin of the modern orders ofmammals pp 19ndash34 in M I Ghaznavi S M Raza and M T Hasan(eds) Siwaliks of South Asia Proceedings of the 3rd GEOSASWorkshop Islamabad Pakistan 1ndash5 March 1997 Geological Sur-vey of Pakistan Islamabad Pakistan

Uhen M D 1998 Middle to late Eocene basilosaurines and dorudon-tines pp 29ndash61 in J G M Thewissen (ed) The Emergence ofWhales Plenum Press New York

Uhen M D 2004 Form function and anatomy of Dorudon atrox(Mammalia Cetacea) an archaeocete from the middle to lateEocene of Egypt University of Michigan Papers on Paleontology341ndash222

West R M 1980 Middle Eocene large mammal assemblage withTethyan affinities Ganda Kas region Pakistan Journal of Paleon-tology 54508ndash533

West R M and J R Lukacs 1979 Geology and vertebrate-fossil lo-calities Tertiary continental rocks Kala Chitta Hills Attock Dis-trict Pakistan Milwaukee Public Museum Contributions in Biologyand Geology 261ndash20

Williams E M 1998 Synopsis of the earliest cetaceans PakicetidaeAmbulocetidae Remingtonocetidae and Protocetidae pp 1ndash28 inJ G M Thewissen (ed) The Emergence of Whales Plenum PressNew York

Submitted 7 October 2005 accepted 24 February 2006

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APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

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Kellogg (1936109 oph f) identified a lsquochannel for ophthalmicarteryrsquo and lsquoforamen for ophthalmic arteryrsquo in Zygorhiza kochiilocated dorsal to the channel for optic nerve and optic foramenrespectively The ophthalmic artery is a branch of the internalcarotid artery (in mammals lacking a stapedial artery) and is

thus located ventral to the optic chiasm and optic nerve in mam-mals on the floor of the braincase Usually this artery passesthrough the optic foramen The vascular structure passingthrough the foramen described by Kellogg (ophf) instead passesposterodorsally (see Kellogg 1936fig 31bc) opposite from the

FIGURE 5 Pakicetid cranial anatomy A Pakicetus attocki cranium H-GSP 96231 ventral view B Nalacetus ratimitus cranium H-GSP 96386dorsal view C ventral view Scale bar equals 5 cm

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FIGURE 6 Pakicetid cranial anatomy A Pakicetus attocki cranium H-GSP 96623 ventral view B dorsal view C right lateral view D Pakicetusattocki H-GSP 96231 caudal view E Ichthyolestes pinfoldi H-GSP 98134 caudal view Scale bar equals 5 cm

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direction that the ophthalmic artery would have taken We in-terpret Kelloggrsquos lsquoforamen for the ophthalmic arteryrsquo as the an-terior opening for the sinus canal consistent with our interpre-tation of a similarly located foramen in pakicetids

PresphenoidmdashThe presphenoid is elongate and narrow inP attocki N ratimitus and I pinfoldi (H-GSP 96231 9638696623 98134 Figs 5AC 6A 7B) The ventral surface of the

presphenoid is rounded and smooth in Ichthyolestes whereasin Pakicetus and Nalacetus there is a distinct median crest ven-trally In Ichthyolestes much of the lateral side of the pre-sphenoid is covered by an anterior process of the alisphenoid(Fig 8A) A very small foramen occurs in this part of the pre-sphenoid and a groove extends anteriorly from this foramenWe interpret this foramen as the anterior foramen of the ptery-

FIGURE 7 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A dorsal view B ventral view C ventral view ear region in detail Dventral view outline of ear region Scale bar for AndashB equals 5 cm scale bar for CndashD equals 2 cm

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goid canal (Figs 8C 9) Anteriorly the lateral side of the pre-sphenoid reaches dorsal to touch the orbitosphenoid (Fig 8C)Ventrally the presphenoid is not fused to the basisphenoid in themidline and there is an ossification defect of 45 mm in Ich-thyolestes (Fig 7B)

AlisphenoidmdashThe alisphenoid shares sutures with pre-sphenoid orbitosphenoid parietal squamosal petrosal tym-panic basisphenoid and probably with pterygoid in P attockiN ratimitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Fig 8C) The anterior part of the alisphenoid is a rectan-gular process that lies lateral to much of the presphenoid andorbitosphenoid (H-GSP 98134 Fig 8A) and the sphenorbitalfissure is located just dorsal to this (Figs 6C 8C 9) Posteriorlythe alisphenoid forms a narrow wedge-like process betweensquamosal and basisphenoid in Pakicetus Nalacetus and Ich-thyolestes (H-GSP 30432 96231 96386 96623 98134 Figs 7D

8A) Part of this narrow process forms a prominently raised jointfacet the alisphenoid facet for the anterior process of the tym-panic (Figs 5A 6A 7D) This joint facet is continuous with asimilarly sized facet of the squamosal (Fig 5A 6A 7D) Thealisphenoid surrounds the entire oval foramen (Figs 5C 6A7D) There is no round foramen

The pterygoid process of the alisphenoid in pakicetids (H-GSP30432 96231 96386 96623 98134 Fig 7D) divides posteriorlyinto medial and lateral pterygoid laminae (Fig 7D) that sur-round a low pterygoid fossa (Fig 7D) This structure is compa-rable to the pterygoid fossa of primates and many other terres-trial mammals and should not be confused with the pterygoidsinus fossa of Basilosauridae Odontoceti and Mysticeti Themedial pterygoid lamina is low The pterygoid bone is not pre-served but was probably attached to the medial pterygoidlamina

FIGURE 9 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) right ventrolateral view showing the sphenorbital region with the foraminafrom an oblique angle The zygomatic arch is out of the focal plane of this photograph Scale bar equals 5 cm

FIGURE 8 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A right lateral view B sphenorbital region in detail C outline ofsphenorbital region Scale bar for A equals 5 cm scale bar for BndashC equals 2 cm

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BasisphenoidmdashThe basisphenoid shares sutures with pre-sphenoid alisphenoid and basioccipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Figs 5AC 6A 7B) The basisphenoid is convex vent-rally in cross section and concave in the median plane antero-posteriorly in juvenile Pakicetus and in Ichthyolestes (H-GSP96623 98134 respectively Figs 6A 7B) in adult Pakicetus(H-GSP 96231) and Nalacetus (H-GSP 96386) the basisphenoidis more or less flat in both these orientations (Figs 5AC) Pos-teriorly the suture between basisphenoid and basioccipital israised as bilateral tubercles in all pakicetid genera (H-GSP30432 96231 96386 98134 Figs 5AC 7B) One of the flexors ofthe neck rectus capitis ventralis originates from these tuberclesThere is no contact between the petrosal and the basisphenoidand the medial pterygoid sinus lies between these two bones(Fig 7D)

SquamosalmdashThe squamosal contacts the parietal alisphe-noid tympanic petrosal and occipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 98199 9625496386 96407 96623 98134 Figs 4B 5AB 6A 7AB 8A)making up most of the wall of the braincase Several smalltemporal foramina pierce the area near the occipito-squamosalsuture (Fig 6C) Posteriorly the squamosal forms part of thenuchal crest in all pakicetids (H-GSP 96231 96254 9638696623 Figs 4B 5B 6BD) Laterally the squamosal forms theposterior part of the zygomatic arch (Figs 4B 5AC 6BC 7AB8A 9) and the glenoid fossa (also called mandibular fossaFigs 5AC 6A 7Dglfs) in Pakicetus Nalacetus and Ichthyo-lestes (H-GSP 30432 96231 96386 96407 96623 98134) Theanterior part of the glenoid fossa is flat and extensive medio-laterally The postglenoid process (Figs 4A 5AC 6AC 7D8A) arches posteriorly into the external auditory meatus (Figs4A 5AC 6AC 7D 8A) The squamosal makes up the entireroof of the external auditory meatus and there is a prominentposttympanic process (Figs 4B 5B 6B 7A) The postglenoidforamen (also called retroarticular foramen Figs 5AC 6A 7D)is small and located posteromedial to the postglenoid processThe glenoid fossa extends medial to the postglenoid process andends posteromedially into a shallow groove directed to themiddle ear Medial to this is the squamosal contribution to theprominently raised joint facet (H-GSP 30432 96231 9638696623 98134 Figs 5A 6A 7D) for the anterior process of thetympanic The external auditory meatus ends medially in a re-cessed area in Pakicetus Nalacetus and Ichthyolestes (H-GSP30432 96231 96386 96407 96623 98134) The sharply depressedcenter of this recessed area represents the epitympanic recess(Figs 7D)

PetrosalmdashThe petrosal (periotic) contacts the alisphenoidsquamosal tympanic and occipital in P attocki N ratimitus andI pinfoldi (H-GSP 30432 96231 96386 96407 96431 9662398134 Fig 7D) The petrosal forms the roof of the middle earcavity The promontorium (Figs 5C 6A 7D) which houses theinner ear is a triangular structure on the ventral side The facialcanal runs anterior to the promontorium Medial to the prom-ontorium is the groove for the inferior petrosal sinus with thepiriform fenestra (H-GSP 30432 96386 96623 98134 Figs 5C7D) Rostrolateral to the promontorium is the tensor tympanifossa (H-GSP 30432 96231 96386 96407 96431 96623 98134Fig 7D) There are two foramina on the promontorium the ovalwindow for the stapes (fenestra vestibuli Fig 7D) and caudal toit the round window (fenestra cochleae Fig 7D) The mastoidprocess or posterior process of the petrosal flanks through thesquamosal posteriorly (H-GSP 30432 96231 96386 9640796623 98134 Figs 5AC 6A 7D 8A) A more detailed descrip-tion of the petrosal and tympanic of pakicetid cetaceans (Luoand Gingerich 1999) was based in part on material from H-GSPLocality 62

TympanicmdashThe tympanic (synonyms tympanic bulla audi-tory bulla ectotympanic) contacts the petrosal squamosal ali-sphenoid and exoccipital (GSP-UM 084) The tympanic sur-rounds the middle ear cavity ventrally laterally and medially(see Appendix 1 for all tympanics) The anterior part providesthe incisure for the eustachian tube (Fig 10) The medial pachy-osteosclerotic massive part of the tympanic is the involucrum(Fig 10) The involucrum is not in contact with any skull boneand is only connected to the rest of the skull by the dorsolateraland caudal parts of the tympanic The morphology described byGingerich and colleagues (1983) where the involucrum contactsthe basioccipital was based on the type specimen of P inachusGSP-UM 084 (sometimes listed as UM-GSP 081 by Luo 1998) inwhich this area is deformed

The shape and height of the anteromedial angle of the invo-lucrum (as defined by Luo 1998 Luo and Gingerich 1999Fig10) as well as the shape and orientation of the incisure forthe eustachian tube is species-specific (see Table 2) The lateralpart of the tympanic is a thin plate here the sigmoid process isa narrow and small plate of bone on the dorsal side of the lateralwall at least in I pinfoldi (H-GSP 92003) There is a small con-tact between the sigmoid process and the squamosal (see Luoand Gingerich 1999) Posterior to the sigmoid process the ex-ternal auditory meatus opens to the middle ear cavity Anteriorto the sigmoid process is the anterior process of the tympanicalso called processus tubarius which articulates with a facet onthe suture of squamosal and alisphenoid The posterior processof the tympanic contacts the mastoid (posterior) process of thepetrosal and more medially a small exoccipital facet makescontact with the tympanic (Fig 7D) this facet is the proximalpart of the paroccipital process of exoccipital (Figs 4B 5AC6ADE 7D)

There are four distinct morphologies among the tympanicsfrom H-GSP Locality 62 listed in Table 2 Three of these mor-phologies can be associated with cranial and dental material andthus be identified as P attocki I pinfoldi and N ratimitus Thefourth morphology represents a currently unidentified species ofthe family Pakicetidae Thewissen and Hussain (1998) describeda single tooth from Locality 62 tentatively as pertaining to Pinachus and it is possible that the unidentified bullae are fromthis species

In P attocki tympanics (see Appendix 1 and Table 2 Figs10AndashC) the morphology of the incisure for the eustachian tubeand of the anteromedial angle of the involucrum is similar to thatin the holotype of P inachus and therefore we refer these tym-panics to P attocki ndash which based on teeth (Thewissen andHussain 1998) is our most common species of Pakicetus Theorientation of the incisure for the eustachian tube cannot beobserved in GSP-UM 084 because the tympanic in this specimenis deformed and compressed against the basicranium

In the I pinfoldi specimen H-GSP 96431 (see Appendix 1 andTable 2 Figs 10DndashF) the fragmentary basicranium indicates asize considerably smaller than P attocki This is consistent withdental evidence (Thewissen and Hussain 1998) that indicatesthat the genus Ichthyolestes is smaller than Pakicetus We there-fore refer these specimens to I pinfoldi Although the adjoin-ing tympanic (H-GSP 96431) is the smallest one of our I pinfolditympanics two other tympanics with similar morphologyfor which the tympanic bullar length could be measured areboth within the length range of P attocki (Appendix 1 and Ta-ble 2)

The N ratimitus tympanics (see Appendix 1 and Table 2 Figs10GndashI) with a morphology clearly distinct from the two firstones described in Table 2 are larger than those of P attocki andmost of I pinfoldi and this serves to identify H-GSP 30325 and98140 Articular facets of Nalacetus tympanics (H-GSP 3032530459 96384 96385) match the facets on the exoccipital and

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 755

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squamosal of a Nalacetus skull (H-GSP 96386) The Nalacetusskull differs from Pakicetus (see previous) even though theseskulls are similar in size (Table 1) The Nalacetus skull is muchlarger than the Ichthyolestes skull (Table 1) This further cor-

roborates our identification of Nalacetus as a different genusand allocation of these tympanics to it

Three other remaining pakicetid tympanics (see Appendix 1and Table 2 Figs 10JndashL) are distinct from the three common

FIGURE 10 Pakicetid tympanics vertical columns showing the anterior lateral and dorsal views respectively AndashC Pakicetus attocki (H-GSP30386 left) DndashF Ichthyolestes pinfoldi (H-GSP 30223 left) GndashI Nalacetus ratimitus (H-GSP 96384 left) JndashL pakicetid n sp (H-GSP 98141 right)Scale bar equals 2 cm

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species of pakicetids at H-GSP Locality 62 and probably repre-sent a fourth new species of pakicetid These tympanics areclosest to P attocki tympanics in their size

OccipitalmdashThe most anterior point of the basioccipital of Pattocki N ratimitus and I pinfoldi (H-GSP 96231 96386 98134Figs 5AC 7B) is its suture with the basisphenoid which israised into bilateral tubercles (H-GSP 30432 96231 9638696623 98134 Figs 5AC 7B lsquobasioccipital crestrsquo of Luo 1998lsquoincipient falcate process of basioccipitalrsquo of Luo and Gingerich1999) These tubercles for neck flexors (see basisphenoid above)extend onto the anterior side of the basioccipital Posterior tothis the basioccipital projects laterally as a flat process in Paki-cetus (H-GSP 96231) and in Ichthyolestes (H-GSP 98134 Figs5A 7B) but this process is poorly defined in Nalacetus (H-GSP96386) The lateralmost part of this process is formed by theexoccipital (Figs 5AC 7B) and this process also forms the dor-solateral wall of the jugular foramen of pakicetids (H-GSP96231 96386 96407 96431 98134 Figs 5AC 7D) The exoc-cipitals bear the occipital condyles (H-GSP 96231 96386 9643198134 98199 Figs 4A 5AC 6DE 7B 8A 9) which are com-pressed dorsoventrally do not touch in the median plane andare somewhat raised above the level of the exoccipital The dor-sal and ventral condyloid fossae are of equal size (H-GSP 9623198134 98199 Figs 5AC 6E 7D) The hypoglossal foramen inall pakicetid genera (H-GSP 96231 96386 96407 96431 98134Figs 5AC 7D) is situated approximately halfway between thejugular foramen and the occipital condyle One or two smallcondylar foramina (Figs 5AC 7D) pierce the basioccipital lat-eral to the hypoglossal foramen The exoccipital forms the me-diolaterally elongate paroccipital process in all pakicetids (H-GSP 30432 96231 96386 96431 96623 98134 Figs 4B 5AC6ADE 7D) The medial extremity of this process bears a flatfacet for the tympanic (Fig 7D lsquobullar process of the exoccipitalrsquoof Luo and Gingerich 1999) The anterior part of the paroccipi-tal process shares a long suture with the mastoid process of pe-trosal The paroccipital process projects posterolaterally and thepart adjacent to the occipital condyle is recessed

The supraoccipital makes up most of the posterior side of thenuchal crest in all pakicetids (H-GSP 96231 96254 96386 96623Figs 4B 5B 6BD) although this structure is clearly worn insome specimens eg in I pinfoldi (H-GSP 98134) A small bi-lateral process of supraoccipital visible in Pakicetus and Ich-thyolestes (H-GSP 96231 98134 Figs 6DE) for the epaxial ex-tensors of the neck projects from the dorsal edge of the foramenmagnum (H-GSP 96231 96386 98134 Figs 6DE)Variable em-issary foramina (H-GSP 96231 96254 96386 96623 98134 Figs6DE) pierce the occipital dorsal to the foramen magnum Abilateral oval depression (H-GSP 96231 96386 98134 Figs6DE) occurs just lateral to the midline well above the foramenmagnum This depression could be for musculus longissimuscapitis The precise shape of the supraoccipital cannot be deter-mined because the suture between the exoccipital and the su-praoccipital is not visible

DISCUSSION

The pakicetid crania described here help to understand varia-tion in skull morphology of the earliest cetaceans As such theycan be expected to play a significant role in systematic studiesbut a detailed analysis of character scores for these specimensfalls outside the scope of the present description Here discus-sion will focus on a few aspects of cranial morphology that arerarely involved in cetacean systematics although they displayvariation to an extent that may be relevant systematically andinteresting evolutionarily

Supraorbital Morphology

The supraorbital region of protocetids basilosaurids anddorudontids displays a supraorbital shield (Kellogg 1936 Gin-

gerich et al 1994 1995 Bajpai and Gingerich 1998 Hulbert1998 Uhen 1998 2004) However the basal families of ceta-ceans lack a supraorbital shield pakicetids (Thewissen et al2001b) ambulocetids (Thewissen et al 1996) and remingtono-cetids (Gingerich et al 1994 1995 Bajpai and Thewissen 1998)The new pakicetid specimens indicate that there is much varia-tion in supraorbital morphology within these families In paki-cetids the orbits are convergent dorsally The area between theorbits is recessed over the superior orbital rim (Figs 4ndash8) Assuch pakicetid eyes are directed dorsolaterally There are dif-ferences between the genera (Fig 3 Table 1) in Pakicetus andIchthyolestes the supraorbital region is cuplike and cradles thedorsal part of the eye and the dorsal side of the supraorbitalregion is deeply incised in the medial plane in Pakicetus Thisincisure is less strong in Nalacetus and Ichthyolestes and in Nala-cetus the contact between supraorbital region and eye is toonarrow to be cup-shaped In Ambulocetus (Fig 3 Table 1Thewissen et al 1996fig 10 for H-GSP 18490) the orbits arepositioned near the dorsal side of the skull but they face later-ally leaving the supraorbital region more or less flat (in dorsalview) The eyes here appear to be directed laterally and are nothoused in the bony orbits as deeply as in pakicetids as indicatedby the shape of the postorbital processes As a comparison inRemingtonocetus the eyes are small are set laterally and facelaterally and the supraorbital region is strongly convex (Fig 3Sahni and Mishra 1972 Kumar and Sahni 1986 Bajpai andThewissen 1998) In protocetids the orbit is large faces later-ally and is covered by a large supraorbital shield (Fig 3 Table1) Dorudontids and basilosaurids resemble protocetids in thepresence and shape of their supraorbital shield as indicated bythe ratio between the minimum and maximum supraorbitalwidth (Table 1) and in the orientation of their orbits the latterare still clearly different from modern odontocetes and mystice-tes (Kellogg 1936 Uhen 1998 2004)

Intertemporal Morphology

In early and middle Eocene cetaceans a very narrow regioncan be seen in the skull posterior to the frontoparietal suturenear the sphenorbital fissure (Figs 1 2 4ndash8) This morphology isnot seen in most land mammals but is common in early ceta-ceans such as pakicetids ambulocetids (eg AmbulocetusThewissen et al 1996) and remingtonocetids (Kumar and Sahni1986) and may represent an important diagnostic character forthe order

Tympanic Bullar Morphology

The differences in the tympanic bullar morphology seen be-tween P attocki I pinfoldi N ratimitus and a fourth unnamedspecies can be used for specific recognition However in generalall these tympanics display a similar morphology indicating thatthese early archaeocetes used their peripheral ear structures likeland mammals while hearing in air and relied on bone conduc-tion mechanism while hearing in water (Thewissen and Hussain1993 Luo 1998 Luo and Gingerich 1999 Nummela et al 2004)

Pakicetus attocki is clearly larger than I pinfoldi whereas Pa-kicetus and Nalacetus have similar body sizes (Table 1) Thewis-sen and Hussain (1998) studied the size of upper and lower mo-lars in P attocki P inachus I pinfoldi and N ratimitus Theyfound Ichthyolestes to be the smallest P attocki and Nalacetus tobe of similar size and P inachus to be the largest Our studyshows that the absolute tympanic bullar size is similar in Paki-cetus and Ichthyolestes but larger in Nalacetus (Table 2 Appen-dix 1) This implies that in relative terms Pakicetus tympanicswhen related to body size (Table 2) are smaller than those ofIchthyolestes and Nalacetus

