PALAIOS, 2008, v. 23, p. 645–647 Research Article DOI: 10.2110/palo.2008.p08-030r
A PROBABLE TYRANNOSAURID TRACK FROM THE HELL CREEK FORMATION (UPPER CRETACEOUS), MONTANA, UNITED STATES PHILLIP L. MANNING,1,2* CHRISTOPHER OTT,1 and PETER L. FALKINGHAM1 1
University of Manchester, School of Earth, Atmospheric and Environmental Sciences, Williamson Building, Oxford Road, Manchester, M13 9PL, UK; 2University of Manchester, The Manchester Museum, Oxford Road, Manchester, M13 9PL, UK e-mail:
[email protected]
ABSTRACT
MATERIAL
Large theropod tracks have previously been attributed to Tyrannosaurus rex. Most identifications however, have not been supported by either clear comparison with T. rex osteology or the stratigraphic position of the track. There is a conspicuous absence of tracks in the Upper Cretaceous (Maastrichtian) Hell Creek, Lance, Scollard, Frenchman, and Denver Formations (Lancian, North American Land Mammal Age), where T. rex body fossils have been found. A large tridactyl track is described here from the Hell Creek Formation of Carter County, Montana, United States. This find constitutes the first record of a large theropod track from the Hell Creek Formation, which could have potentially been made by T. rex or another large theropod, based on the track morphology and stratigraphic position.
The new track from the Hell Creek Formation of Carter County, Montana, United States, is preserved in a mudstone interpreted as an overbank deposit, which is also a productive microvertebrate site. The mineralogy of the track is quite distinct, with a high a concentration of siderite (Fe2CO3) precipitated within the track, albeit weathered to goethite (FeO(OH)) and limonite (FeO(OH)·nH2O). The weathered siderite makes the track feature conspicuous in the surrounding gray silty mudstone (Fig. 1). This difference in mineralogy has caused the track to weather and erode at a slower rate than the surrounding sediments, producing a relief feature raised by ⬃50 mm relative to the bedding. This type of mineralogy has been observed with dinosaur tracks from the Belly River Group in Canada (McCrea et al., 2005), where similar sedimentology and environments prevailed, inferring analogous taphonomy. The horizon on which the track is preserved is partially buried by a shallow (⬃20–30 cm) layer of mudstone of the same formation. The site is on Bureau of Land Management land, and precise locality data are available, though ongoing research at the site precludes release at this time.
INTRODUCTION Tyrannosaurid and large theropod dinosaur tracks are very rare in deposits of latest Maastrichtian age (Lockley, 2008; Manning, 2008). Tracks initially attributed to Tyrannosaurus rex have subsequently been identified as belonging to their prey, hadrosaurs (Thulborn, 1990; Lockley and Hunt, 1994). Tracks that were purportedly made by T. rex from the Mesa Verde Group of Carbon County (Utah) were given the suitable ichnospecies Tyrannosauropus petersoni (Haubold, 1971). Lockley and Hunt (1994), however, noted that this ichnospecies was Campanian in age, which predates T. rex by several million years, and indicated that it might have been made by a hadrosaur. The ichnogenus Tyrannosauropus (Lockley and Hunt, 1994) is now considered nomen dubium. Tyrannosauripus pillmorei was described from the Upper Cretaceous Raton Formation (Northern Mexico) by Lockley and Hunt (1994), who made a convincing case for a tyrannosaur affinity, based on track size, age, and a distinctive hallux trace. No Tyrannosaurus rex bones, however, have yet been discovered from the Raton Formation. While this does not reduce the importance or validity of the Raton track, until now there have been no reports of large theropod tracks in rocks containing tyrannosaur body fossils in close geographic and stratigraphic proximity. Lockley et al. (2004) described a tridactyl theropod trackway from the Lance Formation of Wyoming, which was attributed to a possible tyrannosaurid affinity based on size alone. Subsequent fieldwork by the authors of this paper at the Wyoming trackway locality supports the theropod interpretation (Lockley et al., 2004), but given the transmitted nature of the tracks, it is difficult to assign a clear taxonomic affinity for the traces. The track described here is the first to be found in a formation bearing tyrannosaurid (T. rex and Nanotyrannus) body fossils that displays clear features consistent with the morphology and geometry of a large theropod pes.
