253SAURORNITHOLESTES ROBUSTUS, N. SP. (THEROPODA: DROMAEOSAURIDAE) FROM

THE UPPER CRETACEOUS KIRTLAND FORMATION (DE-NA-ZIN MEMBER),SAN JUAN BASIN, NEW MEXICO

ROBERT M. SULLIVAN

Section of Paleontology and Geology, The State Museum of Pennsylvania, 300 North Street, Harrisburg, PA 17120-0024

Abstract—Saurornitholestes robustus, n. sp., is named based on new frontal material from the upper part of theKirtland Formation (De-na-zin Member), San Juan Basin, New Mexico. The frontal is larger, and unusually thick,compared to specimens of Saurornitholestes langstoni. All specimens of Saurornitholestes previously collected fromthe San Juan Basin, and referred to the Judithian taxon S. langstoni, are now provisionally referred to this newspecies. A single tooth and a left second pedal ungual are referred to cf. Saurornitholestes robustus. The recognitionof a new species of Saurornitholestes from the Upper Cretaceous strata of the San Juan Basin, further supports thedistinct temporal position of the upper Fruitland and Kirtland vertebrate faunas, and is consistent in the recognition ofa Kirtlandian land-vertebrate “age” fauna.

INTRODUCTION

Ongoing fieldwork in the Upper Cretaceous strata of the San JuanBasin, New Mexico, continues to add to our knowledge of dinosaur andother fossil vertebrates during the Kirtlandian time interval (Sullivan andLucas, 2003, 2006). Previously, Sullivan and Lucas (2000) reported on awater-worn left frontal of a small theropod dinosaur that compared favor-ably to specimens of Saurornitholestes langstoni, with the exception thatthe anterior part of the New Mexico specimen was slightly more constrictedanteriorly. In the summer of 2005, a larger, more robust, left frontal, refer-able to the genus Saurornitholestes, was recovered from the upper part ofthe Kirtland Formation (De-na-zin Member) in Alamo Wash. This newfrontal forces a reconsideration of the identity of the previously reportedspecimen (SMP VP-1270). Here, I report on this new specimen (SMP VP-1955), and designate a new species of Saurornitholestes, S. robustus, thatadds to our knowledge of Late Cretaceous North American velociraptorinesand specifically the Kirtlandian vertebrate fauna. In this paper, AMNH =American Museum of Natural History (New York); GIN (IGM) = Mongo-lian Institute of Geology (Ulan Bataar); SMP = The State Museum of Penn-sylvania, Harrisburg; and TMP = Royal Tyrrell Museum of Palaeontology,Drumheller.

SYSTEMATIC PALEONTOLOGY

DINOSAURIASAURISCHIA Seeley, 1888

THEROPODA Marsh, 1881DROMAEOSAURIDAE Matthew and Brown, 1922

Velociraptorinae Barsbold, 1983SAURORNITHOLESTES Sues, 1978

Type species— Saurornitholestes langstoni Sues, 1978.Comments— Previously, Saurornitholestes has been considered

to a monotypic taxon known from skeletal material from the DinosaurPark Formation of Alberta, Two Medicine Formation of Montana and theKirtland Formation of New Mexico (Currie, 2005; Sullivan and Lucas,2000). Other reports, based solely on teeth, have also been made by Baszio(1997a,b), Currie et al. (1990), Peng et al. (2001), Rowe et al. (1992),Sankey (1998, 2003) and Sankey et al. (2002, 2005). The referral of speci-mens to cf. Saurornitholestes langstoni from the Horseshoe Canyon andScollard formations by Baszio (1997b) should be viewed with skepticism.Baszio (1997b), himself, noted that isolated teeth from the Scollard, Lanceand Frenchman formations may actually belong to another theropod. Infact, the newly named Atrociraptor from the Horseshoe Canyon Forma-tion of Alberta, has a skull that is very different from Saurornitholestes

(e.g., a shorter and deeper face), but its teeth are almost indistinguishablefrom Saurornitholestes (Currie and Varricchio, 2004). For the present, itseems that the genus Saurornitholestes is restricted to the Late Campanian(Judithian and Kirtlandian).

Saurornitholestes robustus, n. sp.

Holotype— SMP VP-1955, nearly complete left frontal.Horizon and Type Locality— De-na–zin Member of the Kirtland

Formation. The type locality is SMP loc. no. 388 (Alamo Wash [north fork]);UTM data is on file and available to qualified researchers.