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ACKNOWLEDGMENTS

We thank Ellen Byron and Jeanette Killius for their technicalassistance Linda Spurlock prepared the drawings of the skullswhich is gratefully acknowledged Sunil Bajpai kindly gave usaccess to Remingtonocetus specimens in the IITR-SB collectionsunder his care We thank the editor Ewan Fordyce and Zhe-XiLuo and an anonymous reviewer for constructive suggestions forimprovement of the manuscript Financial support was providedby the National Science Foundation (current grant number EAR0207370) to JGMT and by the Ella and Georg Ehrnrooth Foun-dation to SN This article is a contribution to the Howard Uni-versity-Geological Survey of Pakistan projected entitled ldquoCeno-zoic Mammals of Pakistanrdquo We thank the Geological Survey ofPakistan for their support and help in the field

LITERATURE CITED

Aslan A and J G M Thewissen 1996 Preliminary evaluation of pa-leosols and implications for interpreting vertebrate fossil assem-blages Kuldana Formation Northern Pakistan Palaeovertebrata25261ndash277

Bajpai S and P D Gingerich 1998 A new Eocene archaeocete (Mam-malia Cetacea) from India and the time of origin of whales Pro-ceedings of the National Academy of Sciences USA 9515464ndash15468

Bajpai S and J G M Thewissen 1998 Middle Eocene cetaceans fromthe Harudi and Subathu formations of India pp 213ndash233 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Dehm R and T zu Oettingen-Spielberg 1958 Palaumlontologische undgeologische Untersuchungen im Tertiaumlr von Pakistan 2 Die mitte-leocaumlnen Saumlugetiere von Ganda Kas bei Basal in Nordwest-Pakistan Bayerische Akademie der Wissenschaften 911ndash54

Geisler J H and Z-X Luo 1998 Relationships of Cetacea to terres-trial ungulates and the evolution of cranial vasculature in Cete pp163ndash212 in J G M Thewissen (ed) The Emergence of WhalesPlenum Press New York

Geisler J H A E Sanders and Z-X Luo 2005 A new protocetidwhale (Cetacea Archaeoceti) from the late Middle Eocene of SouthCarolina American Museum Novitates 34801ndash65

Gingerich P D 2003 Land-to-sea transition in early whales evolutionof Eocene Archaeoceti (Cetacea) in relation to skeletal proportionsand locomotion of living semiaquatic mammals Paleobiology 29429ndash454

Gingerich P D and D E Russell 1981 Pakicetus inachus a newarchaeocete (Mammalia Cetacea) from the early-middle EoceneKuldana Formation of Kohat (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 25235ndash246

Gingerich P D and D E Russell 1990 Dentition of early EocenePakicetus (Mammalia Cetacea) Contributions from the Museum ofPaleontology University of Michigan 281ndash20

Gingerich P D N A Wells D E Russell and S M I Shah 1983Origin of whales in epicontinental remnant seas new evidence fromthe early Eocene of Pakistan Science 220403ndash406

Gingerich P D S M Raza M Arif M Anwar and X Zhou 1994New whale from the Eocene of Pakistan and the origin of cetaceanswimming Nature 368844ndash847

Gingerich P D M Arif and W C Clyde 1995 New archaeocetes(Mammalia Cetacea) from the middle Eocene Domanda Formationof the Sulaiman Range Punjab (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 29291ndash330

Gingerich P D M ul-Haq I S Zalmout I H Khan and M S Mal-kani 2001 Origin of whales from early artiodactyls hands and feetof Eocene Protocetidae from Pakistan Science 2932239ndash2242

Hulbert R Jr 1998 Postcranial osteology of the North American

middle Eocene protocetid Georgiacetus pp 235ndash267 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Kellogg A R 1936 A review of the Archaeoceti Carnegie Institute ofWashington Publications 4821ndash366

Kumar K and A Sahni 1986 Remingtonocetus harudiensis new com-bination a middle Eocene archaeocete (Mammalia Cetacea) fromwestern Kutch India Journal of Vertebrate Paleontology 6326ndash349

Luo Z-X 1998 Homology and transformation of cetacean ectotym-panic structures pp 269ndash301 in J G M Thewissen (ed) The Emer-gence of Whales Plenum Press New York

Luo Z-X and P D Gingerich 1999 Terrestrial Mesonychia to aquaticCetacea transformation of the basicranium and evolution of hearingin whales University of Michigan Papers on Paleontology 311ndash98

Madar S I In press The postcranial skeleton of early Eocene pakicetidcetaceans Journal of Paleontology

Nummela S J G M Thewissen S Bajpai S T Hussain and K Ku-mar 2004 Eocene evolution of whale hearing Nature 430776ndash778

Sahni A and V P Mishra 1972 A new species of Protocetus (Cetacea)from the middle Eocene of Kutch Western India Palaeontology15490ndash495

Thewissen J G M and S T Hussain 1993 Origin of underwaterhearing in whales Nature 361444ndash445

Thewissen J G M and S T Hussain 1998 Systematic review of thePakicetidae early and middle Eocene Cetacea (Mammalia) fromPakistan and India Bulletin of Carnegie Museum of Natural History34220ndash238

Thewissen J G M and E M Williams 2002 The early radiations ofCetacea (Mammalia) evolutionary patterns and developmental cor-relations Annual Review of Ecology and Systematics 3373ndash90

Thewissen J G M S I Madar and S T Hussain 1996 Ambulocetusnatans an Eocene cetacean (Mammalia) from Pakistan CourierForschungsinstitut Senckenberg 191 1ndash86

Thewissen J G M E M Williams and S T Hussain 2001a Eocenemammal faunas from Northern Indo-Pakistan Journal of Verte-brate Paleontology 21347ndash366

Thewissen J G M E M Williams L J Roe and S T Hussain 2001bSkeletons of terrestrial cetaceans and the relationship of whales toartiodactyls Nature 413277ndash281

Thewissen J G M S T Hussain M Arif A Aslan S I Madar andL J Roe 1998 New localities of Eocene vertebrates in NorthernPakistan and their significance for the origin of the modern orders ofmammals pp 19ndash34 in M I Ghaznavi S M Raza and M T Hasan(eds) Siwaliks of South Asia Proceedings of the 3rd GEOSASWorkshop Islamabad Pakistan 1ndash5 March 1997 Geological Sur-vey of Pakistan Islamabad Pakistan

Uhen M D 1998 Middle to late Eocene basilosaurines and dorudon-tines pp 29ndash61 in J G M Thewissen (ed) The Emergence ofWhales Plenum Press New York

Uhen M D 2004 Form function and anatomy of Dorudon atrox(Mammalia Cetacea) an archaeocete from the middle to lateEocene of Egypt University of Michigan Papers on Paleontology341ndash222

West R M 1980 Middle Eocene large mammal assemblage withTethyan affinities Ganda Kas region Pakistan Journal of Paleon-tology 54508ndash533

West R M and J R Lukacs 1979 Geology and vertebrate-fossil lo-calities Tertiary continental rocks Kala Chitta Hills Attock Dis-trict Pakistan Milwaukee Public Museum Contributions in Biologyand Geology 261ndash20

Williams E M 1998 Synopsis of the earliest cetaceans PakicetidaeAmbulocetidae Remingtonocetidae and Protocetidae pp 1ndash28 inJ G M Thewissen (ed) The Emergence of Whales Plenum PressNew York

Submitted 7 October 2005 accepted 24 February 2006

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APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

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FIGURE 6 Pakicetid cranial anatomy A Pakicetus attocki cranium H-GSP 96623 ventral view B dorsal view C right lateral view D Pakicetusattocki H-GSP 96231 caudal view E Ichthyolestes pinfoldi H-GSP 98134 caudal view Scale bar equals 5 cm

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direction that the ophthalmic artery would have taken We in-terpret Kelloggrsquos lsquoforamen for the ophthalmic arteryrsquo as the an-terior opening for the sinus canal consistent with our interpre-tation of a similarly located foramen in pakicetids

PresphenoidmdashThe presphenoid is elongate and narrow inP attocki N ratimitus and I pinfoldi (H-GSP 96231 9638696623 98134 Figs 5AC 6A 7B) The ventral surface of the

presphenoid is rounded and smooth in Ichthyolestes whereasin Pakicetus and Nalacetus there is a distinct median crest ven-trally In Ichthyolestes much of the lateral side of the pre-sphenoid is covered by an anterior process of the alisphenoid(Fig 8A) A very small foramen occurs in this part of the pre-sphenoid and a groove extends anteriorly from this foramenWe interpret this foramen as the anterior foramen of the ptery-

FIGURE 7 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A dorsal view B ventral view C ventral view ear region in detail Dventral view outline of ear region Scale bar for AndashB equals 5 cm scale bar for CndashD equals 2 cm

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goid canal (Figs 8C 9) Anteriorly the lateral side of the pre-sphenoid reaches dorsal to touch the orbitosphenoid (Fig 8C)Ventrally the presphenoid is not fused to the basisphenoid in themidline and there is an ossification defect of 45 mm in Ich-thyolestes (Fig 7B)

AlisphenoidmdashThe alisphenoid shares sutures with pre-sphenoid orbitosphenoid parietal squamosal petrosal tym-panic basisphenoid and probably with pterygoid in P attockiN ratimitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Fig 8C) The anterior part of the alisphenoid is a rectan-gular process that lies lateral to much of the presphenoid andorbitosphenoid (H-GSP 98134 Fig 8A) and the sphenorbitalfissure is located just dorsal to this (Figs 6C 8C 9) Posteriorlythe alisphenoid forms a narrow wedge-like process betweensquamosal and basisphenoid in Pakicetus Nalacetus and Ich-thyolestes (H-GSP 30432 96231 96386 96623 98134 Figs 7D

8A) Part of this narrow process forms a prominently raised jointfacet the alisphenoid facet for the anterior process of the tym-panic (Figs 5A 6A 7D) This joint facet is continuous with asimilarly sized facet of the squamosal (Fig 5A 6A 7D) Thealisphenoid surrounds the entire oval foramen (Figs 5C 6A7D) There is no round foramen

The pterygoid process of the alisphenoid in pakicetids (H-GSP30432 96231 96386 96623 98134 Fig 7D) divides posteriorlyinto medial and lateral pterygoid laminae (Fig 7D) that sur-round a low pterygoid fossa (Fig 7D) This structure is compa-rable to the pterygoid fossa of primates and many other terres-trial mammals and should not be confused with the pterygoidsinus fossa of Basilosauridae Odontoceti and Mysticeti Themedial pterygoid lamina is low The pterygoid bone is not pre-served but was probably attached to the medial pterygoidlamina

FIGURE 9 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) right ventrolateral view showing the sphenorbital region with the foraminafrom an oblique angle The zygomatic arch is out of the focal plane of this photograph Scale bar equals 5 cm

FIGURE 8 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A right lateral view B sphenorbital region in detail C outline ofsphenorbital region Scale bar for A equals 5 cm scale bar for BndashC equals 2 cm

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BasisphenoidmdashThe basisphenoid shares sutures with pre-sphenoid alisphenoid and basioccipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Figs 5AC 6A 7B) The basisphenoid is convex vent-rally in cross section and concave in the median plane antero-posteriorly in juvenile Pakicetus and in Ichthyolestes (H-GSP96623 98134 respectively Figs 6A 7B) in adult Pakicetus(H-GSP 96231) and Nalacetus (H-GSP 96386) the basisphenoidis more or less flat in both these orientations (Figs 5AC) Pos-teriorly the suture between basisphenoid and basioccipital israised as bilateral tubercles in all pakicetid genera (H-GSP30432 96231 96386 98134 Figs 5AC 7B) One of the flexors ofthe neck rectus capitis ventralis originates from these tuberclesThere is no contact between the petrosal and the basisphenoidand the medial pterygoid sinus lies between these two bones(Fig 7D)

SquamosalmdashThe squamosal contacts the parietal alisphe-noid tympanic petrosal and occipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 98199 9625496386 96407 96623 98134 Figs 4B 5AB 6A 7AB 8A)making up most of the wall of the braincase Several smalltemporal foramina pierce the area near the occipito-squamosalsuture (Fig 6C) Posteriorly the squamosal forms part of thenuchal crest in all pakicetids (H-GSP 96231 96254 9638696623 Figs 4B 5B 6BD) Laterally the squamosal forms theposterior part of the zygomatic arch (Figs 4B 5AC 6BC 7AB8A 9) and the glenoid fossa (also called mandibular fossaFigs 5AC 6A 7Dglfs) in Pakicetus Nalacetus and Ichthyo-lestes (H-GSP 30432 96231 96386 96407 96623 98134) Theanterior part of the glenoid fossa is flat and extensive medio-laterally The postglenoid process (Figs 4A 5AC 6AC 7D8A) arches posteriorly into the external auditory meatus (Figs4A 5AC 6AC 7D 8A) The squamosal makes up the entireroof of the external auditory meatus and there is a prominentposttympanic process (Figs 4B 5B 6B 7A) The postglenoidforamen (also called retroarticular foramen Figs 5AC 6A 7D)is small and located posteromedial to the postglenoid processThe glenoid fossa extends medial to the postglenoid process andends posteromedially into a shallow groove directed to themiddle ear Medial to this is the squamosal contribution to theprominently raised joint facet (H-GSP 30432 96231 9638696623 98134 Figs 5A 6A 7D) for the anterior process of thetympanic The external auditory meatus ends medially in a re-cessed area in Pakicetus Nalacetus and Ichthyolestes (H-GSP30432 96231 96386 96407 96623 98134) The sharply depressedcenter of this recessed area represents the epitympanic recess(Figs 7D)

PetrosalmdashThe petrosal (periotic) contacts the alisphenoidsquamosal tympanic and occipital in P attocki N ratimitus andI pinfoldi (H-GSP 30432 96231 96386 96407 96431 9662398134 Fig 7D) The petrosal forms the roof of the middle earcavity The promontorium (Figs 5C 6A 7D) which houses theinner ear is a triangular structure on the ventral side The facialcanal runs anterior to the promontorium Medial to the prom-ontorium is the groove for the inferior petrosal sinus with thepiriform fenestra (H-GSP 30432 96386 96623 98134 Figs 5C7D) Rostrolateral to the promontorium is the tensor tympanifossa (H-GSP 30432 96231 96386 96407 96431 96623 98134Fig 7D) There are two foramina on the promontorium the ovalwindow for the stapes (fenestra vestibuli Fig 7D) and caudal toit the round window (fenestra cochleae Fig 7D) The mastoidprocess or posterior process of the petrosal flanks through thesquamosal posteriorly (H-GSP 30432 96231 96386 9640796623 98134 Figs 5AC 6A 7D 8A) A more detailed descrip-tion of the petrosal and tympanic of pakicetid cetaceans (Luoand Gingerich 1999) was based in part on material from H-GSPLocality 62

TympanicmdashThe tympanic (synonyms tympanic bulla audi-tory bulla ectotympanic) contacts the petrosal squamosal ali-sphenoid and exoccipital (GSP-UM 084) The tympanic sur-rounds the middle ear cavity ventrally laterally and medially(see Appendix 1 for all tympanics) The anterior part providesthe incisure for the eustachian tube (Fig 10) The medial pachy-osteosclerotic massive part of the tympanic is the involucrum(Fig 10) The involucrum is not in contact with any skull boneand is only connected to the rest of the skull by the dorsolateraland caudal parts of the tympanic The morphology described byGingerich and colleagues (1983) where the involucrum contactsthe basioccipital was based on the type specimen of P inachusGSP-UM 084 (sometimes listed as UM-GSP 081 by Luo 1998) inwhich this area is deformed

The shape and height of the anteromedial angle of the invo-lucrum (as defined by Luo 1998 Luo and Gingerich 1999Fig10) as well as the shape and orientation of the incisure forthe eustachian tube is species-specific (see Table 2) The lateralpart of the tympanic is a thin plate here the sigmoid process isa narrow and small plate of bone on the dorsal side of the lateralwall at least in I pinfoldi (H-GSP 92003) There is a small con-tact between the sigmoid process and the squamosal (see Luoand Gingerich 1999) Posterior to the sigmoid process the ex-ternal auditory meatus opens to the middle ear cavity Anteriorto the sigmoid process is the anterior process of the tympanicalso called processus tubarius which articulates with a facet onthe suture of squamosal and alisphenoid The posterior processof the tympanic contacts the mastoid (posterior) process of thepetrosal and more medially a small exoccipital facet makescontact with the tympanic (Fig 7D) this facet is the proximalpart of the paroccipital process of exoccipital (Figs 4B 5AC6ADE 7D)

There are four distinct morphologies among the tympanicsfrom H-GSP Locality 62 listed in Table 2 Three of these mor-phologies can be associated with cranial and dental material andthus be identified as P attocki I pinfoldi and N ratimitus Thefourth morphology represents a currently unidentified species ofthe family Pakicetidae Thewissen and Hussain (1998) describeda single tooth from Locality 62 tentatively as pertaining to Pinachus and it is possible that the unidentified bullae are fromthis species

In P attocki tympanics (see Appendix 1 and Table 2 Figs10AndashC) the morphology of the incisure for the eustachian tubeand of the anteromedial angle of the involucrum is similar to thatin the holotype of P inachus and therefore we refer these tym-panics to P attocki ndash which based on teeth (Thewissen andHussain 1998) is our most common species of Pakicetus Theorientation of the incisure for the eustachian tube cannot beobserved in GSP-UM 084 because the tympanic in this specimenis deformed and compressed against the basicranium

In the I pinfoldi specimen H-GSP 96431 (see Appendix 1 andTable 2 Figs 10DndashF) the fragmentary basicranium indicates asize considerably smaller than P attocki This is consistent withdental evidence (Thewissen and Hussain 1998) that indicatesthat the genus Ichthyolestes is smaller than Pakicetus We there-fore refer these specimens to I pinfoldi Although the adjoin-ing tympanic (H-GSP 96431) is the smallest one of our I pinfolditympanics two other tympanics with similar morphologyfor which the tympanic bullar length could be measured areboth within the length range of P attocki (Appendix 1 and Ta-ble 2)

The N ratimitus tympanics (see Appendix 1 and Table 2 Figs10GndashI) with a morphology clearly distinct from the two firstones described in Table 2 are larger than those of P attocki andmost of I pinfoldi and this serves to identify H-GSP 30325 and98140 Articular facets of Nalacetus tympanics (H-GSP 3032530459 96384 96385) match the facets on the exoccipital and

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squamosal of a Nalacetus skull (H-GSP 96386) The Nalacetusskull differs from Pakicetus (see previous) even though theseskulls are similar in size (Table 1) The Nalacetus skull is muchlarger than the Ichthyolestes skull (Table 1) This further cor-

roborates our identification of Nalacetus as a different genusand allocation of these tympanics to it

Three other remaining pakicetid tympanics (see Appendix 1and Table 2 Figs 10JndashL) are distinct from the three common

FIGURE 10 Pakicetid tympanics vertical columns showing the anterior lateral and dorsal views respectively AndashC Pakicetus attocki (H-GSP30386 left) DndashF Ichthyolestes pinfoldi (H-GSP 30223 left) GndashI Nalacetus ratimitus (H-GSP 96384 left) JndashL pakicetid n sp (H-GSP 98141 right)Scale bar equals 2 cm

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species of pakicetids at H-GSP Locality 62 and probably repre-sent a fourth new species of pakicetid These tympanics areclosest to P attocki tympanics in their size

OccipitalmdashThe most anterior point of the basioccipital of Pattocki N ratimitus and I pinfoldi (H-GSP 96231 96386 98134Figs 5AC 7B) is its suture with the basisphenoid which israised into bilateral tubercles (H-GSP 30432 96231 9638696623 98134 Figs 5AC 7B lsquobasioccipital crestrsquo of Luo 1998lsquoincipient falcate process of basioccipitalrsquo of Luo and Gingerich1999) These tubercles for neck flexors (see basisphenoid above)extend onto the anterior side of the basioccipital Posterior tothis the basioccipital projects laterally as a flat process in Paki-cetus (H-GSP 96231) and in Ichthyolestes (H-GSP 98134 Figs5A 7B) but this process is poorly defined in Nalacetus (H-GSP96386) The lateralmost part of this process is formed by theexoccipital (Figs 5AC 7B) and this process also forms the dor-solateral wall of the jugular foramen of pakicetids (H-GSP96231 96386 96407 96431 98134 Figs 5AC 7D) The exoc-cipitals bear the occipital condyles (H-GSP 96231 96386 9643198134 98199 Figs 4A 5AC 6DE 7B 8A 9) which are com-pressed dorsoventrally do not touch in the median plane andare somewhat raised above the level of the exoccipital The dor-sal and ventral condyloid fossae are of equal size (H-GSP 9623198134 98199 Figs 5AC 6E 7D) The hypoglossal foramen inall pakicetid genera (H-GSP 96231 96386 96407 96431 98134Figs 5AC 7D) is situated approximately halfway between thejugular foramen and the occipital condyle One or two smallcondylar foramina (Figs 5AC 7D) pierce the basioccipital lat-eral to the hypoglossal foramen The exoccipital forms the me-diolaterally elongate paroccipital process in all pakicetids (H-GSP 30432 96231 96386 96431 96623 98134 Figs 4B 5AC6ADE 7D) The medial extremity of this process bears a flatfacet for the tympanic (Fig 7D lsquobullar process of the exoccipitalrsquoof Luo and Gingerich 1999) The anterior part of the paroccipi-tal process shares a long suture with the mastoid process of pe-trosal The paroccipital process projects posterolaterally and thepart adjacent to the occipital condyle is recessed

