Dinosaur and turtle tracks from the Laramie/Arapahoe formations (Upper Cretaceous), near Denver, Colorado, USA

Cretaceous Research (2001) 22, 365–376doi:10.1006/cres.2001.0262, available online at http://www.idealibrary.com on

Dinosaur and turtle tracks from theLaramie/Arapahoe formations (UpperCretaceous), near Denver, Colorado, USA

Joanna Wright and Martin Lockley

Department of Geology, University of Colorado at Denver, Campus Box 172, PO Box 173364, Denver,Colorado 80217-3364, USA

Revised manuscript accepted 23 March 2001

A large slab showing more than two dozen dinosaur footprints discovered in the Upper Cretaceous (Maastrichtian) Laramieor Arapahoe Formation near Denver, Colorado, represents the first significant find of dinosaur tracks in the greater Denvermetropolitan area in more than a decade. The tracks are all of tridactyl bipeds and appear to be mainly attributable totheropod dinosaurs, although some may be attributed to ornithopods. The track-bearing slab is undoubtedly the highestconcentration and best preserved example of small dinosaur tracks ever discovered in the Laramie Formation, and addssignificantly to our understanding of the range of size and shape of small bipedal tracks in the Denver area during the latestCretaceous. At another site, small vertebrate tracks attributed to turtles were recently discovered. They consist of very shortwide tracks with deep claw impressions. � 2001 Academic Press

K W: dinosaur; turtle; tracks; Laramie Formation; Arapahoe Formation; Cretaceous; Colorado.

1. Introduction

In the spring of 1999, Suzy Langston and JeanneHoiem, employees at Sun Microsystems Inc., noticeddinosaur tracks on a large slab of rock incorporatedinto the newly constructed inner-courtyard land-scaping at a building on a large complex (Interlockenbusiness park) just west of Broomfield, Colorado, anorthwest suburb of Denver (Figure 1). The rock wasno longer in situ; the outcrop from which it wasextracted has not been precisely identified, and is nolonger exposed. The rock also shows a number oflarge plant impressions, including some ‘palm’ fronds,cf. Sabalites.

Owing to the relatively friable nature of the rock,and its exposed location outside the building, latexmolds of two track-bearing portions of the specimenwere made in April 1999, to provide permanentreplicas for the fossil footprint collections of theUniversity of Colorado at Denver. One replica(CU-MWC 223.5) was made of a single footprint thatis separated from the other tracks by a distance of onemetre. The second replica (CU-MWC 223.6) is of themain track-bearing section, which consists of at least20 partial or relatively complete tracks of which 14 arecomplete enough to provide measurements.

0195–6671/01/030365+12 $35.00/0

The objective of this account is to document thetracks and place them in the broader context ofthe fossil footprint record of the Upper Cretaceous(Laramie-Arapahoe-Denver) formations in theDenver area (Figure 1C), and other Upper Cre-taceous track and dinosaur/vertebrate bearing for-mations in the region. No attempt is made to describethe plant fossils.

In order to preserve the tracks, Sun Microsystemscut out the main track-bearing portion of the slab anddonated it to the Public Library in Broomfield, whereit will go on display in the near future.

2. General geological and palaeontologicalsetting

The tracks are preserved in a light-grey to light-orange-brown (weathered) or light-brown (weath-ered) fine-grained sandstone. The plant frondsare preserved as impressions covered by an organicfilm.

The precise origin of the track-bearing slab cannotbe determined, although it was probably excavatedclose to the location where the footprints were exam-ined. Efforts to identify the source of the rock throughdiscussion with the engineers involved in the

� 2001 Academic Press

366 J. Wright and M. Lockley

Figure 1. A, simple map of Colorado showing the location of Denver. B, the vicinity of Denver showing the relative positionsof Broomfield and Golden; shaded areas represent urban development. C, generalized stratigraphic section showing thenames and positions of the geological formations mentioned, relative to the K/T boundary.

construction of the complex suggest that the rockprobably originated from a depth of about 15 m atwhat is now the foundation of Building 2 on theSun Microsystems corporate campus in Interlockenbusiness park.