Age— late Kirtlandian (73 Ma; see Sullivan and Lucas, 2006).Diagnosis— A species of Saurornitholestes distinguished from

Saurornitholestes langstoni by its larger and more robust frontal(twice as thick).

Description— The holotype (SMP VP-1955, Fig. 1) is a nearlycomplete left frontal. There is some damage behind the orbital region to-ward the posterior process and there is also some minor damage along theanterior-most part of the orbital adjacent to the lacrimal facet. The dorsaland lateral (orbital) surfaces are weathered and splintered, giving the fron-tal a rather rough appearance. The ventral and medial surfaces are largelyunweathered. Despite the weathered and less than complete condition, thefrontal is adequate for diagnostic purposes.

The frontal measures (anteroposteriorly along the midline) 62 mmin length. Maximum width cannot be established with certainty because ofthe damage along the lateral portion of the postorbital, but I estimate it atabout 30 mm.

Dorsally, the frontal is relatively flat, rising slightly toward the rimof the orbital border. There is a slight depression on the dorsal surface abovewhere the confluence of the posterior margin of the olfactory bulb and theanterior margin of the cerebral hemisphere impression occur (on the ven-tral side). The area of the frontal-nasal contact is damaged, both mediallyand laterally. The lacrimal contact surface is intact, and is well-developedon the anterolateral side of the frontal, but it is slightly damaged along itslateroposterior-most portion at the front of the orbital rim.

Ventrally, the olfactory bulb and cerebral hemisphere impressionsare present, although the former is weakly developed. Anteriorly, the fron-tal is relatively thick and presumably had a strong contact with the nasal.The olfactory bulb impression is less pronounced than that of SMP VP-1270 (Sullivan and Lucas, 2000), but that may be due to the fact that SMPVP-1270 is water-worn. The bridge between the olfactory bulb impressionand the cerebral hemisphere impression is not as constricted as in SMPVP-1270, again probably for the same reasons. The cerebral hemisphereimpression is well-defined, bordered laterally by the prominent descendingorbital border. The frontoparietal articulation surface is weakly developedand slightly damaged.

Lucas, S. G. and Sullivan, R.M., eds., 2006, Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin 35.

254

FIGURE 1. Saurornitholestes robustus, n. sp. SMP VP-1955 (holotype), nearly complete left frontal, from SMP loc. # 388, De-na-zin Member, KirtlandFormation. A, dorsal view (stereo pair); B, ventral view (stereo pair); C, orbital rim (lateral view) (dorsal direction is to the right); D, medial view of midlinesutural surface (dorsal direction is to the left). Bar scale = 10 mm.

255

Median suture thickness is 8 mm, measured at the confluence ofthe posterior margin of the olfactory bulb and the anterior margin of thecerebral hemisphere impression. The maximum thickness of the frontal iswhere the posterior border descends ventrally. Here, the frontal measures10 mm thick, and is more than twice the thickness of the holotype (TMP74.10.5) in this region. The median suture surface is very rugose.

The lateral surface of the orbital wall is weathered. No foramina arevisible, due to the condition of the lateral surface.

Comments— The massive nature of the holotype frontal (SMP VP-1955) clearly distinguishes it from the holotype of Saurornithololesteslangstoni (TMP 74.10.5), leaving little doubt as to its specific distinctive-ness. The ratio of the length (measured along the midline) to the thickness(posterior part of the frontal) is 6:1 for S. robustus and 10:1 for S. langstoni.Moreover, the holotype of S. robustus differs from referred specimens ofVelociraptor mongoliensis (Barsbold and Osmólska, 1999), although theyshare a number of features (i.e., length/width ratio; shallow depression alongmid-length of frontal; strong frontal nasal contact; lacrimal overlapping thefrontal dorsally, etc.), which are herein considered characters ofvelociraptorines in general. Thus, there are sufficient reasons to retain bothSaurornitholestes and Velociraptor as distinct genera.

The frontal, once attributed to the species S. langstoni, has all themorphological attributes of the genus. These include: 1) triangular shape;2) posteroventral projection of the frontoparietal contact; 3) sigmoidallycurved ridge running anterolaterally onto the frontal margin of the postor-bital process; and 4) posterior concavity around the supratemporal fossae(Sues, 1978; Currie, 1987; Sullivan and Lucas, 2000). The specific differ-ence is the massive thickness of the frontal and somewhat larger size.