The supraoccipital makes up most of the posterior side of thenuchal crest in all pakicetids (H-GSP 96231 96254 96386 96623Figs 4B 5B 6BD) although this structure is clearly worn insome specimens eg in I pinfoldi (H-GSP 98134) A small bi-lateral process of supraoccipital visible in Pakicetus and Ich-thyolestes (H-GSP 96231 98134 Figs 6DE) for the epaxial ex-tensors of the neck projects from the dorsal edge of the foramenmagnum (H-GSP 96231 96386 98134 Figs 6DE)Variable em-issary foramina (H-GSP 96231 96254 96386 96623 98134 Figs6DE) pierce the occipital dorsal to the foramen magnum Abilateral oval depression (H-GSP 96231 96386 98134 Figs6DE) occurs just lateral to the midline well above the foramenmagnum This depression could be for musculus longissimuscapitis The precise shape of the supraoccipital cannot be deter-mined because the suture between the exoccipital and the su-praoccipital is not visible

DISCUSSION

The pakicetid crania described here help to understand varia-tion in skull morphology of the earliest cetaceans As such theycan be expected to play a significant role in systematic studiesbut a detailed analysis of character scores for these specimensfalls outside the scope of the present description Here discus-sion will focus on a few aspects of cranial morphology that arerarely involved in cetacean systematics although they displayvariation to an extent that may be relevant systematically andinteresting evolutionarily

Supraorbital Morphology

The supraorbital region of protocetids basilosaurids anddorudontids displays a supraorbital shield (Kellogg 1936 Gin-

gerich et al 1994 1995 Bajpai and Gingerich 1998 Hulbert1998 Uhen 1998 2004) However the basal families of ceta-ceans lack a supraorbital shield pakicetids (Thewissen et al2001b) ambulocetids (Thewissen et al 1996) and remingtono-cetids (Gingerich et al 1994 1995 Bajpai and Thewissen 1998)The new pakicetid specimens indicate that there is much varia-tion in supraorbital morphology within these families In paki-cetids the orbits are convergent dorsally The area between theorbits is recessed over the superior orbital rim (Figs 4ndash8) Assuch pakicetid eyes are directed dorsolaterally There are dif-ferences between the genera (Fig 3 Table 1) in Pakicetus andIchthyolestes the supraorbital region is cuplike and cradles thedorsal part of the eye and the dorsal side of the supraorbitalregion is deeply incised in the medial plane in Pakicetus Thisincisure is less strong in Nalacetus and Ichthyolestes and in Nala-cetus the contact between supraorbital region and eye is toonarrow to be cup-shaped In Ambulocetus (Fig 3 Table 1Thewissen et al 1996fig 10 for H-GSP 18490) the orbits arepositioned near the dorsal side of the skull but they face later-ally leaving the supraorbital region more or less flat (in dorsalview) The eyes here appear to be directed laterally and are nothoused in the bony orbits as deeply as in pakicetids as indicatedby the shape of the postorbital processes As a comparison inRemingtonocetus the eyes are small are set laterally and facelaterally and the supraorbital region is strongly convex (Fig 3Sahni and Mishra 1972 Kumar and Sahni 1986 Bajpai andThewissen 1998) In protocetids the orbit is large faces later-ally and is covered by a large supraorbital shield (Fig 3 Table1) Dorudontids and basilosaurids resemble protocetids in thepresence and shape of their supraorbital shield as indicated bythe ratio between the minimum and maximum supraorbitalwidth (Table 1) and in the orientation of their orbits the latterare still clearly different from modern odontocetes and mystice-tes (Kellogg 1936 Uhen 1998 2004)

Intertemporal Morphology

In early and middle Eocene cetaceans a very narrow regioncan be seen in the skull posterior to the frontoparietal suturenear the sphenorbital fissure (Figs 1 2 4ndash8) This morphology isnot seen in most land mammals but is common in early ceta-ceans such as pakicetids ambulocetids (eg AmbulocetusThewissen et al 1996) and remingtonocetids (Kumar and Sahni1986) and may represent an important diagnostic character forthe order

Tympanic Bullar Morphology

The differences in the tympanic bullar morphology seen be-tween P attocki I pinfoldi N ratimitus and a fourth unnamedspecies can be used for specific recognition However in generalall these tympanics display a similar morphology indicating thatthese early archaeocetes used their peripheral ear structures likeland mammals while hearing in air and relied on bone conduc-tion mechanism while hearing in water (Thewissen and Hussain1993 Luo 1998 Luo and Gingerich 1999 Nummela et al 2004)

Pakicetus attocki is clearly larger than I pinfoldi whereas Pa-kicetus and Nalacetus have similar body sizes (Table 1) Thewis-sen and Hussain (1998) studied the size of upper and lower mo-lars in P attocki P inachus I pinfoldi and N ratimitus Theyfound Ichthyolestes to be the smallest P attocki and Nalacetus tobe of similar size and P inachus to be the largest Our studyshows that the absolute tympanic bullar size is similar in Paki-cetus and Ichthyolestes but larger in Nalacetus (Table 2 Appen-dix 1) This implies that in relative terms Pakicetus tympanicswhen related to body size (Table 2) are smaller than those ofIchthyolestes and Nalacetus

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ACKNOWLEDGMENTS

We thank Ellen Byron and Jeanette Killius for their technicalassistance Linda Spurlock prepared the drawings of the skullswhich is gratefully acknowledged Sunil Bajpai kindly gave usaccess to Remingtonocetus specimens in the IITR-SB collectionsunder his care We thank the editor Ewan Fordyce and Zhe-XiLuo and an anonymous reviewer for constructive suggestions forimprovement of the manuscript Financial support was providedby the National Science Foundation (current grant number EAR0207370) to JGMT and by the Ella and Georg Ehrnrooth Foun-dation to SN This article is a contribution to the Howard Uni-versity-Geological Survey of Pakistan projected entitled ldquoCeno-zoic Mammals of Pakistanrdquo We thank the Geological Survey ofPakistan for their support and help in the field

LITERATURE CITED

Aslan A and J G M Thewissen 1996 Preliminary evaluation of pa-leosols and implications for interpreting vertebrate fossil assem-blages Kuldana Formation Northern Pakistan Palaeovertebrata25261ndash277

Bajpai S and P D Gingerich 1998 A new Eocene archaeocete (Mam-malia Cetacea) from India and the time of origin of whales Pro-ceedings of the National Academy of Sciences USA 9515464ndash15468

Bajpai S and J G M Thewissen 1998 Middle Eocene cetaceans fromthe Harudi and Subathu formations of India pp 213ndash233 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Dehm R and T zu Oettingen-Spielberg 1958 Palaumlontologische undgeologische Untersuchungen im Tertiaumlr von Pakistan 2 Die mitte-leocaumlnen Saumlugetiere von Ganda Kas bei Basal in Nordwest-Pakistan Bayerische Akademie der Wissenschaften 911ndash54

Geisler J H and Z-X Luo 1998 Relationships of Cetacea to terres-trial ungulates and the evolution of cranial vasculature in Cete pp163ndash212 in J G M Thewissen (ed) The Emergence of WhalesPlenum Press New York

Geisler J H A E Sanders and Z-X Luo 2005 A new protocetidwhale (Cetacea Archaeoceti) from the late Middle Eocene of SouthCarolina American Museum Novitates 34801ndash65

Gingerich P D 2003 Land-to-sea transition in early whales evolutionof Eocene Archaeoceti (Cetacea) in relation to skeletal proportionsand locomotion of living semiaquatic mammals Paleobiology 29429ndash454

Gingerich P D and D E Russell 1981 Pakicetus inachus a newarchaeocete (Mammalia Cetacea) from the early-middle EoceneKuldana Formation of Kohat (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 25235ndash246

Gingerich P D and D E Russell 1990 Dentition of early EocenePakicetus (Mammalia Cetacea) Contributions from the Museum ofPaleontology University of Michigan 281ndash20

Gingerich P D N A Wells D E Russell and S M I Shah 1983Origin of whales in epicontinental remnant seas new evidence fromthe early Eocene of Pakistan Science 220403ndash406

Gingerich P D S M Raza M Arif M Anwar and X Zhou 1994New whale from the Eocene of Pakistan and the origin of cetaceanswimming Nature 368844ndash847

Gingerich P D M Arif and W C Clyde 1995 New archaeocetes(Mammalia Cetacea) from the middle Eocene Domanda Formationof the Sulaiman Range Punjab (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 29291ndash330

Gingerich P D M ul-Haq I S Zalmout I H Khan and M S Mal-kani 2001 Origin of whales from early artiodactyls hands and feetof Eocene Protocetidae from Pakistan Science 2932239ndash2242

Hulbert R Jr 1998 Postcranial osteology of the North American

middle Eocene protocetid Georgiacetus pp 235ndash267 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Kellogg A R 1936 A review of the Archaeoceti Carnegie Institute ofWashington Publications 4821ndash366

Kumar K and A Sahni 1986 Remingtonocetus harudiensis new com-bination a middle Eocene archaeocete (Mammalia Cetacea) fromwestern Kutch India Journal of Vertebrate Paleontology 6326ndash349

Luo Z-X 1998 Homology and transformation of cetacean ectotym-panic structures pp 269ndash301 in J G M Thewissen (ed) The Emer-gence of Whales Plenum Press New York

Luo Z-X and P D Gingerich 1999 Terrestrial Mesonychia to aquaticCetacea transformation of the basicranium and evolution of hearingin whales University of Michigan Papers on Paleontology 311ndash98

Madar S I In press The postcranial skeleton of early Eocene pakicetidcetaceans Journal of Paleontology

Nummela S J G M Thewissen S Bajpai S T Hussain and K Ku-mar 2004 Eocene evolution of whale hearing Nature 430776ndash778

Sahni A and V P Mishra 1972 A new species of Protocetus (Cetacea)from the middle Eocene of Kutch Western India Palaeontology15490ndash495

Thewissen J G M and S T Hussain 1993 Origin of underwaterhearing in whales Nature 361444ndash445

Thewissen J G M and S T Hussain 1998 Systematic review of thePakicetidae early and middle Eocene Cetacea (Mammalia) fromPakistan and India Bulletin of Carnegie Museum of Natural History34220ndash238

Thewissen J G M and E M Williams 2002 The early radiations ofCetacea (Mammalia) evolutionary patterns and developmental cor-relations Annual Review of Ecology and Systematics 3373ndash90

Thewissen J G M S I Madar and S T Hussain 1996 Ambulocetusnatans an Eocene cetacean (Mammalia) from Pakistan CourierForschungsinstitut Senckenberg 191 1ndash86

Thewissen J G M E M Williams and S T Hussain 2001a Eocenemammal faunas from Northern Indo-Pakistan Journal of Verte-brate Paleontology 21347ndash366

Thewissen J G M E M Williams L J Roe and S T Hussain 2001bSkeletons of terrestrial cetaceans and the relationship of whales toartiodactyls Nature 413277ndash281

Thewissen J G M S T Hussain M Arif A Aslan S I Madar andL J Roe 1998 New localities of Eocene vertebrates in NorthernPakistan and their significance for the origin of the modern orders ofmammals pp 19ndash34 in M I Ghaznavi S M Raza and M T Hasan(eds) Siwaliks of South Asia Proceedings of the 3rd GEOSASWorkshop Islamabad Pakistan 1ndash5 March 1997 Geological Sur-vey of Pakistan Islamabad Pakistan

Uhen M D 1998 Middle to late Eocene basilosaurines and dorudon-tines pp 29ndash61 in J G M Thewissen (ed) The Emergence ofWhales Plenum Press New York

Uhen M D 2004 Form function and anatomy of Dorudon atrox(Mammalia Cetacea) an archaeocete from the middle to lateEocene of Egypt University of Michigan Papers on Paleontology341ndash222

West R M 1980 Middle Eocene large mammal assemblage withTethyan affinities Ganda Kas region Pakistan Journal of Paleon-tology 54508ndash533

West R M and J R Lukacs 1979 Geology and vertebrate-fossil lo-calities Tertiary continental rocks Kala Chitta Hills Attock Dis-trict Pakistan Milwaukee Public Museum Contributions in Biologyand Geology 261ndash20

Williams E M 1998 Synopsis of the earliest cetaceans PakicetidaeAmbulocetidae Remingtonocetidae and Protocetidae pp 1ndash28 inJ G M Thewissen (ed) The Emergence of Whales Plenum PressNew York

Submitted 7 October 2005 accepted 24 February 2006

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APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

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direction that the ophthalmic artery would have taken We in-terpret Kelloggrsquos lsquoforamen for the ophthalmic arteryrsquo as the an-terior opening for the sinus canal consistent with our interpre-tation of a similarly located foramen in pakicetids

PresphenoidmdashThe presphenoid is elongate and narrow inP attocki N ratimitus and I pinfoldi (H-GSP 96231 9638696623 98134 Figs 5AC 6A 7B) The ventral surface of the

presphenoid is rounded and smooth in Ichthyolestes whereasin Pakicetus and Nalacetus there is a distinct median crest ven-trally In Ichthyolestes much of the lateral side of the pre-sphenoid is covered by an anterior process of the alisphenoid(Fig 8A) A very small foramen occurs in this part of the pre-sphenoid and a groove extends anteriorly from this foramenWe interpret this foramen as the anterior foramen of the ptery-

FIGURE 7 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A dorsal view B ventral view C ventral view ear region in detail Dventral view outline of ear region Scale bar for AndashB equals 5 cm scale bar for CndashD equals 2 cm

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goid canal (Figs 8C 9) Anteriorly the lateral side of the pre-sphenoid reaches dorsal to touch the orbitosphenoid (Fig 8C)Ventrally the presphenoid is not fused to the basisphenoid in themidline and there is an ossification defect of 45 mm in Ich-thyolestes (Fig 7B)

AlisphenoidmdashThe alisphenoid shares sutures with pre-sphenoid orbitosphenoid parietal squamosal petrosal tym-panic basisphenoid and probably with pterygoid in P attockiN ratimitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Fig 8C) The anterior part of the alisphenoid is a rectan-gular process that lies lateral to much of the presphenoid andorbitosphenoid (H-GSP 98134 Fig 8A) and the sphenorbitalfissure is located just dorsal to this (Figs 6C 8C 9) Posteriorlythe alisphenoid forms a narrow wedge-like process betweensquamosal and basisphenoid in Pakicetus Nalacetus and Ich-thyolestes (H-GSP 30432 96231 96386 96623 98134 Figs 7D

8A) Part of this narrow process forms a prominently raised jointfacet the alisphenoid facet for the anterior process of the tym-panic (Figs 5A 6A 7D) This joint facet is continuous with asimilarly sized facet of the squamosal (Fig 5A 6A 7D) Thealisphenoid surrounds the entire oval foramen (Figs 5C 6A7D) There is no round foramen

The pterygoid process of the alisphenoid in pakicetids (H-GSP30432 96231 96386 96623 98134 Fig 7D) divides posteriorlyinto medial and lateral pterygoid laminae (Fig 7D) that sur-round a low pterygoid fossa (Fig 7D) This structure is compa-rable to the pterygoid fossa of primates and many other terres-trial mammals and should not be confused with the pterygoidsinus fossa of Basilosauridae Odontoceti and Mysticeti Themedial pterygoid lamina is low The pterygoid bone is not pre-served but was probably attached to the medial pterygoidlamina

FIGURE 9 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) right ventrolateral view showing the sphenorbital region with the foraminafrom an oblique angle The zygomatic arch is out of the focal plane of this photograph Scale bar equals 5 cm

FIGURE 8 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A right lateral view B sphenorbital region in detail C outline ofsphenorbital region Scale bar for A equals 5 cm scale bar for BndashC equals 2 cm

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BasisphenoidmdashThe basisphenoid shares sutures with pre-sphenoid alisphenoid and basioccipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Figs 5AC 6A 7B) The basisphenoid is convex vent-rally in cross section and concave in the median plane antero-posteriorly in juvenile Pakicetus and in Ichthyolestes (H-GSP96623 98134 respectively Figs 6A 7B) in adult Pakicetus(H-GSP 96231) and Nalacetus (H-GSP 96386) the basisphenoidis more or less flat in both these orientations (Figs 5AC) Pos-teriorly the suture between basisphenoid and basioccipital israised as bilateral tubercles in all pakicetid genera (H-GSP30432 96231 96386 98134 Figs 5AC 7B) One of the flexors ofthe neck rectus capitis ventralis originates from these tuberclesThere is no contact between the petrosal and the basisphenoidand the medial pterygoid sinus lies between these two bones(Fig 7D)

SquamosalmdashThe squamosal contacts the parietal alisphe-noid tympanic petrosal and occipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 98199 9625496386 96407 96623 98134 Figs 4B 5AB 6A 7AB 8A)making up most of the wall of the braincase Several smalltemporal foramina pierce the area near the occipito-squamosalsuture (Fig 6C) Posteriorly the squamosal forms part of thenuchal crest in all pakicetids (H-GSP 96231 96254 9638696623 Figs 4B 5B 6BD) Laterally the squamosal forms theposterior part of the zygomatic arch (Figs 4B 5AC 6BC 7AB8A 9) and the glenoid fossa (also called mandibular fossaFigs 5AC 6A 7Dglfs) in Pakicetus Nalacetus and Ichthyo-lestes (H-GSP 30432 96231 96386 96407 96623 98134) Theanterior part of the glenoid fossa is flat and extensive medio-laterally The postglenoid process (Figs 4A 5AC 6AC 7D8A) arches posteriorly into the external auditory meatus (Figs4A 5AC 6AC 7D 8A) The squamosal makes up the entireroof of the external auditory meatus and there is a prominentposttympanic process (Figs 4B 5B 6B 7A) The postglenoidforamen (also called retroarticular foramen Figs 5AC 6A 7D)is small and located posteromedial to the postglenoid processThe glenoid fossa extends medial to the postglenoid process andends posteromedially into a shallow groove directed to themiddle ear Medial to this is the squamosal contribution to theprominently raised joint facet (H-GSP 30432 96231 9638696623 98134 Figs 5A 6A 7D) for the anterior process of thetympanic The external auditory meatus ends medially in a re-cessed area in Pakicetus Nalacetus and Ichthyolestes (H-GSP30432 96231 96386 96407 96623 98134) The sharply depressedcenter of this recessed area represents the epitympanic recess(Figs 7D)

PetrosalmdashThe petrosal (periotic) contacts the alisphenoidsquamosal tympanic and occipital in P attocki N ratimitus andI pinfoldi (H-GSP 30432 96231 96386 96407 96431 9662398134 Fig 7D) The petrosal forms the roof of the middle earcavity The promontorium (Figs 5C 6A 7D) which houses theinner ear is a triangular structure on the ventral side The facialcanal runs anterior to the promontorium Medial to the prom-ontorium is the groove for the inferior petrosal sinus with thepiriform fenestra (H-GSP 30432 96386 96623 98134 Figs 5C7D) Rostrolateral to the promontorium is the tensor tympanifossa (H-GSP 30432 96231 96386 96407 96431 96623 98134Fig 7D) There are two foramina on the promontorium the ovalwindow for the stapes (fenestra vestibuli Fig 7D) and caudal toit the round window (fenestra cochleae Fig 7D) The mastoidprocess or posterior process of the petrosal flanks through thesquamosal posteriorly (H-GSP 30432 96231 96386 9640796623 98134 Figs 5AC 6A 7D 8A) A more detailed descrip-tion of the petrosal and tympanic of pakicetid cetaceans (Luoand Gingerich 1999) was based in part on material from H-GSPLocality 62

TympanicmdashThe tympanic (synonyms tympanic bulla audi-tory bulla ectotympanic) contacts the petrosal squamosal ali-sphenoid and exoccipital (GSP-UM 084) The tympanic sur-rounds the middle ear cavity ventrally laterally and medially(see Appendix 1 for all tympanics) The anterior part providesthe incisure for the eustachian tube (Fig 10) The medial pachy-osteosclerotic massive part of the tympanic is the involucrum(Fig 10) The involucrum is not in contact with any skull boneand is only connected to the rest of the skull by the dorsolateraland caudal parts of the tympanic The morphology described byGingerich and colleagues (1983) where the involucrum contactsthe basioccipital was based on the type specimen of P inachusGSP-UM 084 (sometimes listed as UM-GSP 081 by Luo 1998) inwhich this area is deformed

The shape and height of the anteromedial angle of the invo-lucrum (as defined by Luo 1998 Luo and Gingerich 1999Fig10) as well as the shape and orientation of the incisure forthe eustachian tube is species-specific (see Table 2) The lateralpart of the tympanic is a thin plate here the sigmoid process isa narrow and small plate of bone on the dorsal side of the lateralwall at least in I pinfoldi (H-GSP 92003) There is a small con-tact between the sigmoid process and the squamosal (see Luoand Gingerich 1999) Posterior to the sigmoid process the ex-ternal auditory meatus opens to the middle ear cavity Anteriorto the sigmoid process is the anterior process of the tympanicalso called processus tubarius which articulates with a facet onthe suture of squamosal and alisphenoid The posterior processof the tympanic contacts the mastoid (posterior) process of thepetrosal and more medially a small exoccipital facet makescontact with the tympanic (Fig 7D) this facet is the proximalpart of the paroccipital process of exoccipital (Figs 4B 5AC6ADE 7D)

There are four distinct morphologies among the tympanicsfrom H-GSP Locality 62 listed in Table 2 Three of these mor-phologies can be associated with cranial and dental material andthus be identified as P attocki I pinfoldi and N ratimitus Thefourth morphology represents a currently unidentified species ofthe family Pakicetidae Thewissen and Hussain (1998) describeda single tooth from Locality 62 tentatively as pertaining to Pinachus and it is possible that the unidentified bullae are fromthis species

In P attocki tympanics (see Appendix 1 and Table 2 Figs10AndashC) the morphology of the incisure for the eustachian tubeand of the anteromedial angle of the involucrum is similar to thatin the holotype of P inachus and therefore we refer these tym-panics to P attocki ndash which based on teeth (Thewissen andHussain 1998) is our most common species of Pakicetus Theorientation of the incisure for the eustachian tube cannot beobserved in GSP-UM 084 because the tympanic in this specimenis deformed and compressed against the basicranium