The Interlocken business park is located close to thecontact between the Laramie and Arapahoe for-mations, which dip southward at angles from 10 to35�. The exact line of the contact is uncertain owing topoor exposure of the rocks in this area. The lower partof the Laramie Formation in this area is 210–225 mthick and consists of light-grey, well-sorted, fine tomedium grained ‘salt-and-pepper’ micaceous quartz

sandstone. The upper part of this formation is183 m thick and is composed of olive-grey to darkgreyish brown shale, siltstone, lignitic claystoneand coal, interbedded with regularly bedded, finelylaminated, light-grey to light-brown (weathered)sandstone (Machette, 1977). The lower part of theLaramie Formation, near Golden, Colorado, hasbeen interpreted as part of a deltaic complex con-sisting of muddy swamps cut by channels (Weimer& Land, 1975), while the upper part has beeninterpreted as representing lacustrine or freshwaterbay and swamp environments (Weimer & Land,1975).

Dinosaur and turtle tracks in Colorado 367

The overlying Arapahoe Formation (about 120 mthick) consists of olive-grey to bluish grey claystoneand siltstone, light grey to light orange-brown (weath-ered) fine-grained sandstone, and sparse granule con-glomerate composed of claystone fragments, whichweather orange brown, and/or igneous and sedimen-tary rock fragments. Dinosaur bones have been foundin this formation north of Broomfield (Machette,1977).

It is difficult to determine whether the Sun speci-men came from the top of the Laramie Formation orthe base of the Arapahoe Formation, though if it wasexcavated from a depth of 15 m the possibility ofpenetrating the Laramie Formation increases. Bothformations contain lithologies similar to that of thetrack-bearing slab. A significant number of blockswith claystone fragments which weather orange brownwere removed during excavation of the site, and usedfor landscaping; they strongly resemble Arapahoe For-mation lithologies. Whether excavations went deepenough to remove the Arapahoe Formation down tothe upper part of the Laramie is uncertain. Sincedinosaur tracks are known from the Laramie For-mation and not from the Arapahoe Formation, it istempting to infer that the slab is from the Laramie.The partial palm fronds are not particularly wellpreserved; however, they resemble Sabalites morpho-logically, which is common in the dinosaur-track-bearing beds in Golden, and in younger formations.Dinosaur skeletal remains are also known from theLaramie, Arapahoe and the Cretaceous part of theoverlying Denver Formation, so we predict that trackscould be found in any of these formations (althoughthe Denver Formation is not shown on geologicalmaps of the Interlocken area). The Laramie,Arapahoe and Denver formations were all depositedin a narrow time-window; they contain some similarlithologies even though the Arapahoe and Denverformations are synorogenic whereas the Laramie For-mation was deposited before the start of the Laramideorogeny (Raynolds, 1997; Figure 1C). In theBroomfield area and the Denver Basin in general theyare not well exposed or easy to differentiate precisely.

3. Track preservation and previous trackdiscoveries

The tracks are preserved as natural casts; they occuron relatively smooth but mudcracked surfaces. Therock surface is so smooth as to suggest that it mayhave been ‘planed off’ before burial. One possibility toaccount for this smoothness is that the tracks mayhave originally been made on surfaces that wereslightly raised relative to the surrounding areas.

Alternatively they may have been made on a surfacethat was eroded before the overlying sands weredeposited, so that the areas with preserved tracksrepresent remnant patches that were not washed outand destroyed. Unfortunately the composition of theoriginal layer on which the tracks were made is notknown. However, the presence of the mudcracksindicates that it was muddier and finer-grained thanthe sand layer which subsequently infilled and pre-served the tracks and desiccation cracks. The presenceof both the tracks and the desiccation cracks indicate adepositional hiatus.

The tracks are all similar in preservational qualityand thus are considered to have been made over arelatively short period of time. Tracks made overlonger periods are likely to show more variation inpreservational quality as those made first woulddeteriorate whereas those made later would be clearer(Cohen et al., 1993). At least 22 tracks or partialtracks can be seen on this specimen, 14 of which weremeasured (Table 1). All the footprints are tridactyl,and most have similar orientations.

A diverse assemblage of vertebrate footprints isknown from the Laramie Formation (Lockley &Hunt, 1994a, 1995a, b). The tracks have been attrib-uted to large hadrosaurs, large ceratopsians (Ceratop-sipes goldenensis), large and small theropods, andchampsosaurs (Champsosaurichnus parfeti) (Lockley &Hunt, 1995a). Most of these tracks come from thecomplex of clay pits east of Highway 6, on the westside of Golden (Figure 1). At this location the strataare nearly vertical and large natural track casts canbe seen from the roadside at one outcrop west ofHighway 6 and a few hundred yards north of 19thStreet in Golden.