Cf. Saurornitholestes robustus

Material— SMP VP-1901, tooth (Fig. 2), SMP loc #350; and SMPVP-1741, left second pedal ungual (Fig. 3), SMP loc. #361. Both speci-mens were collected from the De-na-zin Member, Kirtland Formation, SanJuan Basin, New Mexico

Comments— The tooth (SMP VP-1901) is nearly complete and ischaracteristically laterally compressed and strongly recurved. It has a FABL(fore-aft basal length) of 6.5 mm. The height of the tooth is 12 mm, mea-

sured from the base to the tip of the crown along the posterior carina; 15mm along the anterior carina. Denticles are present along both the anteriorand posterior carinae, and they are slightly hooked toward the tip. Some ofthe denticles are damaged, especially along the anterior edge towards thebase. The enamel veneer is largely missing, with only remnants adheringto the tooth in the vicinity of the denticles on both the anterior and posteriorcarinae. Although somewhat larger than most Saurornitholetes teeth, thistooth cannot be distinguished from those of Saurornitholestes (S. langstoni)as illustrated by Currie et al. (1990), Sankey et al. (2002) and Brinkman(2005).

The second pedal ungual (SMP VP-1741) is incomplete both proxi-mally and distally, making it difficult to accurately determine its total size.However, it measures 69 mm along the preserved outside curvature. It islaterally compressed and possesses a prominent lateral groove on each side.This groove is situated slightly higher on the left lateral side compared tothe right side, thus it is from the left pes (Kirtland et al., 1993; Rahut andWerner, 1995; Norell and Makovicky, 1997). The pedal ungual is deeperthan that of Velociraptor mongoliensis (IGM 100/985) described and il-lustrated by Norell and Makovicky (1997, fig. 6).

DISCUSSION

This is the second frontal of Saurornitholestes known from the SanJuan Basin, New Mexico, and arguably it is the most important, as it docu-ments features that serve to distinguish it from its presumed predecessorSaurornitholestes langstoni. The previous report of Saurornitholestes fromthe Kirtland Formation was also based on a left frontal from the San JuanBasin (Sullivan and Lucas, 2000). However, this specimen (SMP VP-1270)was smaller, slightly water-worn, and in some respects, similar to the holo-type of S. langstoni. At the time of its report, some minor differences werenoted, but these were relegated to individual variation (Sullivan and Lucas,2000). A re-evaluation of SMP VP-1270, compared to the holotype of S.langstoni (TMP 74.10.5), which are the same size, shows it to be morelike the holotype of S. robustus in having a thicker frontal. A single tooth(SMP VP-1901) and a left second pedal ungual (SMP VP-1714) are bothfrom the De-na-zin Member of the Kirtland Formation and clearly pertainto Saurornitholestes (Figs. 2, 3). They are provisionally referred to S.

FIGURE 2. Cf. Saurornitholestes robustus, n. sp. SMP VP-1901 (referredspecimen), islolated tooth, from SMP loc. # 350, De-na-zin Member, KirtlandFormation. Bar Scale = 5 mm.

FIGURE 3. Cf. Saurornitholestes robustus, n. sp. SMP VP-1741 (referredspecimen), left pes ungual (digit II), from SMP loc. # 361, De-na-zin Member,Kirtland Formation. A, medial view; B, lateral view. Bar Scale = 10 mm.

256robustus based on geographic and stratigraphic parsimony.

Two species of Saurornitholestes are now known: S. langstoni andS. robustus. Currie (2005) cited three undescribed partial skeletons, twofrom the Dinosaur Park Formation of Alberta and one from the Two Medi-cine Formation of Montana.

Although Saurornitholestes is considered to be similar to the Asiandromaeosaurid Velociraptor, recognition of two separate genera has beenthe consensus of most workers (Currie, 2005). Differences in the postcra-nial skeleton have already been documented by Norell and Makovicky(1997), and the deeper pedal ungual in Saurornitholestes robustus, if prop-erly referred, further reinforces the argument for generic and specific dis-tinction. Although the frontals are not preserved in the holotype ofVelociraptor mongoliensis (AMNH 6515) (Osborn, 1924; Sues, 1977),frontals are known from referred material (GIN 1000/24, 100/25) describedin detail by Barsbold and Osmólska (1999). Comparison of the frontal ma-terial to that seen in a cast of skull of Velociraptor mongoliensis from aprivate collection, also verify the taxonomic distinction between the generaVelociraptor and Sauronitholestes.