In the I pinfoldi specimen H-GSP 96431 (see Appendix 1 andTable 2 Figs 10DndashF) the fragmentary basicranium indicates asize considerably smaller than P attocki This is consistent withdental evidence (Thewissen and Hussain 1998) that indicatesthat the genus Ichthyolestes is smaller than Pakicetus We there-fore refer these specimens to I pinfoldi Although the adjoin-ing tympanic (H-GSP 96431) is the smallest one of our I pinfolditympanics two other tympanics with similar morphologyfor which the tympanic bullar length could be measured areboth within the length range of P attocki (Appendix 1 and Ta-ble 2)

The N ratimitus tympanics (see Appendix 1 and Table 2 Figs10GndashI) with a morphology clearly distinct from the two firstones described in Table 2 are larger than those of P attocki andmost of I pinfoldi and this serves to identify H-GSP 30325 and98140 Articular facets of Nalacetus tympanics (H-GSP 3032530459 96384 96385) match the facets on the exoccipital and

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 755

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squamosal of a Nalacetus skull (H-GSP 96386) The Nalacetusskull differs from Pakicetus (see previous) even though theseskulls are similar in size (Table 1) The Nalacetus skull is muchlarger than the Ichthyolestes skull (Table 1) This further cor-

roborates our identification of Nalacetus as a different genusand allocation of these tympanics to it

Three other remaining pakicetid tympanics (see Appendix 1and Table 2 Figs 10JndashL) are distinct from the three common

FIGURE 10 Pakicetid tympanics vertical columns showing the anterior lateral and dorsal views respectively AndashC Pakicetus attocki (H-GSP30386 left) DndashF Ichthyolestes pinfoldi (H-GSP 30223 left) GndashI Nalacetus ratimitus (H-GSP 96384 left) JndashL pakicetid n sp (H-GSP 98141 right)Scale bar equals 2 cm

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species of pakicetids at H-GSP Locality 62 and probably repre-sent a fourth new species of pakicetid These tympanics areclosest to P attocki tympanics in their size

OccipitalmdashThe most anterior point of the basioccipital of Pattocki N ratimitus and I pinfoldi (H-GSP 96231 96386 98134Figs 5AC 7B) is its suture with the basisphenoid which israised into bilateral tubercles (H-GSP 30432 96231 9638696623 98134 Figs 5AC 7B lsquobasioccipital crestrsquo of Luo 1998lsquoincipient falcate process of basioccipitalrsquo of Luo and Gingerich1999) These tubercles for neck flexors (see basisphenoid above)extend onto the anterior side of the basioccipital Posterior tothis the basioccipital projects laterally as a flat process in Paki-cetus (H-GSP 96231) and in Ichthyolestes (H-GSP 98134 Figs5A 7B) but this process is poorly defined in Nalacetus (H-GSP96386) The lateralmost part of this process is formed by theexoccipital (Figs 5AC 7B) and this process also forms the dor-solateral wall of the jugular foramen of pakicetids (H-GSP96231 96386 96407 96431 98134 Figs 5AC 7D) The exoc-cipitals bear the occipital condyles (H-GSP 96231 96386 9643198134 98199 Figs 4A 5AC 6DE 7B 8A 9) which are com-pressed dorsoventrally do not touch in the median plane andare somewhat raised above the level of the exoccipital The dor-sal and ventral condyloid fossae are of equal size (H-GSP 9623198134 98199 Figs 5AC 6E 7D) The hypoglossal foramen inall pakicetid genera (H-GSP 96231 96386 96407 96431 98134Figs 5AC 7D) is situated approximately halfway between thejugular foramen and the occipital condyle One or two smallcondylar foramina (Figs 5AC 7D) pierce the basioccipital lat-eral to the hypoglossal foramen The exoccipital forms the me-diolaterally elongate paroccipital process in all pakicetids (H-GSP 30432 96231 96386 96431 96623 98134 Figs 4B 5AC6ADE 7D) The medial extremity of this process bears a flatfacet for the tympanic (Fig 7D lsquobullar process of the exoccipitalrsquoof Luo and Gingerich 1999) The anterior part of the paroccipi-tal process shares a long suture with the mastoid process of pe-trosal The paroccipital process projects posterolaterally and thepart adjacent to the occipital condyle is recessed

The supraoccipital makes up most of the posterior side of thenuchal crest in all pakicetids (H-GSP 96231 96254 96386 96623Figs 4B 5B 6BD) although this structure is clearly worn insome specimens eg in I pinfoldi (H-GSP 98134) A small bi-lateral process of supraoccipital visible in Pakicetus and Ich-thyolestes (H-GSP 96231 98134 Figs 6DE) for the epaxial ex-tensors of the neck projects from the dorsal edge of the foramenmagnum (H-GSP 96231 96386 98134 Figs 6DE)Variable em-issary foramina (H-GSP 96231 96254 96386 96623 98134 Figs6DE) pierce the occipital dorsal to the foramen magnum Abilateral oval depression (H-GSP 96231 96386 98134 Figs6DE) occurs just lateral to the midline well above the foramenmagnum This depression could be for musculus longissimuscapitis The precise shape of the supraoccipital cannot be deter-mined because the suture between the exoccipital and the su-praoccipital is not visible

DISCUSSION

The pakicetid crania described here help to understand varia-tion in skull morphology of the earliest cetaceans As such theycan be expected to play a significant role in systematic studiesbut a detailed analysis of character scores for these specimensfalls outside the scope of the present description Here discus-sion will focus on a few aspects of cranial morphology that arerarely involved in cetacean systematics although they displayvariation to an extent that may be relevant systematically andinteresting evolutionarily

Supraorbital Morphology

The supraorbital region of protocetids basilosaurids anddorudontids displays a supraorbital shield (Kellogg 1936 Gin-

gerich et al 1994 1995 Bajpai and Gingerich 1998 Hulbert1998 Uhen 1998 2004) However the basal families of ceta-ceans lack a supraorbital shield pakicetids (Thewissen et al2001b) ambulocetids (Thewissen et al 1996) and remingtono-cetids (Gingerich et al 1994 1995 Bajpai and Thewissen 1998)The new pakicetid specimens indicate that there is much varia-tion in supraorbital morphology within these families In paki-cetids the orbits are convergent dorsally The area between theorbits is recessed over the superior orbital rim (Figs 4ndash8) Assuch pakicetid eyes are directed dorsolaterally There are dif-ferences between the genera (Fig 3 Table 1) in Pakicetus andIchthyolestes the supraorbital region is cuplike and cradles thedorsal part of the eye and the dorsal side of the supraorbitalregion is deeply incised in the medial plane in Pakicetus Thisincisure is less strong in Nalacetus and Ichthyolestes and in Nala-cetus the contact between supraorbital region and eye is toonarrow to be cup-shaped In Ambulocetus (Fig 3 Table 1Thewissen et al 1996fig 10 for H-GSP 18490) the orbits arepositioned near the dorsal side of the skull but they face later-ally leaving the supraorbital region more or less flat (in dorsalview) The eyes here appear to be directed laterally and are nothoused in the bony orbits as deeply as in pakicetids as indicatedby the shape of the postorbital processes As a comparison inRemingtonocetus the eyes are small are set laterally and facelaterally and the supraorbital region is strongly convex (Fig 3Sahni and Mishra 1972 Kumar and Sahni 1986 Bajpai andThewissen 1998) In protocetids the orbit is large faces later-ally and is covered by a large supraorbital shield (Fig 3 Table1) Dorudontids and basilosaurids resemble protocetids in thepresence and shape of their supraorbital shield as indicated bythe ratio between the minimum and maximum supraorbitalwidth (Table 1) and in the orientation of their orbits the latterare still clearly different from modern odontocetes and mystice-tes (Kellogg 1936 Uhen 1998 2004)

Intertemporal Morphology

In early and middle Eocene cetaceans a very narrow regioncan be seen in the skull posterior to the frontoparietal suturenear the sphenorbital fissure (Figs 1 2 4ndash8) This morphology isnot seen in most land mammals but is common in early ceta-ceans such as pakicetids ambulocetids (eg AmbulocetusThewissen et al 1996) and remingtonocetids (Kumar and Sahni1986) and may represent an important diagnostic character forthe order

Tympanic Bullar Morphology

The differences in the tympanic bullar morphology seen be-tween P attocki I pinfoldi N ratimitus and a fourth unnamedspecies can be used for specific recognition However in generalall these tympanics display a similar morphology indicating thatthese early archaeocetes used their peripheral ear structures likeland mammals while hearing in air and relied on bone conduc-tion mechanism while hearing in water (Thewissen and Hussain1993 Luo 1998 Luo and Gingerich 1999 Nummela et al 2004)

Pakicetus attocki is clearly larger than I pinfoldi whereas Pa-kicetus and Nalacetus have similar body sizes (Table 1) Thewis-sen and Hussain (1998) studied the size of upper and lower mo-lars in P attocki P inachus I pinfoldi and N ratimitus Theyfound Ichthyolestes to be the smallest P attocki and Nalacetus tobe of similar size and P inachus to be the largest Our studyshows that the absolute tympanic bullar size is similar in Paki-cetus and Ichthyolestes but larger in Nalacetus (Table 2 Appen-dix 1) This implies that in relative terms Pakicetus tympanicswhen related to body size (Table 2) are smaller than those ofIchthyolestes and Nalacetus

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 757

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ACKNOWLEDGMENTS

We thank Ellen Byron and Jeanette Killius for their technicalassistance Linda Spurlock prepared the drawings of the skullswhich is gratefully acknowledged Sunil Bajpai kindly gave usaccess to Remingtonocetus specimens in the IITR-SB collectionsunder his care We thank the editor Ewan Fordyce and Zhe-XiLuo and an anonymous reviewer for constructive suggestions forimprovement of the manuscript Financial support was providedby the National Science Foundation (current grant number EAR0207370) to JGMT and by the Ella and Georg Ehrnrooth Foun-dation to SN This article is a contribution to the Howard Uni-versity-Geological Survey of Pakistan projected entitled ldquoCeno-zoic Mammals of Pakistanrdquo We thank the Geological Survey ofPakistan for their support and help in the field

LITERATURE CITED

Aslan A and J G M Thewissen 1996 Preliminary evaluation of pa-leosols and implications for interpreting vertebrate fossil assem-blages Kuldana Formation Northern Pakistan Palaeovertebrata25261ndash277

Bajpai S and P D Gingerich 1998 A new Eocene archaeocete (Mam-malia Cetacea) from India and the time of origin of whales Pro-ceedings of the National Academy of Sciences USA 9515464ndash15468

Bajpai S and J G M Thewissen 1998 Middle Eocene cetaceans fromthe Harudi and Subathu formations of India pp 213ndash233 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Dehm R and T zu Oettingen-Spielberg 1958 Palaumlontologische undgeologische Untersuchungen im Tertiaumlr von Pakistan 2 Die mitte-leocaumlnen Saumlugetiere von Ganda Kas bei Basal in Nordwest-Pakistan Bayerische Akademie der Wissenschaften 911ndash54

Geisler J H and Z-X Luo 1998 Relationships of Cetacea to terres-trial ungulates and the evolution of cranial vasculature in Cete pp163ndash212 in J G M Thewissen (ed) The Emergence of WhalesPlenum Press New York

Geisler J H A E Sanders and Z-X Luo 2005 A new protocetidwhale (Cetacea Archaeoceti) from the late Middle Eocene of SouthCarolina American Museum Novitates 34801ndash65

Gingerich P D 2003 Land-to-sea transition in early whales evolutionof Eocene Archaeoceti (Cetacea) in relation to skeletal proportionsand locomotion of living semiaquatic mammals Paleobiology 29429ndash454

Gingerich P D and D E Russell 1981 Pakicetus inachus a newarchaeocete (Mammalia Cetacea) from the early-middle EoceneKuldana Formation of Kohat (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 25235ndash246

Gingerich P D and D E Russell 1990 Dentition of early EocenePakicetus (Mammalia Cetacea) Contributions from the Museum ofPaleontology University of Michigan 281ndash20

Gingerich P D N A Wells D E Russell and S M I Shah 1983Origin of whales in epicontinental remnant seas new evidence fromthe early Eocene of Pakistan Science 220403ndash406

Gingerich P D S M Raza M Arif M Anwar and X Zhou 1994New whale from the Eocene of Pakistan and the origin of cetaceanswimming Nature 368844ndash847

Gingerich P D M Arif and W C Clyde 1995 New archaeocetes(Mammalia Cetacea) from the middle Eocene Domanda Formationof the Sulaiman Range Punjab (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 29291ndash330

Gingerich P D M ul-Haq I S Zalmout I H Khan and M S Mal-kani 2001 Origin of whales from early artiodactyls hands and feetof Eocene Protocetidae from Pakistan Science 2932239ndash2242

Hulbert R Jr 1998 Postcranial osteology of the North American

middle Eocene protocetid Georgiacetus pp 235ndash267 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Kellogg A R 1936 A review of the Archaeoceti Carnegie Institute ofWashington Publications 4821ndash366

Kumar K and A Sahni 1986 Remingtonocetus harudiensis new com-bination a middle Eocene archaeocete (Mammalia Cetacea) fromwestern Kutch India Journal of Vertebrate Paleontology 6326ndash349

Luo Z-X 1998 Homology and transformation of cetacean ectotym-panic structures pp 269ndash301 in J G M Thewissen (ed) The Emer-gence of Whales Plenum Press New York

Luo Z-X and P D Gingerich 1999 Terrestrial Mesonychia to aquaticCetacea transformation of the basicranium and evolution of hearingin whales University of Michigan Papers on Paleontology 311ndash98

Madar S I In press The postcranial skeleton of early Eocene pakicetidcetaceans Journal of Paleontology

Nummela S J G M Thewissen S Bajpai S T Hussain and K Ku-mar 2004 Eocene evolution of whale hearing Nature 430776ndash778

Sahni A and V P Mishra 1972 A new species of Protocetus (Cetacea)from the middle Eocene of Kutch Western India Palaeontology15490ndash495

Thewissen J G M and S T Hussain 1993 Origin of underwaterhearing in whales Nature 361444ndash445

Thewissen J G M and S T Hussain 1998 Systematic review of thePakicetidae early and middle Eocene Cetacea (Mammalia) fromPakistan and India Bulletin of Carnegie Museum of Natural History34220ndash238

Thewissen J G M and E M Williams 2002 The early radiations ofCetacea (Mammalia) evolutionary patterns and developmental cor-relations Annual Review of Ecology and Systematics 3373ndash90

Thewissen J G M S I Madar and S T Hussain 1996 Ambulocetusnatans an Eocene cetacean (Mammalia) from Pakistan CourierForschungsinstitut Senckenberg 191 1ndash86

Thewissen J G M E M Williams and S T Hussain 2001a Eocenemammal faunas from Northern Indo-Pakistan Journal of Verte-brate Paleontology 21347ndash366

Thewissen J G M E M Williams L J Roe and S T Hussain 2001bSkeletons of terrestrial cetaceans and the relationship of whales toartiodactyls Nature 413277ndash281

Thewissen J G M S T Hussain M Arif A Aslan S I Madar andL J Roe 1998 New localities of Eocene vertebrates in NorthernPakistan and their significance for the origin of the modern orders ofmammals pp 19ndash34 in M I Ghaznavi S M Raza and M T Hasan(eds) Siwaliks of South Asia Proceedings of the 3rd GEOSASWorkshop Islamabad Pakistan 1ndash5 March 1997 Geological Sur-vey of Pakistan Islamabad Pakistan

Uhen M D 1998 Middle to late Eocene basilosaurines and dorudon-tines pp 29ndash61 in J G M Thewissen (ed) The Emergence ofWhales Plenum Press New York

Uhen M D 2004 Form function and anatomy of Dorudon atrox(Mammalia Cetacea) an archaeocete from the middle to lateEocene of Egypt University of Michigan Papers on Paleontology341ndash222

West R M 1980 Middle Eocene large mammal assemblage withTethyan affinities Ganda Kas region Pakistan Journal of Paleon-tology 54508ndash533

West R M and J R Lukacs 1979 Geology and vertebrate-fossil lo-calities Tertiary continental rocks Kala Chitta Hills Attock Dis-trict Pakistan Milwaukee Public Museum Contributions in Biologyand Geology 261ndash20

Williams E M 1998 Synopsis of the earliest cetaceans PakicetidaeAmbulocetidae Remingtonocetidae and Protocetidae pp 1ndash28 inJ G M Thewissen (ed) The Emergence of Whales Plenum PressNew York

Submitted 7 October 2005 accepted 24 February 2006

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APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 759

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goid canal (Figs 8C 9) Anteriorly the lateral side of the pre-sphenoid reaches dorsal to touch the orbitosphenoid (Fig 8C)Ventrally the presphenoid is not fused to the basisphenoid in themidline and there is an ossification defect of 45 mm in Ich-thyolestes (Fig 7B)

AlisphenoidmdashThe alisphenoid shares sutures with pre-sphenoid orbitosphenoid parietal squamosal petrosal tym-panic basisphenoid and probably with pterygoid in P attockiN ratimitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Fig 8C) The anterior part of the alisphenoid is a rectan-gular process that lies lateral to much of the presphenoid andorbitosphenoid (H-GSP 98134 Fig 8A) and the sphenorbitalfissure is located just dorsal to this (Figs 6C 8C 9) Posteriorlythe alisphenoid forms a narrow wedge-like process betweensquamosal and basisphenoid in Pakicetus Nalacetus and Ich-thyolestes (H-GSP 30432 96231 96386 96623 98134 Figs 7D

8A) Part of this narrow process forms a prominently raised jointfacet the alisphenoid facet for the anterior process of the tym-panic (Figs 5A 6A 7D) This joint facet is continuous with asimilarly sized facet of the squamosal (Fig 5A 6A 7D) Thealisphenoid surrounds the entire oval foramen (Figs 5C 6A7D) There is no round foramen

The pterygoid process of the alisphenoid in pakicetids (H-GSP30432 96231 96386 96623 98134 Fig 7D) divides posteriorlyinto medial and lateral pterygoid laminae (Fig 7D) that sur-round a low pterygoid fossa (Fig 7D) This structure is compa-rable to the pterygoid fossa of primates and many other terres-trial mammals and should not be confused with the pterygoidsinus fossa of Basilosauridae Odontoceti and Mysticeti Themedial pterygoid lamina is low The pterygoid bone is not pre-served but was probably attached to the medial pterygoidlamina

FIGURE 9 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) right ventrolateral view showing the sphenorbital region with the foraminafrom an oblique angle The zygomatic arch is out of the focal plane of this photograph Scale bar equals 5 cm

FIGURE 8 Cranial anatomy of Ichthyolestes pinfoldi (H-GSP 98134) A right lateral view B sphenorbital region in detail C outline ofsphenorbital region Scale bar for A equals 5 cm scale bar for BndashC equals 2 cm

JOURNAL OF VERTEBRATE PALEONTOLOGY VOL 26 NO 3 2006754

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BasisphenoidmdashThe basisphenoid shares sutures with pre-sphenoid alisphenoid and basioccipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Figs 5AC 6A 7B) The basisphenoid is convex vent-rally in cross section and concave in the median plane antero-posteriorly in juvenile Pakicetus and in Ichthyolestes (H-GSP96623 98134 respectively Figs 6A 7B) in adult Pakicetus(H-GSP 96231) and Nalacetus (H-GSP 96386) the basisphenoidis more or less flat in both these orientations (Figs 5AC) Pos-teriorly the suture between basisphenoid and basioccipital israised as bilateral tubercles in all pakicetid genera (H-GSP30432 96231 96386 98134 Figs 5AC 7B) One of the flexors ofthe neck rectus capitis ventralis originates from these tuberclesThere is no contact between the petrosal and the basisphenoidand the medial pterygoid sinus lies between these two bones(Fig 7D)

SquamosalmdashThe squamosal contacts the parietal alisphe-noid tympanic petrosal and occipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 98199 9625496386 96407 96623 98134 Figs 4B 5AB 6A 7AB 8A)making up most of the wall of the braincase Several smalltemporal foramina pierce the area near the occipito-squamosalsuture (Fig 6C) Posteriorly the squamosal forms part of thenuchal crest in all pakicetids (H-GSP 96231 96254 9638696623 Figs 4B 5B 6BD) Laterally the squamosal forms theposterior part of the zygomatic arch (Figs 4B 5AC 6BC 7AB8A 9) and the glenoid fossa (also called mandibular fossaFigs 5AC 6A 7Dglfs) in Pakicetus Nalacetus and Ichthyo-lestes (H-GSP 30432 96231 96386 96407 96623 98134) Theanterior part of the glenoid fossa is flat and extensive medio-laterally The postglenoid process (Figs 4A 5AC 6AC 7D8A) arches posteriorly into the external auditory meatus (Figs4A 5AC 6AC 7D 8A) The squamosal makes up the entireroof of the external auditory meatus and there is a prominentposttympanic process (Figs 4B 5B 6B 7A) The postglenoidforamen (also called retroarticular foramen Figs 5AC 6A 7D)is small and located posteromedial to the postglenoid processThe glenoid fossa extends medial to the postglenoid process andends posteromedially into a shallow groove directed to themiddle ear Medial to this is the squamosal contribution to theprominently raised joint facet (H-GSP 30432 96231 9638696623 98134 Figs 5A 6A 7D) for the anterior process of thetympanic The external auditory meatus ends medially in a re-cessed area in Pakicetus Nalacetus and Ichthyolestes (H-GSP30432 96231 96386 96407 96623 98134) The sharply depressedcenter of this recessed area represents the epitympanic recess(Figs 7D)