The theropod tracks from the clay pits were classi-fied into six morphotypes (Lockley & Hunt, 1995a).Of these only three fall into the same size range as thetracks found in Broomfield. None of these morpho-types was named, although the largest, an isolatedtrack some 30 inches (76 cm) long, might be thefootprint of a tyrannosaur (perhaps Tyrannosaurus).

Only one other significant tracksite is known fromthe Laramie Formation; it lies to the north of Golden,in the vicinity of Leyden Gulch (Lockley & Hunt,1995a). Here the strata are nearly vertical. Only a fewof the tracks from this locality are attributable todinosaurs, mainly hadrosaurs; other tracks discoveredrecently were probably made by turtles swimming orwalking underwater (see below).

Previous studies of the Laramie Formation fauna inColorado include the description by Carpenter (1979)of skeletal remains of fish, amphibians, reptiles andmammals from central Weld County. Among the


reptiles, Carpenter (1979) reported the remains oftwo types of theropod dinosaur (a dromaeosaur and atyrannosaur, possibly Tyrannosaurus), a hypsilopho-dontid ornithopod (Thescelosaurus sp.) and the re-mains of large hadrosaurs and ceratopsians. Similarfaunal assemblages are typical of the Lance Formationin Wyoming, where a new tracksite has recently beendiscovered (see below).

4. The track-bearing slab

All the relatively complete tracks on the slab weremeasured, amounting to 15 specimens out of anestimated total of 23 (Figure 2). Tracks that were notmeasured either did not show all three digits clearly,or were preserved in such a way as to make theirmorphology unclear. In four cases we measured tracksin which only two digits were clearly visible by meas-uring half widths and then doubling (* in Table 1).

CU-MWC 223.6 (Figures 2, 3A). The section of theslab on which all the other footprints were foundmeasures approximately 1.5�0.75 m. Associatedwith the tracks are two fossilized partial palm fronds(Figures 2, 3A, B), a fossilized stem segment (Figures2, 3A, C) and desiccation cracks. The plant fossils arepreserved as impressions coated by an organic film.The desiccation cracks cut across the tracks and werethus formed after the tracks were impressed.

The tracks from Leyden Gulch are preserved ascasts up to 10 mm deep and associated with numer-ous small casts of what may be burrows, and alsodesiccation cracks.

5. Systematic palaeontology

Although these tracks are moderately well preserved,complete trackways are not known. In addition, theirmorphology is too generalised to allow confidentdifferentiation from other theropod footprints. Thesame applies to the probable turtle tracks. Thereforeno formal nomenclature has been attempted; instead,three dinosaurian track morphotypes have been des-ignated morphotypes A, B and C. All measured trackshave been numbered (2–15 on Figure 2: CU-MWC223.6), with number 1 referring to specimen CU-MWC 223.5 (Figure 4; see Table 1).

5.1. Morphotype A (numbered tracks 3–5, 7, 9–14)

Diagnosis. Tracks of a bipedal vertebrate, footprintstridactyl, digits strongly tapering from rounded proxi-mally to a sharp point distally. Digits completelyseparate from one another proximally. Impression ofdigit III longer than those of the lateral digits.

Table 1. Footprint length and width measurements of the Sun Microsystems specimen, Broomfield, Colorado. Tracknumbers correspond to those on Figure 2. Numbers in italics indicate that the exact measurement is uncertain owing to poorpreservation; an asterisk in front of a width measurement indicates that it is a half-width measurement because the print wasincomplete. Track 1 is off to the side of the main area of the slab, although on the same horizon; track 2 is a large trackpartially obscured by part of a fossilized palm frond.