All specimens of Saurornitholestes robustus come from the De-na-zin Member of the Kirtland Formation, which is Kirtlandian age. The dura-tion of the Kirtlandian is approximately 2 million years (74.8 to 72.8 Ma)based on recent correlations (see Sullivan and Lucas, 2006). The fossilsfrom the De-na-zin Member can be more precisely dated at 73 Ma basedon the 40Ar/39Ar dates of 73.04 Ma (Ash J) and 73.37 Ma (Ash H), pub-

lished by Fassett and Steiner (1999), which bracket the upper and lowerportions of the De-na-zin Member. By contrast, specimens ofSaurornitholestes (S. langstoni) from the Dinosaur Park and Two Medi-cine formations are considerably older, 75 Ma at the very least (Eberth,2005). It is not unreasonable to conclude that given the age difference, thatthe morphological differences between the species of Saurornitholestesare significant. Despite ongoing claims of endemic faunas and provincial-ism of Western Interior dinosaurs during the Late Cretaceous (see Sullivanand Lucas, 2006, for citations), these differences are mostly due to tempo-ral differences, not biogeographic ones. The cosmopolitan nature of thedinosaur faunas during the Late Cretaceous is borne out by the genericsimilarities of faunas from the north to the south. The coastal plain alongthe western margin of the Western Interior seaway provided a barrier freecorridor for dinosaur dispersal.

ACKNOWLEDGMENTS

I thank James Gardner (Royal Tyrrell Museum of Palaeontology,Drumheller) and Phil J. Currie (University of Alberta, Edmonton) for ac-cess to comparative material; and Phil J. Currie and Don Brinkman (RoyalTyrrell Museum of Palaeontology, Drumheller) for discussions concerningSaurornitholestes. Don Brinkman and Spencer G. Lucas reviewed thiscontribution and I thank them for the comments and suggestions.

Barsbold, R. and Osmólska, H., 1999, The skull of Velociraptor (Theropoda) fromthe Late Cretaceous of Mongolia: Acta Palaeontologica Polonica, v. 44, p.189-219.

Baszio, S., 1997a, Palaeoecology of dinosaur assemblages throughout the Late Cre-taceous of South Alberta, Canada: Courier Forschungsinstitut Senckenberg, v.196, p. 1-31.

Baszio, S., 1997b, Systematic Palaeontology isolated dinosaur teeth from the latestCretaceous of south Alberta, Canada: Courier Forschungsinstitut Senckenberg,v. 196, p. 33-77.

Brinkman, D., 2005, An illustrated guide to the vertebrate microfossils from theDinosaur Park Formation: Unpublished guide, prepared for the Alberta Paleon-tological Society, Workshop on vertebrate microfossils (Jan. 26, 2002); revisedApril 3, 2005, 141 p.

Currie, P.J., 1987, Theropods of the Judith River Formation of Dinosaur ProvincialPark, Alberta, Canada; in Currie, P.J. and Koster, E.H., eds., Fourth Symposiumon Mesozoic Terrestrial Ecosystems, Short Papers, v. 3: Drumheller, TyrrellMuseum of Palaeontology, p. 52-60.

Currie, P.J., 2005, Theropoda, including birds; in Currie, P.J. and Koppelhus, E.B.,eds., Dinosaur Provincial Park: Bloomington and Indianapolis, Indiana Univer-sity Press, p. 367-397.

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Eberth, D.A., 2005, The geology; in Currie, P.J. and Koppelhus, E.B., eds., Dino-saur Provincial Park: Bloomington and Indianapolis, Indiana University Press,p. 367-397.

Fassett, J.E. and Steiner, M.B., 1997, Precise age of C33N-C32R magnetic-polarityreversal, San Juan Basin, New Mexico and Colorado: New Mexico GeologicalSociety, Guidebook 48: 239-247.

Kirkland, J.I., Gaston, R., and Burge, D., 1993, A large dromaeosaur (Theropoda)from the Lower Cretaceous of Eastern Utah: Hunteria, v. 2, 1-16.

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