PetrosalmdashThe petrosal (periotic) contacts the alisphenoidsquamosal tympanic and occipital in P attocki N ratimitus andI pinfoldi (H-GSP 30432 96231 96386 96407 96431 9662398134 Fig 7D) The petrosal forms the roof of the middle earcavity The promontorium (Figs 5C 6A 7D) which houses theinner ear is a triangular structure on the ventral side The facialcanal runs anterior to the promontorium Medial to the prom-ontorium is the groove for the inferior petrosal sinus with thepiriform fenestra (H-GSP 30432 96386 96623 98134 Figs 5C7D) Rostrolateral to the promontorium is the tensor tympanifossa (H-GSP 30432 96231 96386 96407 96431 96623 98134Fig 7D) There are two foramina on the promontorium the ovalwindow for the stapes (fenestra vestibuli Fig 7D) and caudal toit the round window (fenestra cochleae Fig 7D) The mastoidprocess or posterior process of the petrosal flanks through thesquamosal posteriorly (H-GSP 30432 96231 96386 9640796623 98134 Figs 5AC 6A 7D 8A) A more detailed descrip-tion of the petrosal and tympanic of pakicetid cetaceans (Luoand Gingerich 1999) was based in part on material from H-GSPLocality 62

TympanicmdashThe tympanic (synonyms tympanic bulla audi-tory bulla ectotympanic) contacts the petrosal squamosal ali-sphenoid and exoccipital (GSP-UM 084) The tympanic sur-rounds the middle ear cavity ventrally laterally and medially(see Appendix 1 for all tympanics) The anterior part providesthe incisure for the eustachian tube (Fig 10) The medial pachy-osteosclerotic massive part of the tympanic is the involucrum(Fig 10) The involucrum is not in contact with any skull boneand is only connected to the rest of the skull by the dorsolateraland caudal parts of the tympanic The morphology described byGingerich and colleagues (1983) where the involucrum contactsthe basioccipital was based on the type specimen of P inachusGSP-UM 084 (sometimes listed as UM-GSP 081 by Luo 1998) inwhich this area is deformed

The shape and height of the anteromedial angle of the invo-lucrum (as defined by Luo 1998 Luo and Gingerich 1999Fig10) as well as the shape and orientation of the incisure forthe eustachian tube is species-specific (see Table 2) The lateralpart of the tympanic is a thin plate here the sigmoid process isa narrow and small plate of bone on the dorsal side of the lateralwall at least in I pinfoldi (H-GSP 92003) There is a small con-tact between the sigmoid process and the squamosal (see Luoand Gingerich 1999) Posterior to the sigmoid process the ex-ternal auditory meatus opens to the middle ear cavity Anteriorto the sigmoid process is the anterior process of the tympanicalso called processus tubarius which articulates with a facet onthe suture of squamosal and alisphenoid The posterior processof the tympanic contacts the mastoid (posterior) process of thepetrosal and more medially a small exoccipital facet makescontact with the tympanic (Fig 7D) this facet is the proximalpart of the paroccipital process of exoccipital (Figs 4B 5AC6ADE 7D)

There are four distinct morphologies among the tympanicsfrom H-GSP Locality 62 listed in Table 2 Three of these mor-phologies can be associated with cranial and dental material andthus be identified as P attocki I pinfoldi and N ratimitus Thefourth morphology represents a currently unidentified species ofthe family Pakicetidae Thewissen and Hussain (1998) describeda single tooth from Locality 62 tentatively as pertaining to Pinachus and it is possible that the unidentified bullae are fromthis species

In P attocki tympanics (see Appendix 1 and Table 2 Figs10AndashC) the morphology of the incisure for the eustachian tubeand of the anteromedial angle of the involucrum is similar to thatin the holotype of P inachus and therefore we refer these tym-panics to P attocki ndash which based on teeth (Thewissen andHussain 1998) is our most common species of Pakicetus Theorientation of the incisure for the eustachian tube cannot beobserved in GSP-UM 084 because the tympanic in this specimenis deformed and compressed against the basicranium

In the I pinfoldi specimen H-GSP 96431 (see Appendix 1 andTable 2 Figs 10DndashF) the fragmentary basicranium indicates asize considerably smaller than P attocki This is consistent withdental evidence (Thewissen and Hussain 1998) that indicatesthat the genus Ichthyolestes is smaller than Pakicetus We there-fore refer these specimens to I pinfoldi Although the adjoin-ing tympanic (H-GSP 96431) is the smallest one of our I pinfolditympanics two other tympanics with similar morphologyfor which the tympanic bullar length could be measured areboth within the length range of P attocki (Appendix 1 and Ta-ble 2)

The N ratimitus tympanics (see Appendix 1 and Table 2 Figs10GndashI) with a morphology clearly distinct from the two firstones described in Table 2 are larger than those of P attocki andmost of I pinfoldi and this serves to identify H-GSP 30325 and98140 Articular facets of Nalacetus tympanics (H-GSP 3032530459 96384 96385) match the facets on the exoccipital and

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 755

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squamosal of a Nalacetus skull (H-GSP 96386) The Nalacetusskull differs from Pakicetus (see previous) even though theseskulls are similar in size (Table 1) The Nalacetus skull is muchlarger than the Ichthyolestes skull (Table 1) This further cor-

roborates our identification of Nalacetus as a different genusand allocation of these tympanics to it

Three other remaining pakicetid tympanics (see Appendix 1and Table 2 Figs 10JndashL) are distinct from the three common

FIGURE 10 Pakicetid tympanics vertical columns showing the anterior lateral and dorsal views respectively AndashC Pakicetus attocki (H-GSP30386 left) DndashF Ichthyolestes pinfoldi (H-GSP 30223 left) GndashI Nalacetus ratimitus (H-GSP 96384 left) JndashL pakicetid n sp (H-GSP 98141 right)Scale bar equals 2 cm

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species of pakicetids at H-GSP Locality 62 and probably repre-sent a fourth new species of pakicetid These tympanics areclosest to P attocki tympanics in their size

OccipitalmdashThe most anterior point of the basioccipital of Pattocki N ratimitus and I pinfoldi (H-GSP 96231 96386 98134Figs 5AC 7B) is its suture with the basisphenoid which israised into bilateral tubercles (H-GSP 30432 96231 9638696623 98134 Figs 5AC 7B lsquobasioccipital crestrsquo of Luo 1998lsquoincipient falcate process of basioccipitalrsquo of Luo and Gingerich1999) These tubercles for neck flexors (see basisphenoid above)extend onto the anterior side of the basioccipital Posterior tothis the basioccipital projects laterally as a flat process in Paki-cetus (H-GSP 96231) and in Ichthyolestes (H-GSP 98134 Figs5A 7B) but this process is poorly defined in Nalacetus (H-GSP96386) The lateralmost part of this process is formed by theexoccipital (Figs 5AC 7B) and this process also forms the dor-solateral wall of the jugular foramen of pakicetids (H-GSP96231 96386 96407 96431 98134 Figs 5AC 7D) The exoc-cipitals bear the occipital condyles (H-GSP 96231 96386 9643198134 98199 Figs 4A 5AC 6DE 7B 8A 9) which are com-pressed dorsoventrally do not touch in the median plane andare somewhat raised above the level of the exoccipital The dor-sal and ventral condyloid fossae are of equal size (H-GSP 9623198134 98199 Figs 5AC 6E 7D) The hypoglossal foramen inall pakicetid genera (H-GSP 96231 96386 96407 96431 98134Figs 5AC 7D) is situated approximately halfway between thejugular foramen and the occipital condyle One or two smallcondylar foramina (Figs 5AC 7D) pierce the basioccipital lat-eral to the hypoglossal foramen The exoccipital forms the me-diolaterally elongate paroccipital process in all pakicetids (H-GSP 30432 96231 96386 96431 96623 98134 Figs 4B 5AC6ADE 7D) The medial extremity of this process bears a flatfacet for the tympanic (Fig 7D lsquobullar process of the exoccipitalrsquoof Luo and Gingerich 1999) The anterior part of the paroccipi-tal process shares a long suture with the mastoid process of pe-trosal The paroccipital process projects posterolaterally and thepart adjacent to the occipital condyle is recessed

The supraoccipital makes up most of the posterior side of thenuchal crest in all pakicetids (H-GSP 96231 96254 96386 96623Figs 4B 5B 6BD) although this structure is clearly worn insome specimens eg in I pinfoldi (H-GSP 98134) A small bi-lateral process of supraoccipital visible in Pakicetus and Ich-thyolestes (H-GSP 96231 98134 Figs 6DE) for the epaxial ex-tensors of the neck projects from the dorsal edge of the foramenmagnum (H-GSP 96231 96386 98134 Figs 6DE)Variable em-issary foramina (H-GSP 96231 96254 96386 96623 98134 Figs6DE) pierce the occipital dorsal to the foramen magnum Abilateral oval depression (H-GSP 96231 96386 98134 Figs6DE) occurs just lateral to the midline well above the foramenmagnum This depression could be for musculus longissimuscapitis The precise shape of the supraoccipital cannot be deter-mined because the suture between the exoccipital and the su-praoccipital is not visible

DISCUSSION

The pakicetid crania described here help to understand varia-tion in skull morphology of the earliest cetaceans As such theycan be expected to play a significant role in systematic studiesbut a detailed analysis of character scores for these specimensfalls outside the scope of the present description Here discus-sion will focus on a few aspects of cranial morphology that arerarely involved in cetacean systematics although they displayvariation to an extent that may be relevant systematically andinteresting evolutionarily

Supraorbital Morphology

The supraorbital region of protocetids basilosaurids anddorudontids displays a supraorbital shield (Kellogg 1936 Gin-

gerich et al 1994 1995 Bajpai and Gingerich 1998 Hulbert1998 Uhen 1998 2004) However the basal families of ceta-ceans lack a supraorbital shield pakicetids (Thewissen et al2001b) ambulocetids (Thewissen et al 1996) and remingtono-cetids (Gingerich et al 1994 1995 Bajpai and Thewissen 1998)The new pakicetid specimens indicate that there is much varia-tion in supraorbital morphology within these families In paki-cetids the orbits are convergent dorsally The area between theorbits is recessed over the superior orbital rim (Figs 4ndash8) Assuch pakicetid eyes are directed dorsolaterally There are dif-ferences between the genera (Fig 3 Table 1) in Pakicetus andIchthyolestes the supraorbital region is cuplike and cradles thedorsal part of the eye and the dorsal side of the supraorbitalregion is deeply incised in the medial plane in Pakicetus Thisincisure is less strong in Nalacetus and Ichthyolestes and in Nala-cetus the contact between supraorbital region and eye is toonarrow to be cup-shaped In Ambulocetus (Fig 3 Table 1Thewissen et al 1996fig 10 for H-GSP 18490) the orbits arepositioned near the dorsal side of the skull but they face later-ally leaving the supraorbital region more or less flat (in dorsalview) The eyes here appear to be directed laterally and are nothoused in the bony orbits as deeply as in pakicetids as indicatedby the shape of the postorbital processes As a comparison inRemingtonocetus the eyes are small are set laterally and facelaterally and the supraorbital region is strongly convex (Fig 3Sahni and Mishra 1972 Kumar and Sahni 1986 Bajpai andThewissen 1998) In protocetids the orbit is large faces later-ally and is covered by a large supraorbital shield (Fig 3 Table1) Dorudontids and basilosaurids resemble protocetids in thepresence and shape of their supraorbital shield as indicated bythe ratio between the minimum and maximum supraorbitalwidth (Table 1) and in the orientation of their orbits the latterare still clearly different from modern odontocetes and mystice-tes (Kellogg 1936 Uhen 1998 2004)

Intertemporal Morphology

In early and middle Eocene cetaceans a very narrow regioncan be seen in the skull posterior to the frontoparietal suturenear the sphenorbital fissure (Figs 1 2 4ndash8) This morphology isnot seen in most land mammals but is common in early ceta-ceans such as pakicetids ambulocetids (eg AmbulocetusThewissen et al 1996) and remingtonocetids (Kumar and Sahni1986) and may represent an important diagnostic character forthe order

Tympanic Bullar Morphology

The differences in the tympanic bullar morphology seen be-tween P attocki I pinfoldi N ratimitus and a fourth unnamedspecies can be used for specific recognition However in generalall these tympanics display a similar morphology indicating thatthese early archaeocetes used their peripheral ear structures likeland mammals while hearing in air and relied on bone conduc-tion mechanism while hearing in water (Thewissen and Hussain1993 Luo 1998 Luo and Gingerich 1999 Nummela et al 2004)

Pakicetus attocki is clearly larger than I pinfoldi whereas Pa-kicetus and Nalacetus have similar body sizes (Table 1) Thewis-sen and Hussain (1998) studied the size of upper and lower mo-lars in P attocki P inachus I pinfoldi and N ratimitus Theyfound Ichthyolestes to be the smallest P attocki and Nalacetus tobe of similar size and P inachus to be the largest Our studyshows that the absolute tympanic bullar size is similar in Paki-cetus and Ichthyolestes but larger in Nalacetus (Table 2 Appen-dix 1) This implies that in relative terms Pakicetus tympanicswhen related to body size (Table 2) are smaller than those ofIchthyolestes and Nalacetus

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 757

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ACKNOWLEDGMENTS

We thank Ellen Byron and Jeanette Killius for their technicalassistance Linda Spurlock prepared the drawings of the skullswhich is gratefully acknowledged Sunil Bajpai kindly gave usaccess to Remingtonocetus specimens in the IITR-SB collectionsunder his care We thank the editor Ewan Fordyce and Zhe-XiLuo and an anonymous reviewer for constructive suggestions forimprovement of the manuscript Financial support was providedby the National Science Foundation (current grant number EAR0207370) to JGMT and by the Ella and Georg Ehrnrooth Foun-dation to SN This article is a contribution to the Howard Uni-versity-Geological Survey of Pakistan projected entitled ldquoCeno-zoic Mammals of Pakistanrdquo We thank the Geological Survey ofPakistan for their support and help in the field

LITERATURE CITED

Aslan A and J G M Thewissen 1996 Preliminary evaluation of pa-leosols and implications for interpreting vertebrate fossil assem-blages Kuldana Formation Northern Pakistan Palaeovertebrata25261ndash277

Bajpai S and P D Gingerich 1998 A new Eocene archaeocete (Mam-malia Cetacea) from India and the time of origin of whales Pro-ceedings of the National Academy of Sciences USA 9515464ndash15468

Bajpai S and J G M Thewissen 1998 Middle Eocene cetaceans fromthe Harudi and Subathu formations of India pp 213ndash233 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Dehm R and T zu Oettingen-Spielberg 1958 Palaumlontologische undgeologische Untersuchungen im Tertiaumlr von Pakistan 2 Die mitte-leocaumlnen Saumlugetiere von Ganda Kas bei Basal in Nordwest-Pakistan Bayerische Akademie der Wissenschaften 911ndash54

Geisler J H and Z-X Luo 1998 Relationships of Cetacea to terres-trial ungulates and the evolution of cranial vasculature in Cete pp163ndash212 in J G M Thewissen (ed) The Emergence of WhalesPlenum Press New York

Geisler J H A E Sanders and Z-X Luo 2005 A new protocetidwhale (Cetacea Archaeoceti) from the late Middle Eocene of SouthCarolina American Museum Novitates 34801ndash65

Gingerich P D 2003 Land-to-sea transition in early whales evolutionof Eocene Archaeoceti (Cetacea) in relation to skeletal proportionsand locomotion of living semiaquatic mammals Paleobiology 29429ndash454

Gingerich P D and D E Russell 1981 Pakicetus inachus a newarchaeocete (Mammalia Cetacea) from the early-middle EoceneKuldana Formation of Kohat (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 25235ndash246

Gingerich P D and D E Russell 1990 Dentition of early EocenePakicetus (Mammalia Cetacea) Contributions from the Museum ofPaleontology University of Michigan 281ndash20

Gingerich P D N A Wells D E Russell and S M I Shah 1983Origin of whales in epicontinental remnant seas new evidence fromthe early Eocene of Pakistan Science 220403ndash406

Gingerich P D S M Raza M Arif M Anwar and X Zhou 1994New whale from the Eocene of Pakistan and the origin of cetaceanswimming Nature 368844ndash847

Gingerich P D M Arif and W C Clyde 1995 New archaeocetes(Mammalia Cetacea) from the middle Eocene Domanda Formationof the Sulaiman Range Punjab (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 29291ndash330

Gingerich P D M ul-Haq I S Zalmout I H Khan and M S Mal-kani 2001 Origin of whales from early artiodactyls hands and feetof Eocene Protocetidae from Pakistan Science 2932239ndash2242

Hulbert R Jr 1998 Postcranial osteology of the North American

middle Eocene protocetid Georgiacetus pp 235ndash267 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Kellogg A R 1936 A review of the Archaeoceti Carnegie Institute ofWashington Publications 4821ndash366

Kumar K and A Sahni 1986 Remingtonocetus harudiensis new com-bination a middle Eocene archaeocete (Mammalia Cetacea) fromwestern Kutch India Journal of Vertebrate Paleontology 6326ndash349

Luo Z-X 1998 Homology and transformation of cetacean ectotym-panic structures pp 269ndash301 in J G M Thewissen (ed) The Emer-gence of Whales Plenum Press New York

Luo Z-X and P D Gingerich 1999 Terrestrial Mesonychia to aquaticCetacea transformation of the basicranium and evolution of hearingin whales University of Michigan Papers on Paleontology 311ndash98

Madar S I In press The postcranial skeleton of early Eocene pakicetidcetaceans Journal of Paleontology

Nummela S J G M Thewissen S Bajpai S T Hussain and K Ku-mar 2004 Eocene evolution of whale hearing Nature 430776ndash778

Sahni A and V P Mishra 1972 A new species of Protocetus (Cetacea)from the middle Eocene of Kutch Western India Palaeontology15490ndash495

Thewissen J G M and S T Hussain 1993 Origin of underwaterhearing in whales Nature 361444ndash445

Thewissen J G M and S T Hussain 1998 Systematic review of thePakicetidae early and middle Eocene Cetacea (Mammalia) fromPakistan and India Bulletin of Carnegie Museum of Natural History34220ndash238

Thewissen J G M and E M Williams 2002 The early radiations ofCetacea (Mammalia) evolutionary patterns and developmental cor-relations Annual Review of Ecology and Systematics 3373ndash90

Thewissen J G M S I Madar and S T Hussain 1996 Ambulocetusnatans an Eocene cetacean (Mammalia) from Pakistan CourierForschungsinstitut Senckenberg 191 1ndash86

Thewissen J G M E M Williams and S T Hussain 2001a Eocenemammal faunas from Northern Indo-Pakistan Journal of Verte-brate Paleontology 21347ndash366

Thewissen J G M E M Williams L J Roe and S T Hussain 2001bSkeletons of terrestrial cetaceans and the relationship of whales toartiodactyls Nature 413277ndash281

Thewissen J G M S T Hussain M Arif A Aslan S I Madar andL J Roe 1998 New localities of Eocene vertebrates in NorthernPakistan and their significance for the origin of the modern orders ofmammals pp 19ndash34 in M I Ghaznavi S M Raza and M T Hasan(eds) Siwaliks of South Asia Proceedings of the 3rd GEOSASWorkshop Islamabad Pakistan 1ndash5 March 1997 Geological Sur-vey of Pakistan Islamabad Pakistan

Uhen M D 1998 Middle to late Eocene basilosaurines and dorudon-tines pp 29ndash61 in J G M Thewissen (ed) The Emergence ofWhales Plenum Press New York

Uhen M D 2004 Form function and anatomy of Dorudon atrox(Mammalia Cetacea) an archaeocete from the middle to lateEocene of Egypt University of Michigan Papers on Paleontology341ndash222

West R M 1980 Middle Eocene large mammal assemblage withTethyan affinities Ganda Kas region Pakistan Journal of Paleon-tology 54508ndash533

West R M and J R Lukacs 1979 Geology and vertebrate-fossil lo-calities Tertiary continental rocks Kala Chitta Hills Attock Dis-trict Pakistan Milwaukee Public Museum Contributions in Biologyand Geology 261ndash20

Williams E M 1998 Synopsis of the earliest cetaceans PakicetidaeAmbulocetidae Remingtonocetidae and Protocetidae pp 1ndash28 inJ G M Thewissen (ed) The Emergence of Whales Plenum PressNew York

Submitted 7 October 2005 accepted 24 February 2006

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APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

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BasisphenoidmdashThe basisphenoid shares sutures with pre-sphenoid alisphenoid and basioccipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 96386 9662398134 Figs 5AC 6A 7B) The basisphenoid is convex vent-rally in cross section and concave in the median plane antero-posteriorly in juvenile Pakicetus and in Ichthyolestes (H-GSP96623 98134 respectively Figs 6A 7B) in adult Pakicetus(H-GSP 96231) and Nalacetus (H-GSP 96386) the basisphenoidis more or less flat in both these orientations (Figs 5AC) Pos-teriorly the suture between basisphenoid and basioccipital israised as bilateral tubercles in all pakicetid genera (H-GSP30432 96231 96386 98134 Figs 5AC 7B) One of the flexors ofthe neck rectus capitis ventralis originates from these tuberclesThere is no contact between the petrosal and the basisphenoidand the medial pterygoid sinus lies between these two bones(Fig 7D)