No. L W

Morphotype A Morphotype B Morphotype C

No. L W No. L W No. L W

1 20.0 18.5 3 11.5 14.0 1 20.0 18.5 6 14.0 10.22 24.0 21.0 4 12.3 14.4 2 24.0 21.0 8 12.7 9.93 11.5 14.0 5 11.5 12.5 15 16.0 *7.5 Av 13.4 10.14 12.3 14.4 6 14.0 10.2 Av 20.0 19.8 SD 0.92 0.215 11.5 12.5 7 12.0 *6.3 SD 4.00 1.776 14.0 10.2 8 12.7 9.97 12.0 *6.3 9 11.7 *5.08 12.7 9.9 10 11.8 12.39 11.7 *5.0 11 11.1 12.5

10 11.8 12.3 12 11.0 *5.511 11.1 12.5 13 12.9 10.412 11.0 *5.5 14 13.7 10.513 12.9 10.4 Av 12.2 11.914 13.7 10.5 SD 0.97 1.6815 16.0 *7.5

Description. The tracks average 12 cm in length with arange of 11–13.7 cm. Widths are more variable. Total


digit divarifications range from about 55 to 85�. Im-pressions shallow, no clear pads visible. All trackspreserved as isolated casts; no trackways discernible(Figures 2, 3A, E, G).

Remarks. This is the most common morphotype,including all but four or five of the tracks on thisspecimen. These tracks are in the same size range astracks described as morphotypes A and B by Lockley& Hunt (1995a, fig 7).

5.2. Morphotype B (tracks 1, 2, 15)

Diagnosis. Tracks of a bipedal vertebrate, footprintstridactyl. The tracks range in length from approxi-mately 16 to 24 cm. All tracks preserved as isolatedcasts; no trackways discernible.

Description. CU-MWC 223.5 (Figure 4A) is thelargest complete track cast on the Broomfield slab,measuring 20 cm long by 18.5 cm wide. The digitimpressions are parallel-sided, without any clear indi-cation of discrete digital pads, or sharp terminations(claw impressions), though traces of these may havebeen removed by breakage of the cast along layers(bedding planes) parallel to the track-bearing surface.The extension of the middle toe (III) beyond the tipsof the lateral toes (II and IV) is considerable. Unfor-tunately tracks 2 and 15 are not very well preserved,and are tentatively placed in this morphotype on thebasis of their size.

Remarks. There is no obvious relationship betweenthese tracks and the morphotypes described byLockley & Hunt (1995a), although track number 1has short lateral digits, which suggests some simi-larities with morphotype D (Figure 1).

5.3. Morphotype C (tracks 6, 8)

Diagnosis. Track of a bipedal vertebrate, footprintstridactyl, digits wide, parallel and rounded (not taper-ing) at both ends. The most obvious feature thatdistinguishes it from morphotypes A and B is thepresence of what is interpreted as the impression of arounded fleshy ‘heel’ pad. Digital impressions andheel impression all discrete. Impression of digit IIImuch longer than those of the two lateral digits.

Description. Well-preserved tracks although track 8does not show the ‘heel’ pad (Figure 3D). It is placedin this morphotype on the basis of the rounded distalends of the digits and the length of digit III relative tothe lateral digits. Total digit divarification is 35–55�.All tracks preserved as isolated casts; no trackwaysdiscernible.

Figure 2. Main track-bearing surface (specimen CU-MWC-223.6) from the Laramie Formation, Sun Microsystemsspecimen, Laramie Formation, Broomfield, Colorado. Numbers correspond to those in Table 1.

Remarks. One of the features of this track type is therelatively long digit III (Figures 3D, F; 4E, F). Bothmorphotypes D and E described by Lockley & Hunt(1995a, fig. 7) have a long digit III, although theywere much larger.


Figure 3. A, main track-bearing surface (specimen CU-MWC-223.6) from the Laramie Formation, Sun Microsystemsspecimen, Laramie Formation, Broomfield, Colorado. Scale bar represents 0.5 m; letters correspond to Figure 3B–G. B,detail of palm frond. C, detail of stem. D, track 8. E, unnumbered partial theropod track, distal end of digit III broken.F, track 6. G, tracks 10 and 11; note that only the distal ends of the toes are impressed. Scale bars in B–G represent10 mm.


Figure 4. A, isolated dinosaur track (CU-MWC-223.5) from Sun Microsystems specimen, herein labelled as morphotypeB, is characterized by a long central digit and lack of clear pad impressions. B, CU-MWC 223.1 from the LaramieFormation, near Golden. C, outline of photograph of track from Sternberg, 1926. D, CU-MWC-224.3 from the LanceFormation, Wyoming. B–D, also have long central digits and indistinct pad impressions. E, F, tracks 6 and 8 respectivelyfrom specimen CU-MWC 223.6, designated morphotype C (herein) and compared with Dinehichnus (Figure G).