DESCRIPTION The tridactyl track displays clear impressions of digits II, III, and IV; there is no evidence of a hallux. Digit II is shifted anteriorly with respect to digits III and IV, displaying a characteristic posteromedian notch, and indicates the track was formed by a left pes. The overall length of the track, as measured parallel to digit III, is 720 mm, and the maximum width of the track, as measured normal to the axis of digit III, is 760 mm. The digit traces measure 140 mm, 180 mm, and 160 mm in width for digits IV, III, and II, respectively. Digits terminate in rounded ends (Fig. 2). INTERPRETATION
* Corresponding author.
The characteristics of the tridactyl track indicate it was formed by a left pes of theropod affinity. Interdigital angle of digits II and III is ⬃18⬚ and ⬃35⬚ between digits III and IV as measured according to Leonardi (1987) and Hasiotis et al. (2007). Laboratory and computer simulations (Manning, 2004, 2008), however, indicate that the interdigital angle can vary considerably with sediment rheology, limb kinematics, and depth within a track volume. The variation in mineralogy between sediment within and outside of the track is clearly a function of formation and taphonomy. As the track formed, sediment was compressed and dewatered. This altered the mechanical and chemical properties of the media, resulting in a zone beneath the track more competent than the surrounding sediment. Track morphology indicates that it may be a shallow transmitted track (Manning, 1999, 2004), as there are no skin impressions, phalangeal pad outlines, or clearly delineated claw marks. These are also known as undertracks (e.g., Lockley and Hunt, 1994). The track displays distinct anterior and posterior boundaries, indicating the original intersection of the
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TABLE 1—Digit lengths in two tyrannosaur specimens, Field Museum of Natural History (FMNH) PR2081 (Sue) and Black Hills Institute (BHI) 3033 (Stan). Digit lengths are approximate values as measured from distal tip of the terminal ungual phalanx to the proximal end of phalanx 1. Note that in life, although the proximal end of the first phalanges would not have been in contact with the ground, a keratinous sheath would have extended the length of the digits, and a fleshy pad at the rear of the foot would have been present. Measurements for BHI 3033 were taken directly from a specimen (cast) held at the Manchester Museum, UK. Measurements for FMNH PR2081 were taken from Brochu (2003; note that measurements not taken for phalanx IV-1 as it was missing).
BHI 3033 (Stan) FMNH PR2081 (Sue)
FIGURE 1—Large theropod track in outcrop. Difference in mineralogy within and outside track define clear outline.
track with the Cretaceous surface may have been a few centimeters higher than that was preserved. Only digits II, III, and IV are preserved, suggesting that, if the hallux touched down, it did not deform sediments enough to register a digital trace. While the possession of a hallux is a diagnostic character for some large theropod dinosaurs, it is not a loadbearing feature. The proximal position of the hallux also reduces its chance of generating any significant surface or subsurface trace.
Digit II length (mm)
Digit III length (mm)
Digit IV length (mm)
550 530
620 650
530 500⫹
The hypex between digits III and IV and the relatively anterior hypex between II and III are what might be expected for the pedal morphology and geometry of a large theropod pes. The pedal morphology of tyrannosaurids is relatively conservative within the group, with a phalangeal formula of 2-2-3-4-0, not including the terminal ungual phalanx. Digit III is the longest, and digits II and IV are subequal in length. Digit I is highly reduced and reversed (the hallux). The hypex between digits II and III is more anterior than between digits III and IV. The geometry and morphology of the Hell Creek Formation track is consistent with these characters. Table 1 shows osteological digit lengths of the left pes in two tyrannosaur specimens. The preservation of the track, combined with the absence of a hallux impression, reduces diagnostic features to the anterior displacement of the hypex between digit II and III, the posteromedial notch behind the proximal margin of digit II, and the size typical of a large theropod. These features likely preclude a hadrosaurian track maker, although this possibility cannot be ruled out absolutely. In the local area surrounding the track site, a few isolated remains of Tyrannosaurus rex have been recovered but are lower in the formation than the track site. A nearly complete specimen of Nanotyrannus (Jane) was recovered in 2001 from a locality approximately 7 miles (⬃11 km)
FIGURE 2—Large theropod track and relevant measurements. Photograph (left) and outline (right) with measurements taken from specimen. Hammer in photograph is 405 mm long. Digits are labelled II–IV. Digit hypices were taken as ‘‘the apex of the re-entrant angle between digits’’ (Peabody, 1948, p. 299). This corresponded with the most posterior presence of sediment between track digits.