SquamosalmdashThe squamosal contacts the parietal alisphe-noid tympanic petrosal and occipital in P attocki N rati-mitus and I pinfoldi (H-GSP 30432 96231 98199 9625496386 96407 96623 98134 Figs 4B 5AB 6A 7AB 8A)making up most of the wall of the braincase Several smalltemporal foramina pierce the area near the occipito-squamosalsuture (Fig 6C) Posteriorly the squamosal forms part of thenuchal crest in all pakicetids (H-GSP 96231 96254 9638696623 Figs 4B 5B 6BD) Laterally the squamosal forms theposterior part of the zygomatic arch (Figs 4B 5AC 6BC 7AB8A 9) and the glenoid fossa (also called mandibular fossaFigs 5AC 6A 7Dglfs) in Pakicetus Nalacetus and Ichthyo-lestes (H-GSP 30432 96231 96386 96407 96623 98134) Theanterior part of the glenoid fossa is flat and extensive medio-laterally The postglenoid process (Figs 4A 5AC 6AC 7D8A) arches posteriorly into the external auditory meatus (Figs4A 5AC 6AC 7D 8A) The squamosal makes up the entireroof of the external auditory meatus and there is a prominentposttympanic process (Figs 4B 5B 6B 7A) The postglenoidforamen (also called retroarticular foramen Figs 5AC 6A 7D)is small and located posteromedial to the postglenoid processThe glenoid fossa extends medial to the postglenoid process andends posteromedially into a shallow groove directed to themiddle ear Medial to this is the squamosal contribution to theprominently raised joint facet (H-GSP 30432 96231 9638696623 98134 Figs 5A 6A 7D) for the anterior process of thetympanic The external auditory meatus ends medially in a re-cessed area in Pakicetus Nalacetus and Ichthyolestes (H-GSP30432 96231 96386 96407 96623 98134) The sharply depressedcenter of this recessed area represents the epitympanic recess(Figs 7D)

PetrosalmdashThe petrosal (periotic) contacts the alisphenoidsquamosal tympanic and occipital in P attocki N ratimitus andI pinfoldi (H-GSP 30432 96231 96386 96407 96431 9662398134 Fig 7D) The petrosal forms the roof of the middle earcavity The promontorium (Figs 5C 6A 7D) which houses theinner ear is a triangular structure on the ventral side The facialcanal runs anterior to the promontorium Medial to the prom-ontorium is the groove for the inferior petrosal sinus with thepiriform fenestra (H-GSP 30432 96386 96623 98134 Figs 5C7D) Rostrolateral to the promontorium is the tensor tympanifossa (H-GSP 30432 96231 96386 96407 96431 96623 98134Fig 7D) There are two foramina on the promontorium the ovalwindow for the stapes (fenestra vestibuli Fig 7D) and caudal toit the round window (fenestra cochleae Fig 7D) The mastoidprocess or posterior process of the petrosal flanks through thesquamosal posteriorly (H-GSP 30432 96231 96386 9640796623 98134 Figs 5AC 6A 7D 8A) A more detailed descrip-tion of the petrosal and tympanic of pakicetid cetaceans (Luoand Gingerich 1999) was based in part on material from H-GSPLocality 62

TympanicmdashThe tympanic (synonyms tympanic bulla audi-tory bulla ectotympanic) contacts the petrosal squamosal ali-sphenoid and exoccipital (GSP-UM 084) The tympanic sur-rounds the middle ear cavity ventrally laterally and medially(see Appendix 1 for all tympanics) The anterior part providesthe incisure for the eustachian tube (Fig 10) The medial pachy-osteosclerotic massive part of the tympanic is the involucrum(Fig 10) The involucrum is not in contact with any skull boneand is only connected to the rest of the skull by the dorsolateraland caudal parts of the tympanic The morphology described byGingerich and colleagues (1983) where the involucrum contactsthe basioccipital was based on the type specimen of P inachusGSP-UM 084 (sometimes listed as UM-GSP 081 by Luo 1998) inwhich this area is deformed

The shape and height of the anteromedial angle of the invo-lucrum (as defined by Luo 1998 Luo and Gingerich 1999Fig10) as well as the shape and orientation of the incisure forthe eustachian tube is species-specific (see Table 2) The lateralpart of the tympanic is a thin plate here the sigmoid process isa narrow and small plate of bone on the dorsal side of the lateralwall at least in I pinfoldi (H-GSP 92003) There is a small con-tact between the sigmoid process and the squamosal (see Luoand Gingerich 1999) Posterior to the sigmoid process the ex-ternal auditory meatus opens to the middle ear cavity Anteriorto the sigmoid process is the anterior process of the tympanicalso called processus tubarius which articulates with a facet onthe suture of squamosal and alisphenoid The posterior processof the tympanic contacts the mastoid (posterior) process of thepetrosal and more medially a small exoccipital facet makescontact with the tympanic (Fig 7D) this facet is the proximalpart of the paroccipital process of exoccipital (Figs 4B 5AC6ADE 7D)

There are four distinct morphologies among the tympanicsfrom H-GSP Locality 62 listed in Table 2 Three of these mor-phologies can be associated with cranial and dental material andthus be identified as P attocki I pinfoldi and N ratimitus Thefourth morphology represents a currently unidentified species ofthe family Pakicetidae Thewissen and Hussain (1998) describeda single tooth from Locality 62 tentatively as pertaining to Pinachus and it is possible that the unidentified bullae are fromthis species

In P attocki tympanics (see Appendix 1 and Table 2 Figs10AndashC) the morphology of the incisure for the eustachian tubeand of the anteromedial angle of the involucrum is similar to thatin the holotype of P inachus and therefore we refer these tym-panics to P attocki ndash which based on teeth (Thewissen andHussain 1998) is our most common species of Pakicetus Theorientation of the incisure for the eustachian tube cannot beobserved in GSP-UM 084 because the tympanic in this specimenis deformed and compressed against the basicranium

In the I pinfoldi specimen H-GSP 96431 (see Appendix 1 andTable 2 Figs 10DndashF) the fragmentary basicranium indicates asize considerably smaller than P attocki This is consistent withdental evidence (Thewissen and Hussain 1998) that indicatesthat the genus Ichthyolestes is smaller than Pakicetus We there-fore refer these specimens to I pinfoldi Although the adjoin-ing tympanic (H-GSP 96431) is the smallest one of our I pinfolditympanics two other tympanics with similar morphologyfor which the tympanic bullar length could be measured areboth within the length range of P attocki (Appendix 1 and Ta-ble 2)

The N ratimitus tympanics (see Appendix 1 and Table 2 Figs10GndashI) with a morphology clearly distinct from the two firstones described in Table 2 are larger than those of P attocki andmost of I pinfoldi and this serves to identify H-GSP 30325 and98140 Articular facets of Nalacetus tympanics (H-GSP 3032530459 96384 96385) match the facets on the exoccipital and

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 755

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squamosal of a Nalacetus skull (H-GSP 96386) The Nalacetusskull differs from Pakicetus (see previous) even though theseskulls are similar in size (Table 1) The Nalacetus skull is muchlarger than the Ichthyolestes skull (Table 1) This further cor-

roborates our identification of Nalacetus as a different genusand allocation of these tympanics to it

Three other remaining pakicetid tympanics (see Appendix 1and Table 2 Figs 10JndashL) are distinct from the three common

FIGURE 10 Pakicetid tympanics vertical columns showing the anterior lateral and dorsal views respectively AndashC Pakicetus attocki (H-GSP30386 left) DndashF Ichthyolestes pinfoldi (H-GSP 30223 left) GndashI Nalacetus ratimitus (H-GSP 96384 left) JndashL pakicetid n sp (H-GSP 98141 right)Scale bar equals 2 cm

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species of pakicetids at H-GSP Locality 62 and probably repre-sent a fourth new species of pakicetid These tympanics areclosest to P attocki tympanics in their size

OccipitalmdashThe most anterior point of the basioccipital of Pattocki N ratimitus and I pinfoldi (H-GSP 96231 96386 98134Figs 5AC 7B) is its suture with the basisphenoid which israised into bilateral tubercles (H-GSP 30432 96231 9638696623 98134 Figs 5AC 7B lsquobasioccipital crestrsquo of Luo 1998lsquoincipient falcate process of basioccipitalrsquo of Luo and Gingerich1999) These tubercles for neck flexors (see basisphenoid above)extend onto the anterior side of the basioccipital Posterior tothis the basioccipital projects laterally as a flat process in Paki-cetus (H-GSP 96231) and in Ichthyolestes (H-GSP 98134 Figs5A 7B) but this process is poorly defined in Nalacetus (H-GSP96386) The lateralmost part of this process is formed by theexoccipital (Figs 5AC 7B) and this process also forms the dor-solateral wall of the jugular foramen of pakicetids (H-GSP96231 96386 96407 96431 98134 Figs 5AC 7D) The exoc-cipitals bear the occipital condyles (H-GSP 96231 96386 9643198134 98199 Figs 4A 5AC 6DE 7B 8A 9) which are com-pressed dorsoventrally do not touch in the median plane andare somewhat raised above the level of the exoccipital The dor-sal and ventral condyloid fossae are of equal size (H-GSP 9623198134 98199 Figs 5AC 6E 7D) The hypoglossal foramen inall pakicetid genera (H-GSP 96231 96386 96407 96431 98134Figs 5AC 7D) is situated approximately halfway between thejugular foramen and the occipital condyle One or two smallcondylar foramina (Figs 5AC 7D) pierce the basioccipital lat-eral to the hypoglossal foramen The exoccipital forms the me-diolaterally elongate paroccipital process in all pakicetids (H-GSP 30432 96231 96386 96431 96623 98134 Figs 4B 5AC6ADE 7D) The medial extremity of this process bears a flatfacet for the tympanic (Fig 7D lsquobullar process of the exoccipitalrsquoof Luo and Gingerich 1999) The anterior part of the paroccipi-tal process shares a long suture with the mastoid process of pe-trosal The paroccipital process projects posterolaterally and thepart adjacent to the occipital condyle is recessed

The supraoccipital makes up most of the posterior side of thenuchal crest in all pakicetids (H-GSP 96231 96254 96386 96623Figs 4B 5B 6BD) although this structure is clearly worn insome specimens eg in I pinfoldi (H-GSP 98134) A small bi-lateral process of supraoccipital visible in Pakicetus and Ich-thyolestes (H-GSP 96231 98134 Figs 6DE) for the epaxial ex-tensors of the neck projects from the dorsal edge of the foramenmagnum (H-GSP 96231 96386 98134 Figs 6DE)Variable em-issary foramina (H-GSP 96231 96254 96386 96623 98134 Figs6DE) pierce the occipital dorsal to the foramen magnum Abilateral oval depression (H-GSP 96231 96386 98134 Figs6DE) occurs just lateral to the midline well above the foramenmagnum This depression could be for musculus longissimuscapitis The precise shape of the supraoccipital cannot be deter-mined because the suture between the exoccipital and the su-praoccipital is not visible

DISCUSSION

The pakicetid crania described here help to understand varia-tion in skull morphology of the earliest cetaceans As such theycan be expected to play a significant role in systematic studiesbut a detailed analysis of character scores for these specimensfalls outside the scope of the present description Here discus-sion will focus on a few aspects of cranial morphology that arerarely involved in cetacean systematics although they displayvariation to an extent that may be relevant systematically andinteresting evolutionarily

Supraorbital Morphology

The supraorbital region of protocetids basilosaurids anddorudontids displays a supraorbital shield (Kellogg 1936 Gin-

gerich et al 1994 1995 Bajpai and Gingerich 1998 Hulbert1998 Uhen 1998 2004) However the basal families of ceta-ceans lack a supraorbital shield pakicetids (Thewissen et al2001b) ambulocetids (Thewissen et al 1996) and remingtono-cetids (Gingerich et al 1994 1995 Bajpai and Thewissen 1998)The new pakicetid specimens indicate that there is much varia-tion in supraorbital morphology within these families In paki-cetids the orbits are convergent dorsally The area between theorbits is recessed over the superior orbital rim (Figs 4ndash8) Assuch pakicetid eyes are directed dorsolaterally There are dif-ferences between the genera (Fig 3 Table 1) in Pakicetus andIchthyolestes the supraorbital region is cuplike and cradles thedorsal part of the eye and the dorsal side of the supraorbitalregion is deeply incised in the medial plane in Pakicetus Thisincisure is less strong in Nalacetus and Ichthyolestes and in Nala-cetus the contact between supraorbital region and eye is toonarrow to be cup-shaped In Ambulocetus (Fig 3 Table 1Thewissen et al 1996fig 10 for H-GSP 18490) the orbits arepositioned near the dorsal side of the skull but they face later-ally leaving the supraorbital region more or less flat (in dorsalview) The eyes here appear to be directed laterally and are nothoused in the bony orbits as deeply as in pakicetids as indicatedby the shape of the postorbital processes As a comparison inRemingtonocetus the eyes are small are set laterally and facelaterally and the supraorbital region is strongly convex (Fig 3Sahni and Mishra 1972 Kumar and Sahni 1986 Bajpai andThewissen 1998) In protocetids the orbit is large faces later-ally and is covered by a large supraorbital shield (Fig 3 Table1) Dorudontids and basilosaurids resemble protocetids in thepresence and shape of their supraorbital shield as indicated bythe ratio between the minimum and maximum supraorbitalwidth (Table 1) and in the orientation of their orbits the latterare still clearly different from modern odontocetes and mystice-tes (Kellogg 1936 Uhen 1998 2004)

Intertemporal Morphology

In early and middle Eocene cetaceans a very narrow regioncan be seen in the skull posterior to the frontoparietal suturenear the sphenorbital fissure (Figs 1 2 4ndash8) This morphology isnot seen in most land mammals but is common in early ceta-ceans such as pakicetids ambulocetids (eg AmbulocetusThewissen et al 1996) and remingtonocetids (Kumar and Sahni1986) and may represent an important diagnostic character forthe order

Tympanic Bullar Morphology

The differences in the tympanic bullar morphology seen be-tween P attocki I pinfoldi N ratimitus and a fourth unnamedspecies can be used for specific recognition However in generalall these tympanics display a similar morphology indicating thatthese early archaeocetes used their peripheral ear structures likeland mammals while hearing in air and relied on bone conduc-tion mechanism while hearing in water (Thewissen and Hussain1993 Luo 1998 Luo and Gingerich 1999 Nummela et al 2004)

Pakicetus attocki is clearly larger than I pinfoldi whereas Pa-kicetus and Nalacetus have similar body sizes (Table 1) Thewis-sen and Hussain (1998) studied the size of upper and lower mo-lars in P attocki P inachus I pinfoldi and N ratimitus Theyfound Ichthyolestes to be the smallest P attocki and Nalacetus tobe of similar size and P inachus to be the largest Our studyshows that the absolute tympanic bullar size is similar in Paki-cetus and Ichthyolestes but larger in Nalacetus (Table 2 Appen-dix 1) This implies that in relative terms Pakicetus tympanicswhen related to body size (Table 2) are smaller than those ofIchthyolestes and Nalacetus

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 757

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ACKNOWLEDGMENTS

We thank Ellen Byron and Jeanette Killius for their technicalassistance Linda Spurlock prepared the drawings of the skullswhich is gratefully acknowledged Sunil Bajpai kindly gave usaccess to Remingtonocetus specimens in the IITR-SB collectionsunder his care We thank the editor Ewan Fordyce and Zhe-XiLuo and an anonymous reviewer for constructive suggestions forimprovement of the manuscript Financial support was providedby the National Science Foundation (current grant number EAR0207370) to JGMT and by the Ella and Georg Ehrnrooth Foun-dation to SN This article is a contribution to the Howard Uni-versity-Geological Survey of Pakistan projected entitled ldquoCeno-zoic Mammals of Pakistanrdquo We thank the Geological Survey ofPakistan for their support and help in the field

LITERATURE CITED

Aslan A and J G M Thewissen 1996 Preliminary evaluation of pa-leosols and implications for interpreting vertebrate fossil assem-blages Kuldana Formation Northern Pakistan Palaeovertebrata25261ndash277

Bajpai S and P D Gingerich 1998 A new Eocene archaeocete (Mam-malia Cetacea) from India and the time of origin of whales Pro-ceedings of the National Academy of Sciences USA 9515464ndash15468

Bajpai S and J G M Thewissen 1998 Middle Eocene cetaceans fromthe Harudi and Subathu formations of India pp 213ndash233 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Dehm R and T zu Oettingen-Spielberg 1958 Palaumlontologische undgeologische Untersuchungen im Tertiaumlr von Pakistan 2 Die mitte-leocaumlnen Saumlugetiere von Ganda Kas bei Basal in Nordwest-Pakistan Bayerische Akademie der Wissenschaften 911ndash54

Geisler J H and Z-X Luo 1998 Relationships of Cetacea to terres-trial ungulates and the evolution of cranial vasculature in Cete pp163ndash212 in J G M Thewissen (ed) The Emergence of WhalesPlenum Press New York

Geisler J H A E Sanders and Z-X Luo 2005 A new protocetidwhale (Cetacea Archaeoceti) from the late Middle Eocene of SouthCarolina American Museum Novitates 34801ndash65

Gingerich P D 2003 Land-to-sea transition in early whales evolutionof Eocene Archaeoceti (Cetacea) in relation to skeletal proportionsand locomotion of living semiaquatic mammals Paleobiology 29429ndash454

Gingerich P D and D E Russell 1981 Pakicetus inachus a newarchaeocete (Mammalia Cetacea) from the early-middle EoceneKuldana Formation of Kohat (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 25235ndash246

Gingerich P D and D E Russell 1990 Dentition of early EocenePakicetus (Mammalia Cetacea) Contributions from the Museum ofPaleontology University of Michigan 281ndash20

Gingerich P D N A Wells D E Russell and S M I Shah 1983Origin of whales in epicontinental remnant seas new evidence fromthe early Eocene of Pakistan Science 220403ndash406

Gingerich P D S M Raza M Arif M Anwar and X Zhou 1994New whale from the Eocene of Pakistan and the origin of cetaceanswimming Nature 368844ndash847

Gingerich P D M Arif and W C Clyde 1995 New archaeocetes(Mammalia Cetacea) from the middle Eocene Domanda Formationof the Sulaiman Range Punjab (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 29291ndash330

Gingerich P D M ul-Haq I S Zalmout I H Khan and M S Mal-kani 2001 Origin of whales from early artiodactyls hands and feetof Eocene Protocetidae from Pakistan Science 2932239ndash2242

Hulbert R Jr 1998 Postcranial osteology of the North American

middle Eocene protocetid Georgiacetus pp 235ndash267 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Kellogg A R 1936 A review of the Archaeoceti Carnegie Institute ofWashington Publications 4821ndash366

Kumar K and A Sahni 1986 Remingtonocetus harudiensis new com-bination a middle Eocene archaeocete (Mammalia Cetacea) fromwestern Kutch India Journal of Vertebrate Paleontology 6326ndash349

Luo Z-X 1998 Homology and transformation of cetacean ectotym-panic structures pp 269ndash301 in J G M Thewissen (ed) The Emer-gence of Whales Plenum Press New York

Luo Z-X and P D Gingerich 1999 Terrestrial Mesonychia to aquaticCetacea transformation of the basicranium and evolution of hearingin whales University of Michigan Papers on Paleontology 311ndash98

Madar S I In press The postcranial skeleton of early Eocene pakicetidcetaceans Journal of Paleontology

Nummela S J G M Thewissen S Bajpai S T Hussain and K Ku-mar 2004 Eocene evolution of whale hearing Nature 430776ndash778

Sahni A and V P Mishra 1972 A new species of Protocetus (Cetacea)from the middle Eocene of Kutch Western India Palaeontology15490ndash495

Thewissen J G M and S T Hussain 1993 Origin of underwaterhearing in whales Nature 361444ndash445

Thewissen J G M and S T Hussain 1998 Systematic review of thePakicetidae early and middle Eocene Cetacea (Mammalia) fromPakistan and India Bulletin of Carnegie Museum of Natural History34220ndash238

Thewissen J G M and E M Williams 2002 The early radiations ofCetacea (Mammalia) evolutionary patterns and developmental cor-relations Annual Review of Ecology and Systematics 3373ndash90

Thewissen J G M S I Madar and S T Hussain 1996 Ambulocetusnatans an Eocene cetacean (Mammalia) from Pakistan CourierForschungsinstitut Senckenberg 191 1ndash86

Thewissen J G M E M Williams and S T Hussain 2001a Eocenemammal faunas from Northern Indo-Pakistan Journal of Verte-brate Paleontology 21347ndash366

Thewissen J G M E M Williams L J Roe and S T Hussain 2001bSkeletons of terrestrial cetaceans and the relationship of whales toartiodactyls Nature 413277ndash281

Thewissen J G M S T Hussain M Arif A Aslan S I Madar andL J Roe 1998 New localities of Eocene vertebrates in NorthernPakistan and their significance for the origin of the modern orders ofmammals pp 19ndash34 in M I Ghaznavi S M Raza and M T Hasan(eds) Siwaliks of South Asia Proceedings of the 3rd GEOSASWorkshop Islamabad Pakistan 1ndash5 March 1997 Geological Sur-vey of Pakistan Islamabad Pakistan

Uhen M D 1998 Middle to late Eocene basilosaurines and dorudon-tines pp 29ndash61 in J G M Thewissen (ed) The Emergence ofWhales Plenum Press New York

Uhen M D 2004 Form function and anatomy of Dorudon atrox(Mammalia Cetacea) an archaeocete from the middle to lateEocene of Egypt University of Michigan Papers on Paleontology341ndash222

West R M 1980 Middle Eocene large mammal assemblage withTethyan affinities Ganda Kas region Pakistan Journal of Paleon-tology 54508ndash533

West R M and J R Lukacs 1979 Geology and vertebrate-fossil lo-calities Tertiary continental rocks Kala Chitta Hills Attock Dis-trict Pakistan Milwaukee Public Museum Contributions in Biologyand Geology 261ndash20

Williams E M 1998 Synopsis of the earliest cetaceans PakicetidaeAmbulocetidae Remingtonocetidae and Protocetidae pp 1ndash28 inJ G M Thewissen (ed) The Emergence of Whales Plenum PressNew York

Submitted 7 October 2005 accepted 24 February 2006

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APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 759

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squamosal of a Nalacetus skull (H-GSP 96386) The Nalacetusskull differs from Pakicetus (see previous) even though theseskulls are similar in size (Table 1) The Nalacetus skull is muchlarger than the Ichthyolestes skull (Table 1) This further cor-

roborates our identification of Nalacetus as a different genusand allocation of these tympanics to it