Figure 5. Part of the track-bearing surface at the Leydon Gulch locality, showing three ?turtle tracks and three discrete clawimpressions. The best preserved track shows five digit impressions. The tracks are preserved as casts.

Leyden Gulch tracks

Description. Short, wide tracks of a quadrupedal ani-mal, consisting of 4–5 claw marks joined by a shallowdepression. The length of the claw impressions com-

prises up to two-thirds of the print length. The clawsform the deepest part of the print; the posterior part ofthe print is transverse perpendicular to the claw axesand shallows into the bedding plane (Figure 5). Size


range from 5–7 cm wide, and 3.5–5 cm long. Otherassociated tracks consist elongate scrape marks, ocur-ring in groups of 3–5. No trackways were discerned.

Remarks. The claw impressions are a dynamic recordof the animals’ interactions with the substrate. Theyshow distinct movement traces, indicating that the tipof the foot was placed down and then pushed back-wards and outwards as the animal was propelledforwards. The direction of movement of the feet isoriented by the configuration of the claws.

6. Interpretation of the tracks andidentification of the trackmakers

The Late Cretaceous was a time of great dinosaurdiversity and therefore the potential trackmakers arenumerous. The tridactyl nature of the footprints nar-rows the trackmaker candidates down somewhat, butfunctional tridactyly is primitive for dinosaurs andmany, especially the smaller ones, had very conserva-tive foot morphology. Track morphotypes A and B areherein considered to have been made by theropodsand morphotype C by a small bipedal ornithopod.Using allometric equations (Thulborn, 1990) to esti-mate the hip heights of the trackmakers, the dinosaursthat made morphotypes A and C would have had hipheights of just over half a metre, and those that mademorphotype B, of around one metre. These are allfairly small animals, especially in dinosaurian terms.The footprint size of morphotypes A and C iscomparable to those of modern turkeys, althoughmorphotype B is closer in size to an emu track.

Such comparisons raise the question of whetherthese tracks could have been made by large birds.Birds existed in the Late Cretaceous and bird trackshave been found at a number of sites. Nearly allknown bird tracks in the fossil record (i.e., about99%) were made by water birds (Lockley et al., 1992).This is probably due in large part to their highpreservation potential next to water and a source ofperiodic flooding and sedimentation. Such tracks al-most always have very narrow digit impressions andtotal divarication angles that range from a minimumof 110� up to about 180� (Lockley et al., 1992). Inaddition such bird tracks sometimes display traces ofwebbing and the reversed hallux (or ‘spur’). None ofthese features is seen in the Broomfield tracks.

Large ground birds were certainly present at thistime. Modern ground bird tracks have a digit divari-fication of around 90�, which is still higher than almostall dinosaur tracks, which are rarely more than 75�.None of the tracks fits this morphology. In addition,the only track with a ‘heel’ impression (really the

impression of the distal end of the metatarsals) is track6, of morphology C. A ‘heel’ impression is character-istic of ground-bird tracks (in this case it is theimpression of the distal end of the tarsometarsus)but they tend to have more pointed distal ends totheir digits, and narrower, more parallel-sided digits(Lockley et al., 1992). Overall it seems most likely thatall of these footprints were made by dinosaurs.

The lack of a heel or metatarsal impression inmorphotypes A and B may be a function of theconsistency of the substrate, the gait of the track-maker, or the functional anatomy of the trackmaker’sfoot. Based on overall morphology, morphotypes Aand B have been attributed to theropods and morpho-type C to an ornithopod. Ornithopod tracks some-times reveal a distinct, centrally located heel pad, andwider, more fleshy toe impressions, as in morphotypeC (Thulborn, 1990). By contrast, strongly taperingdigits and pointed digital terminations (or claw im-pressions) are often interpreted as indicating a thero-podan trackmaker. In addition, digit impressions ofmorphotype A tracks curve slightly inwards towardsthe trackway axis; this is also considered to be indica-tive of a theropod trackmaker (Thulborn, 1990).