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north of the track site (specimen BMRP 2002.4.1 held at the Burpee Museum, Rockford, Illinois, United States). The feet of this specimen are smaller than the track but are of comparable morphology. Other animals with tridactyl feet of appropriate size recovered in the immediate vicinity include both Edmontosaurus annectens and Anatotitan copei, but skeletal morphology is not consistent with the track. Large oviraptorosaur (Caenagnathus?) and ornithomimid (Struthiomimus) skeletal remains are known from the area, as are teeth of dromaeosaurids. These taxa, however, show no osteological evidence for feet large enough to produce this track, even when considering the possibility that the track is an undertrack. CONCLUSION The morphology and stratigraphic position of the large theropod track in the Hell Creek Formation are consistent with what would be predicted for a tyrannosaurid dinosaur. Tyrannosaurus rex and Nanotyrannus are known from this formation and are represented in the area surrounding the site. Such taxa are potential track makers in this case, though given the conservative features of theropod pes, the track may well belong to an as yet unknown large predatory dinosaur. ACKNOWLEDGMENTS We thank the National Geographic Foundation for supporting fieldwork in the Hell Creek Formation (EC0294-06). We are grateful to Professor Martin Lockley and an anonymous reviewer for helpful comments on the manuscript. Special thanks to Jacey Normand and Paul Craven for field support and investigations of additional track features. Thanks also to James Jepson and Karl Bates (University of Manchester) for useful comments during the writing of the manuscript. We acknowledge the Natural Environment Research Council (NERC) for providing a grant with which Falkingham was able to conduct fieldwork (NERC/S/A/2006/14033).
REFERENCES BROCHU, C.A., 2003, Osteology of Tyrannosaurus rex: Insights from a nearly complete skeleton and high-resolution computed topographic analysis of the skull: Memoirs of the Society of Vertebrate Paleontology, v. 7, p. 1–138. HASIOTIS, S.T., PLATT, B.F., HEMBREE, D.L., and EVERHART, M.J., 2007, The trace fossil record of vertebrates, in Miller, W., ed., Trace Fossils—Concepts, Problems, Prospects: Elsevier, Amsterdam, p. 196–218. HAUBOLD, H., 1971, Ichnia amphibiorum et reptiliorum fossilium, in Khun, O., ed., Handbuch der Pala¨oherpetologie: pt. 18, Gustav Fisher Verlag, Stuttgart, p. 1–7. LEONARDI, G., 1987, Glossary and Manual of Tetrapod Footprint Palaeoichnology: Repu´blica Federativa do Brasil, Minaste´rio das Minas e Energia, Departmento Nacional de Produc¸a˘o Mineral, Brasilia, 117 p. LOCKLEY, M.G., 2008, Trouble tracking T. Rex: Dinosaur Ridge, Annual Report 2007, v. 20, p. 13–14. LOCKLEY, M.G., and HUNT, A.P., 1994, A track of the giant theropod dinosaur Tyrannosaurus from close to the Cretaceous/Tertiary boundary, northern New Mexico: Ichnos, v. 3, p. 213. LOCKLEY, M.G., NADON, G., and CURRIE, P.J., 2004, A diverse dinosaur-bird footprint assemblage from the Lance Formation, Upper Cretaceous, Eastern Wyoming: Implications for ichnotaxomomy: Ichnos, v. 11, p. 229–249. MANNING, P.L., 1999, Dinosaur track formation, preservation and interpretation: Fossil and laboratory simulated dinosaur track studies: Unpublished Ph.D. thesis, University of Sheffield, England, 437 p. MANNING, P.L., 2004, A new approach to the analysis and interpretation of tracks: Examples from the dinosaurian, in McIlroy, D., ed., The Application of Ichnology to Palaoenvironmental and Stratigraphic Analysis: Geological Society of London Special Publications, v. 228, p. 93–123. MANNING, P.L., 2008, T. rex speed trap, in Carpenter, K., and Larson, P.L., eds., Tyrannosaurus rex; The Tyrant King: Indiana University Press, Bloomington. MCREA, R.T., CURRIE, P.J., and PEMBERTON, S.G., 2005, Vertebrate ichnology, in Currie, P.J., and Koppelhus, E.B., eds., Dinosaur Provincial Park: Indiana University Press, Bloomington, p. 405–416. PEABODY, F.E., 1948, Reptile and amphibian trackways from the Lower Triassic Moenkopi Formation of Arizona and Utah: University of California Publications in Geological Sciences, v. 27, p. 295–468. THULBORN, R.A., 1990, Dinosaur Tracks: Chapmen & Hall, London, 410 p.
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