Three other remaining pakicetid tympanics (see Appendix 1and Table 2 Figs 10JndashL) are distinct from the three common

FIGURE 10 Pakicetid tympanics vertical columns showing the anterior lateral and dorsal views respectively AndashC Pakicetus attocki (H-GSP30386 left) DndashF Ichthyolestes pinfoldi (H-GSP 30223 left) GndashI Nalacetus ratimitus (H-GSP 96384 left) JndashL pakicetid n sp (H-GSP 98141 right)Scale bar equals 2 cm

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species of pakicetids at H-GSP Locality 62 and probably repre-sent a fourth new species of pakicetid These tympanics areclosest to P attocki tympanics in their size

OccipitalmdashThe most anterior point of the basioccipital of Pattocki N ratimitus and I pinfoldi (H-GSP 96231 96386 98134Figs 5AC 7B) is its suture with the basisphenoid which israised into bilateral tubercles (H-GSP 30432 96231 9638696623 98134 Figs 5AC 7B lsquobasioccipital crestrsquo of Luo 1998lsquoincipient falcate process of basioccipitalrsquo of Luo and Gingerich1999) These tubercles for neck flexors (see basisphenoid above)extend onto the anterior side of the basioccipital Posterior tothis the basioccipital projects laterally as a flat process in Paki-cetus (H-GSP 96231) and in Ichthyolestes (H-GSP 98134 Figs5A 7B) but this process is poorly defined in Nalacetus (H-GSP96386) The lateralmost part of this process is formed by theexoccipital (Figs 5AC 7B) and this process also forms the dor-solateral wall of the jugular foramen of pakicetids (H-GSP96231 96386 96407 96431 98134 Figs 5AC 7D) The exoc-cipitals bear the occipital condyles (H-GSP 96231 96386 9643198134 98199 Figs 4A 5AC 6DE 7B 8A 9) which are com-pressed dorsoventrally do not touch in the median plane andare somewhat raised above the level of the exoccipital The dor-sal and ventral condyloid fossae are of equal size (H-GSP 9623198134 98199 Figs 5AC 6E 7D) The hypoglossal foramen inall pakicetid genera (H-GSP 96231 96386 96407 96431 98134Figs 5AC 7D) is situated approximately halfway between thejugular foramen and the occipital condyle One or two smallcondylar foramina (Figs 5AC 7D) pierce the basioccipital lat-eral to the hypoglossal foramen The exoccipital forms the me-diolaterally elongate paroccipital process in all pakicetids (H-GSP 30432 96231 96386 96431 96623 98134 Figs 4B 5AC6ADE 7D) The medial extremity of this process bears a flatfacet for the tympanic (Fig 7D lsquobullar process of the exoccipitalrsquoof Luo and Gingerich 1999) The anterior part of the paroccipi-tal process shares a long suture with the mastoid process of pe-trosal The paroccipital process projects posterolaterally and thepart adjacent to the occipital condyle is recessed

The supraoccipital makes up most of the posterior side of thenuchal crest in all pakicetids (H-GSP 96231 96254 96386 96623Figs 4B 5B 6BD) although this structure is clearly worn insome specimens eg in I pinfoldi (H-GSP 98134) A small bi-lateral process of supraoccipital visible in Pakicetus and Ich-thyolestes (H-GSP 96231 98134 Figs 6DE) for the epaxial ex-tensors of the neck projects from the dorsal edge of the foramenmagnum (H-GSP 96231 96386 98134 Figs 6DE)Variable em-issary foramina (H-GSP 96231 96254 96386 96623 98134 Figs6DE) pierce the occipital dorsal to the foramen magnum Abilateral oval depression (H-GSP 96231 96386 98134 Figs6DE) occurs just lateral to the midline well above the foramenmagnum This depression could be for musculus longissimuscapitis The precise shape of the supraoccipital cannot be deter-mined because the suture between the exoccipital and the su-praoccipital is not visible

DISCUSSION

The pakicetid crania described here help to understand varia-tion in skull morphology of the earliest cetaceans As such theycan be expected to play a significant role in systematic studiesbut a detailed analysis of character scores for these specimensfalls outside the scope of the present description Here discus-sion will focus on a few aspects of cranial morphology that arerarely involved in cetacean systematics although they displayvariation to an extent that may be relevant systematically andinteresting evolutionarily

Supraorbital Morphology

The supraorbital region of protocetids basilosaurids anddorudontids displays a supraorbital shield (Kellogg 1936 Gin-

gerich et al 1994 1995 Bajpai and Gingerich 1998 Hulbert1998 Uhen 1998 2004) However the basal families of ceta-ceans lack a supraorbital shield pakicetids (Thewissen et al2001b) ambulocetids (Thewissen et al 1996) and remingtono-cetids (Gingerich et al 1994 1995 Bajpai and Thewissen 1998)The new pakicetid specimens indicate that there is much varia-tion in supraorbital morphology within these families In paki-cetids the orbits are convergent dorsally The area between theorbits is recessed over the superior orbital rim (Figs 4ndash8) Assuch pakicetid eyes are directed dorsolaterally There are dif-ferences between the genera (Fig 3 Table 1) in Pakicetus andIchthyolestes the supraorbital region is cuplike and cradles thedorsal part of the eye and the dorsal side of the supraorbitalregion is deeply incised in the medial plane in Pakicetus Thisincisure is less strong in Nalacetus and Ichthyolestes and in Nala-cetus the contact between supraorbital region and eye is toonarrow to be cup-shaped In Ambulocetus (Fig 3 Table 1Thewissen et al 1996fig 10 for H-GSP 18490) the orbits arepositioned near the dorsal side of the skull but they face later-ally leaving the supraorbital region more or less flat (in dorsalview) The eyes here appear to be directed laterally and are nothoused in the bony orbits as deeply as in pakicetids as indicatedby the shape of the postorbital processes As a comparison inRemingtonocetus the eyes are small are set laterally and facelaterally and the supraorbital region is strongly convex (Fig 3Sahni and Mishra 1972 Kumar and Sahni 1986 Bajpai andThewissen 1998) In protocetids the orbit is large faces later-ally and is covered by a large supraorbital shield (Fig 3 Table1) Dorudontids and basilosaurids resemble protocetids in thepresence and shape of their supraorbital shield as indicated bythe ratio between the minimum and maximum supraorbitalwidth (Table 1) and in the orientation of their orbits the latterare still clearly different from modern odontocetes and mystice-tes (Kellogg 1936 Uhen 1998 2004)

Intertemporal Morphology

In early and middle Eocene cetaceans a very narrow regioncan be seen in the skull posterior to the frontoparietal suturenear the sphenorbital fissure (Figs 1 2 4ndash8) This morphology isnot seen in most land mammals but is common in early ceta-ceans such as pakicetids ambulocetids (eg AmbulocetusThewissen et al 1996) and remingtonocetids (Kumar and Sahni1986) and may represent an important diagnostic character forthe order

Tympanic Bullar Morphology

The differences in the tympanic bullar morphology seen be-tween P attocki I pinfoldi N ratimitus and a fourth unnamedspecies can be used for specific recognition However in generalall these tympanics display a similar morphology indicating thatthese early archaeocetes used their peripheral ear structures likeland mammals while hearing in air and relied on bone conduc-tion mechanism while hearing in water (Thewissen and Hussain1993 Luo 1998 Luo and Gingerich 1999 Nummela et al 2004)

Pakicetus attocki is clearly larger than I pinfoldi whereas Pa-kicetus and Nalacetus have similar body sizes (Table 1) Thewis-sen and Hussain (1998) studied the size of upper and lower mo-lars in P attocki P inachus I pinfoldi and N ratimitus Theyfound Ichthyolestes to be the smallest P attocki and Nalacetus tobe of similar size and P inachus to be the largest Our studyshows that the absolute tympanic bullar size is similar in Paki-cetus and Ichthyolestes but larger in Nalacetus (Table 2 Appen-dix 1) This implies that in relative terms Pakicetus tympanicswhen related to body size (Table 2) are smaller than those ofIchthyolestes and Nalacetus

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 757

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ACKNOWLEDGMENTS

We thank Ellen Byron and Jeanette Killius for their technicalassistance Linda Spurlock prepared the drawings of the skullswhich is gratefully acknowledged Sunil Bajpai kindly gave usaccess to Remingtonocetus specimens in the IITR-SB collectionsunder his care We thank the editor Ewan Fordyce and Zhe-XiLuo and an anonymous reviewer for constructive suggestions forimprovement of the manuscript Financial support was providedby the National Science Foundation (current grant number EAR0207370) to JGMT and by the Ella and Georg Ehrnrooth Foun-dation to SN This article is a contribution to the Howard Uni-versity-Geological Survey of Pakistan projected entitled ldquoCeno-zoic Mammals of Pakistanrdquo We thank the Geological Survey ofPakistan for their support and help in the field

LITERATURE CITED

Aslan A and J G M Thewissen 1996 Preliminary evaluation of pa-leosols and implications for interpreting vertebrate fossil assem-blages Kuldana Formation Northern Pakistan Palaeovertebrata25261ndash277

Bajpai S and P D Gingerich 1998 A new Eocene archaeocete (Mam-malia Cetacea) from India and the time of origin of whales Pro-ceedings of the National Academy of Sciences USA 9515464ndash15468

Bajpai S and J G M Thewissen 1998 Middle Eocene cetaceans fromthe Harudi and Subathu formations of India pp 213ndash233 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Dehm R and T zu Oettingen-Spielberg 1958 Palaumlontologische undgeologische Untersuchungen im Tertiaumlr von Pakistan 2 Die mitte-leocaumlnen Saumlugetiere von Ganda Kas bei Basal in Nordwest-Pakistan Bayerische Akademie der Wissenschaften 911ndash54

Geisler J H and Z-X Luo 1998 Relationships of Cetacea to terres-trial ungulates and the evolution of cranial vasculature in Cete pp163ndash212 in J G M Thewissen (ed) The Emergence of WhalesPlenum Press New York

Geisler J H A E Sanders and Z-X Luo 2005 A new protocetidwhale (Cetacea Archaeoceti) from the late Middle Eocene of SouthCarolina American Museum Novitates 34801ndash65

Gingerich P D 2003 Land-to-sea transition in early whales evolutionof Eocene Archaeoceti (Cetacea) in relation to skeletal proportionsand locomotion of living semiaquatic mammals Paleobiology 29429ndash454

Gingerich P D and D E Russell 1981 Pakicetus inachus a newarchaeocete (Mammalia Cetacea) from the early-middle EoceneKuldana Formation of Kohat (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 25235ndash246

Gingerich P D and D E Russell 1990 Dentition of early EocenePakicetus (Mammalia Cetacea) Contributions from the Museum ofPaleontology University of Michigan 281ndash20

Gingerich P D N A Wells D E Russell and S M I Shah 1983Origin of whales in epicontinental remnant seas new evidence fromthe early Eocene of Pakistan Science 220403ndash406

Gingerich P D S M Raza M Arif M Anwar and X Zhou 1994New whale from the Eocene of Pakistan and the origin of cetaceanswimming Nature 368844ndash847

Gingerich P D M Arif and W C Clyde 1995 New archaeocetes(Mammalia Cetacea) from the middle Eocene Domanda Formationof the Sulaiman Range Punjab (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 29291ndash330

Gingerich P D M ul-Haq I S Zalmout I H Khan and M S Mal-kani 2001 Origin of whales from early artiodactyls hands and feetof Eocene Protocetidae from Pakistan Science 2932239ndash2242

Hulbert R Jr 1998 Postcranial osteology of the North American

middle Eocene protocetid Georgiacetus pp 235ndash267 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Kellogg A R 1936 A review of the Archaeoceti Carnegie Institute ofWashington Publications 4821ndash366

Kumar K and A Sahni 1986 Remingtonocetus harudiensis new com-bination a middle Eocene archaeocete (Mammalia Cetacea) fromwestern Kutch India Journal of Vertebrate Paleontology 6326ndash349

Luo Z-X 1998 Homology and transformation of cetacean ectotym-panic structures pp 269ndash301 in J G M Thewissen (ed) The Emer-gence of Whales Plenum Press New York

Luo Z-X and P D Gingerich 1999 Terrestrial Mesonychia to aquaticCetacea transformation of the basicranium and evolution of hearingin whales University of Michigan Papers on Paleontology 311ndash98

Madar S I In press The postcranial skeleton of early Eocene pakicetidcetaceans Journal of Paleontology

Nummela S J G M Thewissen S Bajpai S T Hussain and K Ku-mar 2004 Eocene evolution of whale hearing Nature 430776ndash778

Sahni A and V P Mishra 1972 A new species of Protocetus (Cetacea)from the middle Eocene of Kutch Western India Palaeontology15490ndash495

Thewissen J G M and S T Hussain 1993 Origin of underwaterhearing in whales Nature 361444ndash445

Thewissen J G M and S T Hussain 1998 Systematic review of thePakicetidae early and middle Eocene Cetacea (Mammalia) fromPakistan and India Bulletin of Carnegie Museum of Natural History34220ndash238

Thewissen J G M and E M Williams 2002 The early radiations ofCetacea (Mammalia) evolutionary patterns and developmental cor-relations Annual Review of Ecology and Systematics 3373ndash90

Thewissen J G M S I Madar and S T Hussain 1996 Ambulocetusnatans an Eocene cetacean (Mammalia) from Pakistan CourierForschungsinstitut Senckenberg 191 1ndash86

Thewissen J G M E M Williams and S T Hussain 2001a Eocenemammal faunas from Northern Indo-Pakistan Journal of Verte-brate Paleontology 21347ndash366

Thewissen J G M E M Williams L J Roe and S T Hussain 2001bSkeletons of terrestrial cetaceans and the relationship of whales toartiodactyls Nature 413277ndash281

Thewissen J G M S T Hussain M Arif A Aslan S I Madar andL J Roe 1998 New localities of Eocene vertebrates in NorthernPakistan and their significance for the origin of the modern orders ofmammals pp 19ndash34 in M I Ghaznavi S M Raza and M T Hasan(eds) Siwaliks of South Asia Proceedings of the 3rd GEOSASWorkshop Islamabad Pakistan 1ndash5 March 1997 Geological Sur-vey of Pakistan Islamabad Pakistan

Uhen M D 1998 Middle to late Eocene basilosaurines and dorudon-tines pp 29ndash61 in J G M Thewissen (ed) The Emergence ofWhales Plenum Press New York

Uhen M D 2004 Form function and anatomy of Dorudon atrox(Mammalia Cetacea) an archaeocete from the middle to lateEocene of Egypt University of Michigan Papers on Paleontology341ndash222

West R M 1980 Middle Eocene large mammal assemblage withTethyan affinities Ganda Kas region Pakistan Journal of Paleon-tology 54508ndash533

West R M and J R Lukacs 1979 Geology and vertebrate-fossil lo-calities Tertiary continental rocks Kala Chitta Hills Attock Dis-trict Pakistan Milwaukee Public Museum Contributions in Biologyand Geology 261ndash20

Williams E M 1998 Synopsis of the earliest cetaceans PakicetidaeAmbulocetidae Remingtonocetidae and Protocetidae pp 1ndash28 inJ G M Thewissen (ed) The Emergence of Whales Plenum PressNew York

Submitted 7 October 2005 accepted 24 February 2006

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APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 759

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species of pakicetids at H-GSP Locality 62 and probably repre-sent a fourth new species of pakicetid These tympanics areclosest to P attocki tympanics in their size

OccipitalmdashThe most anterior point of the basioccipital of Pattocki N ratimitus and I pinfoldi (H-GSP 96231 96386 98134Figs 5AC 7B) is its suture with the basisphenoid which israised into bilateral tubercles (H-GSP 30432 96231 9638696623 98134 Figs 5AC 7B lsquobasioccipital crestrsquo of Luo 1998lsquoincipient falcate process of basioccipitalrsquo of Luo and Gingerich1999) These tubercles for neck flexors (see basisphenoid above)extend onto the anterior side of the basioccipital Posterior tothis the basioccipital projects laterally as a flat process in Paki-cetus (H-GSP 96231) and in Ichthyolestes (H-GSP 98134 Figs5A 7B) but this process is poorly defined in Nalacetus (H-GSP96386) The lateralmost part of this process is formed by theexoccipital (Figs 5AC 7B) and this process also forms the dor-solateral wall of the jugular foramen of pakicetids (H-GSP96231 96386 96407 96431 98134 Figs 5AC 7D) The exoc-cipitals bear the occipital condyles (H-GSP 96231 96386 9643198134 98199 Figs 4A 5AC 6DE 7B 8A 9) which are com-pressed dorsoventrally do not touch in the median plane andare somewhat raised above the level of the exoccipital The dor-sal and ventral condyloid fossae are of equal size (H-GSP 9623198134 98199 Figs 5AC 6E 7D) The hypoglossal foramen inall pakicetid genera (H-GSP 96231 96386 96407 96431 98134Figs 5AC 7D) is situated approximately halfway between thejugular foramen and the occipital condyle One or two smallcondylar foramina (Figs 5AC 7D) pierce the basioccipital lat-eral to the hypoglossal foramen The exoccipital forms the me-diolaterally elongate paroccipital process in all pakicetids (H-GSP 30432 96231 96386 96431 96623 98134 Figs 4B 5AC6ADE 7D) The medial extremity of this process bears a flatfacet for the tympanic (Fig 7D lsquobullar process of the exoccipitalrsquoof Luo and Gingerich 1999) The anterior part of the paroccipi-tal process shares a long suture with the mastoid process of pe-trosal The paroccipital process projects posterolaterally and thepart adjacent to the occipital condyle is recessed

The supraoccipital makes up most of the posterior side of thenuchal crest in all pakicetids (H-GSP 96231 96254 96386 96623Figs 4B 5B 6BD) although this structure is clearly worn insome specimens eg in I pinfoldi (H-GSP 98134) A small bi-lateral process of supraoccipital visible in Pakicetus and Ich-thyolestes (H-GSP 96231 98134 Figs 6DE) for the epaxial ex-tensors of the neck projects from the dorsal edge of the foramenmagnum (H-GSP 96231 96386 98134 Figs 6DE)Variable em-issary foramina (H-GSP 96231 96254 96386 96623 98134 Figs6DE) pierce the occipital dorsal to the foramen magnum Abilateral oval depression (H-GSP 96231 96386 98134 Figs6DE) occurs just lateral to the midline well above the foramenmagnum This depression could be for musculus longissimuscapitis The precise shape of the supraoccipital cannot be deter-mined because the suture between the exoccipital and the su-praoccipital is not visible

DISCUSSION

The pakicetid crania described here help to understand varia-tion in skull morphology of the earliest cetaceans As such theycan be expected to play a significant role in systematic studiesbut a detailed analysis of character scores for these specimensfalls outside the scope of the present description Here discus-sion will focus on a few aspects of cranial morphology that arerarely involved in cetacean systematics although they displayvariation to an extent that may be relevant systematically andinteresting evolutionarily

Supraorbital Morphology

The supraorbital region of protocetids basilosaurids anddorudontids displays a supraorbital shield (Kellogg 1936 Gin-

gerich et al 1994 1995 Bajpai and Gingerich 1998 Hulbert1998 Uhen 1998 2004) However the basal families of ceta-ceans lack a supraorbital shield pakicetids (Thewissen et al2001b) ambulocetids (Thewissen et al 1996) and remingtono-cetids (Gingerich et al 1994 1995 Bajpai and Thewissen 1998)The new pakicetid specimens indicate that there is much varia-tion in supraorbital morphology within these families In paki-cetids the orbits are convergent dorsally The area between theorbits is recessed over the superior orbital rim (Figs 4ndash8) Assuch pakicetid eyes are directed dorsolaterally There are dif-ferences between the genera (Fig 3 Table 1) in Pakicetus andIchthyolestes the supraorbital region is cuplike and cradles thedorsal part of the eye and the dorsal side of the supraorbitalregion is deeply incised in the medial plane in Pakicetus Thisincisure is less strong in Nalacetus and Ichthyolestes and in Nala-cetus the contact between supraorbital region and eye is toonarrow to be cup-shaped In Ambulocetus (Fig 3 Table 1Thewissen et al 1996fig 10 for H-GSP 18490) the orbits arepositioned near the dorsal side of the skull but they face later-ally leaving the supraorbital region more or less flat (in dorsalview) The eyes here appear to be directed laterally and are nothoused in the bony orbits as deeply as in pakicetids as indicatedby the shape of the postorbital processes As a comparison inRemingtonocetus the eyes are small are set laterally and facelaterally and the supraorbital region is strongly convex (Fig 3Sahni and Mishra 1972 Kumar and Sahni 1986 Bajpai andThewissen 1998) In protocetids the orbit is large faces later-ally and is covered by a large supraorbital shield (Fig 3 Table1) Dorudontids and basilosaurids resemble protocetids in thepresence and shape of their supraorbital shield as indicated bythe ratio between the minimum and maximum supraorbitalwidth (Table 1) and in the orientation of their orbits the latterare still clearly different from modern odontocetes and mystice-tes (Kellogg 1936 Uhen 1998 2004)

Intertemporal Morphology

In early and middle Eocene cetaceans a very narrow regioncan be seen in the skull posterior to the frontoparietal suturenear the sphenorbital fissure (Figs 1 2 4ndash8) This morphology isnot seen in most land mammals but is common in early ceta-ceans such as pakicetids ambulocetids (eg AmbulocetusThewissen et al 1996) and remingtonocetids (Kumar and Sahni1986) and may represent an important diagnostic character forthe order