Many different types of theropod dinosaurs existedin the Late Cretaceous of North America, but we cannarrow down our selection considerably on the basisof size. For instance, none of the tracks is anywherenear the size that would be expected for Tyrannosaurusrex (Lockley & Hunt, 1994b). But dinosaur hatchlingswould have been considerably smaller than the adults,several orders of magnitude smaller in the most ex-treme examples. However, most trackways may beregarded as those of adults or subadults, becausevery young dinosaurs may have stayed in the nest(Varrichio et al., 1997; Horner, 2000) where they didnot make tracks that were preserved, or may havegrown quickly, like modern birds, with little time tomake tracks that represent their early growth stages.As there were a number of small theropod dinosaurspecies extant in North America during the LateCretaceous (Weishampel, 1992) there is no need toinfer that the trackmakers were juveniles. Thus, if weleave aside the possibility of very immature dinosaursas trackmakers, possible trackmaker candidatesfor morphotypes A and B include ornithomimids,elmisaurids and caenagnathids.

Ornithomimids and elmisaurids had the arctometa-tarsalian condition whereby metatarsal III was‘pinched’ by metatarsals II and IV resulting in a rigidstructure, ideal for running (Barsbold & Osmolska,1992; Currie, 1992; Hutchinson & Padian, 1997).This may have caused these lateral digits to diverge


somewhat more than they do in more primitive thero-pods. In ornithomimids, digits II and IV were sub-equal in length, IV being slightly longer and digit IIIwas relatively elongate. Each pedal digit ended in ablunt claw.

Elmisaurids had more slender pedal digits withsharper claws than ornithomimids. Digits II and IVwere subequal in length but digit III is not as longrelative to the lateral digits as in ornithomimids.

Caenagnathids did not have a pinched third meta-tarsal. Their pedal digits are fairly slender, digits IIand IV are subequal, and digit III is distinctly longerthan the other two (Barsbold et al., 1992). The clawsare moderately sharp.

Caenagnathids and elmisaurids fit the estimatedsize of the trackmaker more closely. On the basis offoot morphology, elmisaurids and ornithomimids maybe a closer match because of their arctometarsaliancondition, although the differences are subtle. Arcto-metarsalian theropods had more symmetrical feet,which fits with the track morphology; however, it hasalso been suggested that the distal end of the metatar-sus would touch the ground, as is the case in birds, butno such impression of a metatarsal can be seen inthese tracks.

Dromaeosaurs and troodontids, though goodmatches in terms of size, are thought to have walkedwith digit II at least partially off the ground, andwould therefore presumably leave very distinctive-looking tracks (Osmolska & Barsbold, 1992; Ostrom,1992), yet to be described by palaeontologists.Dromaeosaurs are known from the Laramie For-mation of Colorado (Carpenter, 1979). Juvenile tyr-annosaurids, especially the juveniles of a gracile formsuch as Albertosaurus, may have left tracks very similarto those of ornithomimids, as their skeletal pedalmorphology is comparable (Hutchinson & Padian,1997).

Morphotype C is distinct from footprints usuallyattributed to theropods, and so prompts speculationthat it was produced by an ornithopodan trackmaker.Possible ornithischian trackmakers include hypsilo-phodontids, pachycephalosaurs and protoceratopsids.

Hypsilophodontids had fairly primitive feet mor-phologically, functionally tridactyl although digit Imay have impressed regularly; it was not load-bearing(Sues & Norman, 1992). Of the three load-bearingdigits, digit II was the shortest, digit III the longestand digit IV was of intermediate length; the digitsended in hoof-like claws. Digit III of hypsilophodon-tids is probably not quite long enough to fit tracks ofmorphotype C. It is generally assumed that tracks ofhypsilophodontids would more closely resemble thoseof the Early Jurassic Anomoepus, which does not have

a distinctive heel pad. However, tracks from theUpper Jurassic and Lower Cretacous of NorthAmerica and Europe that have been interpreted as thetracks of Hypsilophodon or Dryosaurus have been de-scribed under the name Dinehichnus (Lockley et al.,1998a, b) and are very similar in appearance tomorphotype C. The hypsilophodontid Thescelosaurusis known from the Laramie Formation of Colorado(Carpenter, 1979).