Tympanic Bullar Morphology

The differences in the tympanic bullar morphology seen be-tween P attocki I pinfoldi N ratimitus and a fourth unnamedspecies can be used for specific recognition However in generalall these tympanics display a similar morphology indicating thatthese early archaeocetes used their peripheral ear structures likeland mammals while hearing in air and relied on bone conduc-tion mechanism while hearing in water (Thewissen and Hussain1993 Luo 1998 Luo and Gingerich 1999 Nummela et al 2004)

Pakicetus attocki is clearly larger than I pinfoldi whereas Pa-kicetus and Nalacetus have similar body sizes (Table 1) Thewis-sen and Hussain (1998) studied the size of upper and lower mo-lars in P attocki P inachus I pinfoldi and N ratimitus Theyfound Ichthyolestes to be the smallest P attocki and Nalacetus tobe of similar size and P inachus to be the largest Our studyshows that the absolute tympanic bullar size is similar in Paki-cetus and Ichthyolestes but larger in Nalacetus (Table 2 Appen-dix 1) This implies that in relative terms Pakicetus tympanicswhen related to body size (Table 2) are smaller than those ofIchthyolestes and Nalacetus

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 757

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ACKNOWLEDGMENTS

We thank Ellen Byron and Jeanette Killius for their technicalassistance Linda Spurlock prepared the drawings of the skullswhich is gratefully acknowledged Sunil Bajpai kindly gave usaccess to Remingtonocetus specimens in the IITR-SB collectionsunder his care We thank the editor Ewan Fordyce and Zhe-XiLuo and an anonymous reviewer for constructive suggestions forimprovement of the manuscript Financial support was providedby the National Science Foundation (current grant number EAR0207370) to JGMT and by the Ella and Georg Ehrnrooth Foun-dation to SN This article is a contribution to the Howard Uni-versity-Geological Survey of Pakistan projected entitled ldquoCeno-zoic Mammals of Pakistanrdquo We thank the Geological Survey ofPakistan for their support and help in the field

LITERATURE CITED

Aslan A and J G M Thewissen 1996 Preliminary evaluation of pa-leosols and implications for interpreting vertebrate fossil assem-blages Kuldana Formation Northern Pakistan Palaeovertebrata25261ndash277

Bajpai S and P D Gingerich 1998 A new Eocene archaeocete (Mam-malia Cetacea) from India and the time of origin of whales Pro-ceedings of the National Academy of Sciences USA 9515464ndash15468

Bajpai S and J G M Thewissen 1998 Middle Eocene cetaceans fromthe Harudi and Subathu formations of India pp 213ndash233 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Dehm R and T zu Oettingen-Spielberg 1958 Palaumlontologische undgeologische Untersuchungen im Tertiaumlr von Pakistan 2 Die mitte-leocaumlnen Saumlugetiere von Ganda Kas bei Basal in Nordwest-Pakistan Bayerische Akademie der Wissenschaften 911ndash54

Geisler J H and Z-X Luo 1998 Relationships of Cetacea to terres-trial ungulates and the evolution of cranial vasculature in Cete pp163ndash212 in J G M Thewissen (ed) The Emergence of WhalesPlenum Press New York

Geisler J H A E Sanders and Z-X Luo 2005 A new protocetidwhale (Cetacea Archaeoceti) from the late Middle Eocene of SouthCarolina American Museum Novitates 34801ndash65

Gingerich P D 2003 Land-to-sea transition in early whales evolutionof Eocene Archaeoceti (Cetacea) in relation to skeletal proportionsand locomotion of living semiaquatic mammals Paleobiology 29429ndash454

Gingerich P D and D E Russell 1981 Pakicetus inachus a newarchaeocete (Mammalia Cetacea) from the early-middle EoceneKuldana Formation of Kohat (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 25235ndash246

Gingerich P D and D E Russell 1990 Dentition of early EocenePakicetus (Mammalia Cetacea) Contributions from the Museum ofPaleontology University of Michigan 281ndash20

Gingerich P D N A Wells D E Russell and S M I Shah 1983Origin of whales in epicontinental remnant seas new evidence fromthe early Eocene of Pakistan Science 220403ndash406

Gingerich P D S M Raza M Arif M Anwar and X Zhou 1994New whale from the Eocene of Pakistan and the origin of cetaceanswimming Nature 368844ndash847

Gingerich P D M Arif and W C Clyde 1995 New archaeocetes(Mammalia Cetacea) from the middle Eocene Domanda Formationof the Sulaiman Range Punjab (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 29291ndash330

Gingerich P D M ul-Haq I S Zalmout I H Khan and M S Mal-kani 2001 Origin of whales from early artiodactyls hands and feetof Eocene Protocetidae from Pakistan Science 2932239ndash2242

Hulbert R Jr 1998 Postcranial osteology of the North American

middle Eocene protocetid Georgiacetus pp 235ndash267 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Kellogg A R 1936 A review of the Archaeoceti Carnegie Institute ofWashington Publications 4821ndash366

Kumar K and A Sahni 1986 Remingtonocetus harudiensis new com-bination a middle Eocene archaeocete (Mammalia Cetacea) fromwestern Kutch India Journal of Vertebrate Paleontology 6326ndash349

Luo Z-X 1998 Homology and transformation of cetacean ectotym-panic structures pp 269ndash301 in J G M Thewissen (ed) The Emer-gence of Whales Plenum Press New York

Luo Z-X and P D Gingerich 1999 Terrestrial Mesonychia to aquaticCetacea transformation of the basicranium and evolution of hearingin whales University of Michigan Papers on Paleontology 311ndash98

Madar S I In press The postcranial skeleton of early Eocene pakicetidcetaceans Journal of Paleontology

Nummela S J G M Thewissen S Bajpai S T Hussain and K Ku-mar 2004 Eocene evolution of whale hearing Nature 430776ndash778

Sahni A and V P Mishra 1972 A new species of Protocetus (Cetacea)from the middle Eocene of Kutch Western India Palaeontology15490ndash495

Thewissen J G M and S T Hussain 1993 Origin of underwaterhearing in whales Nature 361444ndash445

Thewissen J G M and S T Hussain 1998 Systematic review of thePakicetidae early and middle Eocene Cetacea (Mammalia) fromPakistan and India Bulletin of Carnegie Museum of Natural History34220ndash238

Thewissen J G M and E M Williams 2002 The early radiations ofCetacea (Mammalia) evolutionary patterns and developmental cor-relations Annual Review of Ecology and Systematics 3373ndash90

Thewissen J G M S I Madar and S T Hussain 1996 Ambulocetusnatans an Eocene cetacean (Mammalia) from Pakistan CourierForschungsinstitut Senckenberg 191 1ndash86

Thewissen J G M E M Williams and S T Hussain 2001a Eocenemammal faunas from Northern Indo-Pakistan Journal of Verte-brate Paleontology 21347ndash366

Thewissen J G M E M Williams L J Roe and S T Hussain 2001bSkeletons of terrestrial cetaceans and the relationship of whales toartiodactyls Nature 413277ndash281

Thewissen J G M S T Hussain M Arif A Aslan S I Madar andL J Roe 1998 New localities of Eocene vertebrates in NorthernPakistan and their significance for the origin of the modern orders ofmammals pp 19ndash34 in M I Ghaznavi S M Raza and M T Hasan(eds) Siwaliks of South Asia Proceedings of the 3rd GEOSASWorkshop Islamabad Pakistan 1ndash5 March 1997 Geological Sur-vey of Pakistan Islamabad Pakistan

Uhen M D 1998 Middle to late Eocene basilosaurines and dorudon-tines pp 29ndash61 in J G M Thewissen (ed) The Emergence ofWhales Plenum Press New York

Uhen M D 2004 Form function and anatomy of Dorudon atrox(Mammalia Cetacea) an archaeocete from the middle to lateEocene of Egypt University of Michigan Papers on Paleontology341ndash222

West R M 1980 Middle Eocene large mammal assemblage withTethyan affinities Ganda Kas region Pakistan Journal of Paleon-tology 54508ndash533

West R M and J R Lukacs 1979 Geology and vertebrate-fossil lo-calities Tertiary continental rocks Kala Chitta Hills Attock Dis-trict Pakistan Milwaukee Public Museum Contributions in Biologyand Geology 261ndash20

Williams E M 1998 Synopsis of the earliest cetaceans PakicetidaeAmbulocetidae Remingtonocetidae and Protocetidae pp 1ndash28 inJ G M Thewissen (ed) The Emergence of Whales Plenum PressNew York

Submitted 7 October 2005 accepted 24 February 2006

JOURNAL OF VERTEBRATE PALEONTOLOGY VOL 26 NO 3 2006758

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APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 759

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ACKNOWLEDGMENTS

We thank Ellen Byron and Jeanette Killius for their technicalassistance Linda Spurlock prepared the drawings of the skullswhich is gratefully acknowledged Sunil Bajpai kindly gave usaccess to Remingtonocetus specimens in the IITR-SB collectionsunder his care We thank the editor Ewan Fordyce and Zhe-XiLuo and an anonymous reviewer for constructive suggestions forimprovement of the manuscript Financial support was providedby the National Science Foundation (current grant number EAR0207370) to JGMT and by the Ella and Georg Ehrnrooth Foun-dation to SN This article is a contribution to the Howard Uni-versity-Geological Survey of Pakistan projected entitled ldquoCeno-zoic Mammals of Pakistanrdquo We thank the Geological Survey ofPakistan for their support and help in the field

LITERATURE CITED

Aslan A and J G M Thewissen 1996 Preliminary evaluation of pa-leosols and implications for interpreting vertebrate fossil assem-blages Kuldana Formation Northern Pakistan Palaeovertebrata25261ndash277

Bajpai S and P D Gingerich 1998 A new Eocene archaeocete (Mam-malia Cetacea) from India and the time of origin of whales Pro-ceedings of the National Academy of Sciences USA 9515464ndash15468

Bajpai S and J G M Thewissen 1998 Middle Eocene cetaceans fromthe Harudi and Subathu formations of India pp 213ndash233 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Dehm R and T zu Oettingen-Spielberg 1958 Palaumlontologische undgeologische Untersuchungen im Tertiaumlr von Pakistan 2 Die mitte-leocaumlnen Saumlugetiere von Ganda Kas bei Basal in Nordwest-Pakistan Bayerische Akademie der Wissenschaften 911ndash54

Geisler J H and Z-X Luo 1998 Relationships of Cetacea to terres-trial ungulates and the evolution of cranial vasculature in Cete pp163ndash212 in J G M Thewissen (ed) The Emergence of WhalesPlenum Press New York

Geisler J H A E Sanders and Z-X Luo 2005 A new protocetidwhale (Cetacea Archaeoceti) from the late Middle Eocene of SouthCarolina American Museum Novitates 34801ndash65

Gingerich P D 2003 Land-to-sea transition in early whales evolutionof Eocene Archaeoceti (Cetacea) in relation to skeletal proportionsand locomotion of living semiaquatic mammals Paleobiology 29429ndash454

Gingerich P D and D E Russell 1981 Pakicetus inachus a newarchaeocete (Mammalia Cetacea) from the early-middle EoceneKuldana Formation of Kohat (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 25235ndash246

Gingerich P D and D E Russell 1990 Dentition of early EocenePakicetus (Mammalia Cetacea) Contributions from the Museum ofPaleontology University of Michigan 281ndash20

Gingerich P D N A Wells D E Russell and S M I Shah 1983Origin of whales in epicontinental remnant seas new evidence fromthe early Eocene of Pakistan Science 220403ndash406

Gingerich P D S M Raza M Arif M Anwar and X Zhou 1994New whale from the Eocene of Pakistan and the origin of cetaceanswimming Nature 368844ndash847

Gingerich P D M Arif and W C Clyde 1995 New archaeocetes(Mammalia Cetacea) from the middle Eocene Domanda Formationof the Sulaiman Range Punjab (Pakistan) Contributions from theMuseum of Paleontology University of Michigan 29291ndash330

Gingerich P D M ul-Haq I S Zalmout I H Khan and M S Mal-kani 2001 Origin of whales from early artiodactyls hands and feetof Eocene Protocetidae from Pakistan Science 2932239ndash2242

Hulbert R Jr 1998 Postcranial osteology of the North American

middle Eocene protocetid Georgiacetus pp 235ndash267 in J G MThewissen (ed) The Emergence of Whales Plenum Press NewYork

Kellogg A R 1936 A review of the Archaeoceti Carnegie Institute ofWashington Publications 4821ndash366

Kumar K and A Sahni 1986 Remingtonocetus harudiensis new com-bination a middle Eocene archaeocete (Mammalia Cetacea) fromwestern Kutch India Journal of Vertebrate Paleontology 6326ndash349

Luo Z-X 1998 Homology and transformation of cetacean ectotym-panic structures pp 269ndash301 in J G M Thewissen (ed) The Emer-gence of Whales Plenum Press New York

Luo Z-X and P D Gingerich 1999 Terrestrial Mesonychia to aquaticCetacea transformation of the basicranium and evolution of hearingin whales University of Michigan Papers on Paleontology 311ndash98

Madar S I In press The postcranial skeleton of early Eocene pakicetidcetaceans Journal of Paleontology

Nummela S J G M Thewissen S Bajpai S T Hussain and K Ku-mar 2004 Eocene evolution of whale hearing Nature 430776ndash778

Sahni A and V P Mishra 1972 A new species of Protocetus (Cetacea)from the middle Eocene of Kutch Western India Palaeontology15490ndash495

Thewissen J G M and S T Hussain 1993 Origin of underwaterhearing in whales Nature 361444ndash445

Thewissen J G M and S T Hussain 1998 Systematic review of thePakicetidae early and middle Eocene Cetacea (Mammalia) fromPakistan and India Bulletin of Carnegie Museum of Natural History34220ndash238

Thewissen J G M and E M Williams 2002 The early radiations ofCetacea (Mammalia) evolutionary patterns and developmental cor-relations Annual Review of Ecology and Systematics 3373ndash90

Thewissen J G M S I Madar and S T Hussain 1996 Ambulocetusnatans an Eocene cetacean (Mammalia) from Pakistan CourierForschungsinstitut Senckenberg 191 1ndash86

Thewissen J G M E M Williams and S T Hussain 2001a Eocenemammal faunas from Northern Indo-Pakistan Journal of Verte-brate Paleontology 21347ndash366

Thewissen J G M E M Williams L J Roe and S T Hussain 2001bSkeletons of terrestrial cetaceans and the relationship of whales toartiodactyls Nature 413277ndash281

Thewissen J G M S T Hussain M Arif A Aslan S I Madar andL J Roe 1998 New localities of Eocene vertebrates in NorthernPakistan and their significance for the origin of the modern orders ofmammals pp 19ndash34 in M I Ghaznavi S M Raza and M T Hasan(eds) Siwaliks of South Asia Proceedings of the 3rd GEOSASWorkshop Islamabad Pakistan 1ndash5 March 1997 Geological Sur-vey of Pakistan Islamabad Pakistan

Uhen M D 1998 Middle to late Eocene basilosaurines and dorudon-tines pp 29ndash61 in J G M Thewissen (ed) The Emergence ofWhales Plenum Press New York

Uhen M D 2004 Form function and anatomy of Dorudon atrox(Mammalia Cetacea) an archaeocete from the middle to lateEocene of Egypt University of Michigan Papers on Paleontology341ndash222

West R M 1980 Middle Eocene large mammal assemblage withTethyan affinities Ganda Kas region Pakistan Journal of Paleon-tology 54508ndash533

West R M and J R Lukacs 1979 Geology and vertebrate-fossil lo-calities Tertiary continental rocks Kala Chitta Hills Attock Dis-trict Pakistan Milwaukee Public Museum Contributions in Biologyand Geology 261ndash20

Williams E M 1998 Synopsis of the earliest cetaceans PakicetidaeAmbulocetidae Remingtonocetidae and Protocetidae pp 1ndash28 inJ G M Thewissen (ed) The Emergence of Whales Plenum PressNew York

Submitted 7 October 2005 accepted 24 February 2006

JOURNAL OF VERTEBRATE PALEONTOLOGY VOL 26 NO 3 2006758

Dow

nloa

ded

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McM

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2014

APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

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APPENDIX 1 The material listed with the specimen number the taxon and the bones studied

Specimen no Taxon Skeletal material

H-GSP 18467 Pakicetus attocki Right premaxilla with base of crown for I3 and alveoli for I1-2 and C1

H-GSP 18470 Pakicetus attocki Right maxilla with P4-M3 including full width of right palate and facialfragment of right maxilla1

H-GSP 30119 Pakicetid Left tympanic bulla rostroventral and dorsal parts missingH-GSP 30120 Pakicetid Left tympanic bulla rostral and rostroventral parts missingH-GSP 30142 Pakicetid Right tympanic bulla rostral tip of involucrum and caudal part of lateral wall

missing (2422)H-GSP 30150 Pakicetid Right tympanic bulla dorsal part of involucrum missing ventrolateral and

caudolateral parts missing but internal cast seenH-GSP 30181 Ichthyolestes pinfoldi Right tympanic bulla rostrolateral parts missingH-GSP 30198 Pakicetid n sp Right tympanic bulla caudal and caudomedial parts missing (2639)H-GSP 30223 Ichthyolestes pinfoldi Left tympanic bulla (2737)H-GSP 30306 Nalacetus ratimitus Left maxilla with P2-4 including infraorbital canal and fragment of maxillary

sinusH-GSP 30325 Nalacetus ratimitus Right tympanic bulla medial part (involucrum)H-GSP 30379 Pakicetid Right tympanic bulla dorsal and rostral parts missingH-GSP 30386 Pakicetus attocki Left tympanic bulla dorsolateral part missing (2535)H-GSP 30387 Pakicetid Right tympanic bulla (2355)H-GSP 30389 Pakicetid Left tympanic bulla rostral and dorsal parts missingH-GSP 30398 Pakicetid Unassociated parts of tympanic bullaH-GSP 30424 Pakicetid n sp Right tympanic bulla dorsal part missing (2803)H-GSP 30432 Nalacetus ratimitus Basicranium with left ear region and zygomatic arch but lacking right squamosal

and occipitalH-GSP 30459 Nalacetus ratimitus Right tympanic bulla dorsolateral parts missing (3180)H-GSP 91035 Pakicetus attocki Left tympanic bulla ventral and caudal parts missing (gt2615) Left incus2

H-GSP 92003 Ichthyolestes pinfoldi Left tympanic bulla3 (2636)H-GSP 92100 Pakicetus attocki Left tympanic bulla (2665)H-GSP 96055 Nalacetus ratimitus Left maxilla with P4 fragment and M1-2H-GSP 96192 Pakicetus attocki Left tympanic bulla part of lateral wall missing (2694)H-GSP 96231 Pakicetus attocki Skull including braincase complete sagittal crest orbits and nasals but lacking

palatal and pterygoid regions left lacrimal left ear region and both tympanics4

H-GSP 96254 Pakicetus attocki Braincase and orbital regions with sagittal and nuchal crests but lackingbasicranium and squamosal

H-GSP 96283 Ichthyolestes pinfoldi Left tympanic bulla part of lateral wall missing but with internal castdorsolateral part missing (2536)

H-GSP 96343 Pakicetid Endocast with some bone fragments of the braincase attachedH-GSP 96344 Pakicetus attocki Left tympanic bulla dorsolateral part missing5 (2488)H-GSP 96354 Pakicetus attocki Left superior orbitH-GSP 96384 Nalacetus ratimitus Left tympanic bulla part of lateral wall missing (2873)H-GSP 96385 Nalacetus ratimitus Left tympanic bulla part of ventrolateral wall missing but with internal cast

(2968)H-GSP 96386 Nalacetus ratimitus Braincase with right orbital region including left squamosal and ear region and

fragmentary pterygoid and left and right nasals lacks tympanic6

H-GSP 96407 Ichthyolestes pinfoldi Left temporal region with periotic and cochlea preserved in part as endocastH-GSP 96431 Ichthyolestes pinfoldi Fragment of left basicranium with lateral part of occipital condyle and

paroccipital process left promontorium of petrosal Left tympanic bulla(2337)

H-GSP 96506 Pakicetid Left tympanic bulla rostromedial and caudolateral parts missingH-GSP 96541 Pakicetid Right tympanic bulla lateral wall missing but with internal castH-GSP 96574 Ichthyolestes pinfoldi Orbital fragment including supraorbital and interorbital region7

H-GSP 96623 Pakicetus attocki juvenile Skull with braincase orbits right ear region sagittal crest right side of nuchalcrest but lacking the left temporal region

H-GSP 98134 Ichthyolestes pinfoldi Skull with braincase orbits nasals ear regions but lacking palatine regionsagittal and nuchal crests

H-GSP 98138 Pakicetid Right tympanic bulla rostral part missingH-GSP 98140 Nalacetus ratimitus Internal cast of right tympanic bullaH-GSP 98141 Pakicetid n sp Right tympanic bulla (2676)H-GSP 98143 Pakicetid Internal cast of left tympanic bullaH-GSP 98199 Pakicetus attocki Fragmentary crushed braincase with left occipital condyle and parts of left

tympanic bulla and pterygoidsH-GSP 98220 Pakicetid Right tympanic bulla rostral and dorsal parts missing

The tympanic bullar length (TBL in mm) is given for tympanics in parentheses when available For more data on the TBL values see Table 21Illustrated and described by Thewissen and Hussain (1998)2Incus described by Thewissen and Hussain (1993)3Listed by Luo (1998) under its museum number 18391 but also incorrectly as 1839314Illustrated and summary description in Thewissen et al (2001b)5Described with a transposed number H-GSP 96334 by Luo (1998) and by Luo and Gingerich (1999)6Described by Thewissen and Hussain (1998)7Orbital morphology differs from H-GSP 96231

NUMMELA ET ALmdashPAKICETID CRANIAL ANATOMY 759

Dow

nloa

ded

by [

McM

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r U

nive

rsity

] at

15

31 1

8 D

ecem

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2014