Leptoceratops from the Upper Cretaceous of NorthAmerica has a tetradactyl pes, but if protoceratopsidswere bipedal, they might have produced a tridactylfootprint under the right substrate and gait conditions.However, digit III is barely longer than digits II and IV(Dodson & Currie, 1992) and, therefore, does not fitthis footprint morphology.

The metatarsals and pedal phalanges are poorlyknown in pachycephalosaurs (Maryanska, 1992).There are four metatarsals; the foot probably hadthree functional digits. The unguals are tapering butnot distinctly recurved, and the ungual of digit III isthe most robust. The tracks of pachycephalosaurshave not yet been recognized.

Juvenile hadrosaurs are also possible trackmakercandidates; however, the footprints of these have beendescribed (Lockley & Hunt, 1995b) and are not verysimilar to those of morphotype C in that they consistof a blunt-toed single impression, not four discreteimpressions; the digits are relatively wider and shorter,and digit III is not greatly elongated.

Thus, among purported ornithischian tracks theclosest match to morphotype C is Dinehichnus, whichhas been interpreted as an ornithopod (dryosaurid orhypsilophodontid) track. Morphotype C may, there-fore, have been made by a hypsilophodontid, or evena pachycephalosaur, as we do not have enough data torule out that possibility.

In matching the footprints with the pedal skeletalanatomy of dinosaurs there is always some room fordivergent opinion. It is possible, although probablyunlikely, that all the tracks including morphotype Cwere made by some hitherto unknown dinosaurs. It isnot certain that morphoptypes A and B were made bytheropods and morphotype C by an ornithopod as hasbeen tentatively suggested. It is possible, for example,that morphotype C was made by an ornithomimid,since they tend to have long middle toes, like variousother theropods, and somewhat blunt claws that arealmost hoof-like. The rounded heel impression couldhave been made by the distal end of metatarsal III, ormay not have been made by a part of the foot skeletonbut rather by a fleshy pad.

The short lateral digits and the separation of digit IIin track 1 of morphotype B recalls similar features in


some rather larger tracks from the Laramie Formationin the Golden area (Lockley & Hunt, 1995a), and insome tracks found recently in the Lance Formation ofcentral Wyoming (Figure 4). These tracks from theLance Formation have yet to be described in detail,but they are similar to Ornithomimipus (Sternberg,1926), from the Maastrichtian of Alberta, in that theyhave relatively short lateral digit impressions in re-lation to digit III. The comparison to Ornithimimipus,however, does not mean that these tracks were neces-sarily made by Ornithimimus, as implied by Sternberg(1926), or even a similar dinosaur. Ornithomimipusand the Lance tracks are better preserved than thelarge tracks described herein and thus show clearerindications of the pads in the digits. However, thecomparison indicates the morphological similarity ofthese tracks to other Maastrichtian tracks from NorthAmerica.

The only animals that could have made the LeydenGulch tracks are turtles. The trackmaker had sharpclaws and wide five-toed feet. Crocodiles have onlyfour toes on their hind feet and use their tails morethan their feet for swimming. Any dinosaurs withsharp enough claws to make such tracks have threefunctional toes. Champsosaurs have such length vari-ation in the digits of a single foot (Lockley & Hunt,1995a) that a longer footprint with more variation inclaw impression depth and length would be expected.The tracks are consistent in size and morphology overthe exposed surface and there is nothing to suggestthat they were made by different types of animals.

These tracks are interpreted as having been madeby a turtle partially buoyed up by water but essentiallywalking along the bottom. This is documented behav-iour for turtles (Foster et al., 1998). They also resem-ble the swimming tracks figured by McAllister (1989a,b), particularly in the shape of the transverse posteriormargin.

7. Track orientations

On slab CU-MWC 223.6 nearly all of the tracks areoriented in the same general direction. Only onedeviates significantly from this, going almost at rightangles to the rest. These animals probably took stepssufficiently long to traverse the track-bearing areawithout making more than one footprint; thus, mosttracks probably represent different individuals. Even ifsome animals crossed the area twice, which is pureconjecture, only tracks that are more or less exactlythe same size can be considered as possible evidenceof repeat activity.

There are three possible explanations for paralleltrackways: (1) individual animals could have been

following what has been called a physically controlledpathway, such as a shoreline, river bank, or othernatural passage or thoroughfare through a particularenvironment; (2) parallel trackways may suggest gre-garious or social behaviour among individuals of alarge group; i.e., herding or flocking; (3) paralleltrackways may be the result of a combination of bothof the former explanations.

Given our tentative conclusion that the majority oftrackways are those of small theropods, it is a littleunlikely that they travelled in herds. There is littleevidence for gregarious behaviour in small theropods,although at some sites the skeletal remains of a largenumber of individuals of the same species have beenfound (e.g., Colbert, 1989) and the tracks of smalltheropods are abundant at some sites (Hitchcock,1858). Dinosaur track evidence for gregarious behav-iour among large herbivorous species however is muchmore common than it is among small carnivores(Lockley & Matsukawa, 1999). It is likely that thedifferent morphotypes were made by different species,which would indicate at least three dinosaur speciesrepresented by this slab. Thus, it may be more likelythat the animals were following some physically con-trolled natural pathway. Such pathways exist in manylocal environments, and surely existed along riverbanks and shorelines in the ancient Laramie/Arapahoepalaeoenvironment. Thus of the three explanationssuggested above, the first or third are more likely,although the second cannot be ruled out.

8. Discussion and summary

Our knowledge of Late Cretaceous dinosaurs, es-pecially from Alberta, Montana, and Wyoming, isparticularly good (Weishampel, 1992) but basedalmost entirely on skeletal remains, as in these regionstracks are rare. In Colorado and Utah, skeletal re-mains are a little less abundant, but reports of tracksites are more common (Lockley & Hunt, 1995b).This is probably related in large part to the differencesin lithology between these areas; tracks are more likelyto be preserved and discovered if they are made or castin sand. North of Colorado many Upper Cretaceousformations are mud dominated so the distribution oftracks may be affected by various preservational andgeological factors. The distribution of areas studiedmost frequently by bone and track specialists may alsoaffect this ratio. In general, however, tracks have notbeen studied intensively until the last two decades,so the potential for significant new discoveries ispromising.

Ultimately palaeontologists must integrate datafrom the bone and track records. The skeletal remains


of small theropod dinosaurs are relatively rare; thoseof large theropods are more common but are stilloutnumbered several times over by non-theropods atmost sites (Bakker, 1986; Weishampel, 1992). It istherefore interesting that many track assemblages havea very high proportion of theropod tracks, often out-numbering the tracks of herbivorous dinosaurs. If thisis translated into numbers of individuals it wouldobviously be unsustainable ecologically. This suggeststhat the track record is useful in providing a differentperspective on the abundance of this group, which,because they had thin-walled, hollow bones, had alower skeletal preservation potential. It has been sug-gested that theropod tracks are more abundant thanwould be predicted because they were more active,constantly searching for food, or patrolling areasto make visual contact with potential prey species(Farlow, 1987).

The tracks on the Broomfield slab represent theactivity of two or three different types of trackmakers.The most numerous were small coelurosaurian thero-pods, possibly ornithomimids or caenagnathids. Oneor two tracks on the slab might have been made bysmall ornithopods, possibly hypsilophodontids. Thetracks found so far in the Laramie Formation confirmwhat is known of the Laramie Formation dinosaurassemblage and equivalent deposits from skeletal data,namely that they contain a relatively diverse assem-blage of dinosaurs (and other vertebrates) typical ofthe Late Cretaceous. In Colorado skeletal remains arenot very common in the Laramie Formation, althougha significant fauna is known from Weld County, anddinosaur remains are often reported from the over-lying Arapahoe-Denver formations. In the Golden-Broomfield area the track record is significant andrepresents most of the major groups of dinosaursknown at this time, as well as at least two groups ofaquatic vertebrates (champsosaurs and turtles).

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Acknowledgements

We thank officials at Sun Microsystems Inc. for finan-cial support to conduct this study. We particularlythank Jeanne Hoiem and Suzy Langston for enthusi-astic support and acting as palaeontological liasonsbetween Sun Microsystems Inc. and our group.Jordon Hand helped with the making of latex mouldsand fibreglass replicas. Ken Carpenter and KirkJohnson (both Denver Museum of Natural History)also provided useful discussion on the geology andpalaeontology of the area. The manuscript was greatlyimproved by the comments of two anonymousreferees.


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Documents

Dinosaur and turtle tracks from the Laramie/Arapahoe formations (Upper Cretaceous), near Denver, Colorado, USA