Neonate dinosaurian remains and dinosaurian ...

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Neonate dinosaurian remains and dinosaurian eggshell from the Cloverly Formation, Montana a

W. Desmond Maxwell & John R. Horner a

a

Museum of the Rockies, Montana State University, Bozeman, Montana, 59717

Available online: 24 Aug 2010

To cite this article: W. Desmond Maxwell & John R. Horner (1994): Neonate dinosaurian remains and dinosaurian eggshell from the Cloverly Formation, Montana, Journal of Vertebrate Paleontology, 14:1, 143-146 To link to this article: http://dx.doi.org/10.1080/02724634.1994.10011547

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Journal of Vertebrate Paleontology 14(1):143-146, March 1994 © 1994 by the Society of Vertebrate Paleontology

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NEONATE DINOSAURIAN REMAINS AND DINOSAURIAN EGGSHELL FROM THE CLOVERLY FORMATION, MONTANA

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W. DESMOND MAXWELL and JOHN R . HORNER, Museum ofthe Rockies, Montana State University, Bozeman, Montana 59717

notably different from the three described, but we cannot state with certainty that they represent other distinct varieties. Neonate Dinosaurian Remains-The remains consist of the proximal (MOR 722-1) and distal (MOR 722-2) ends of a right tibia (Fig. 2A, B) and a distal third left metatarsal (MOR 722-3) (Fig. 3A, B). The tibia fragments are black and suitably sized to belong to one bone, but they were found distant from each other on the surface and it cannot be stated with certainty that they belong to the same individual. The tibia is very similar to that of the hypsilophodontids Hypsi/ophodon and Orodromeus and is most likely hypsilophodontid. The proximal end shows little anteroposterior expansion and the flat articular surface is slightly inclined from the convex medial border down to the lateral condyle. The medial condyle is small and separated from the lateral condyle by a shallow, narrow groove, a condition seen in juvenile examples of Hypsilophodon foxii (BMNH R5830) and Orodromeus makelai (MOR 473) and a neonate O. makelai (MOR 661) . By comparison, the tibial medial condyle of a juvenile specimen of the ornithopod Tenontosaurus ti/letti (MOR 678) is larger, projecting caudally, and is separated from the lateral condyle by a deep, narrow groove. The two proximal condyles are rounded and the lateral condyle is much larger than the medial condyle (Fig. 2A) . The cnemial crest forms a low , rounded anterior protrusion. The distal tibia (MOR 722-2) (Fig. 2B) is like that ofjuvenile specimens of Il. foxii (BMNH R5830), O. makelai (MOR 473) and T. tilletti (MOR 679) in that the lateral malleolus is wider and longer than the medial malleolus. The medial malleolus of MOR 722-2 is particularly stout contrasting with the very slender medial malleolus of a neonate O. makelai (MOR 661). The proximal and distal ends (MOR 722-1, 722-2) are 12 mm and II mm across, respectively: if they belong to the same tibia the total length is estimated at approximately 55 mm. The distal end ofa metatarsal (MOR 722-3) is robust and most likely represents a left metatarsal III. The shaft, particularly the medial surface, tapers sharply from the distal surface (Fig. 3A) and the distal articular surface is low and elongate (Fig. 3B). In distal view, the articular surface is deeply concave cranially and slightly convex caudally. A crosssection of the shaft near the distal end has the form of a dorsoventrally flattened circle. The distal surface is 9 mm across and the length of the metatarsal is estimated at 4055 mm . Metatarsal III (MOR 722-3) is unlike metatarsal III of

INTRODUCTION Dinosaurian eggshell is known from the Lower Cretaceous Cedar Mountain and Wyan formations of North America (Jensen, 1970; Dorr, 1985) and from Builijasutuin-Khuduk, Mongolia (Kurzanov and Mikhailov, 1989), but no embryonic or neonate dinosaurian remains have been recorded from any of these eggshell-yielding formations. We report here the first discovery of Early Cretaceous neonate dinosaurian remains and at least three varieties of dinosaurian eggshell from a site in the Cloverly Formation (Aptian-Albian) of Montana. The site (MOR CL-121) is positioned in Unit VII (Ostrom, 1970) of the Cloverly Formation (upper Himes Member of Moberly, 1960), located east of Bridger, Carbon County, Montana. Abbreviations: BMNH, Natural History Museum, London; MOR, Museum of the Rockies, Bozeman, Montana. MATERIAL The site occurs on a steep slope in a weathered, grey mudstone with abundant cal iche . Surface collecting and screen washing of subsurface sediment yielded plant material, gastropods, bivalves, tooth plates of Ceratodus, other fish remains, one shark tooth, turtle plastron (cf. Naomichelys) and bone, abundant crocodilian teeth and bones, three nodosaurid teeth, one sauropod tooth, two teeth of Deinonychus, one tooth of Microvenator, at least three varieties of dinosaurian eggshell, neonate dinosaurian bone, abundant weathered fragments of dinosaurian bone, and one triconodontid tooth. Other finds adjacent to the microsite include several crocodile teeth, a small hypsilophodontid femur, fragmentary remains of Tenontosaurus , and two partial skeletons of Sauropelta. Eggshell-More than 100 eggshell fragments were recovered, mostly from the surface, representing at least three distinct types (Fig. I A-C). The most abundant type of eggshell bears randomly orientated, slender, interlocking ridges with a varying degree of pitting in between (Fig. I A). Thickness varies between 0.9 and 1.1 mm. A more robust variety of eggshell, approximately 2 mm thick, is less abundant and bears thick, interlocking ridges which commonly coalesce to form a broad, mildly undulating surface (Fig. I B). The pitting ofthe surface is prominent and penetrates deep into the shell. A third variety is represented by one fragment. It is only 0.4 mm thick and the surface consists of low, randomly positioned, circular to oblong nodes on a faintly ridged surface (Fig . IC). No pitting is visible on the surface. There are at least five other examples of eggshell that are

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FIGURE 2. Neonate dinosaur bone. A, proximal end of right tibia (MaR 722-1). B , distal end of right tibia (MaR 722-2). Scale bar equals 5 mm.

juvenile specimens of T. tilletti (MaR 678 , 679) , H. foxii (BMNH R5830) and O. make1ai (MaR 623) (Figs. 3, 4) in that the shaft of MaR 722 -3 tapers from the distal end more markedly and the distal articular surface is more elongate. The distal articular surface of T. tilJetti is deeply convex posteriorly, that of H. foxii is almost square with little indentation on the anterior or posterior surface, and that of O. make1ai is compact and bears a distinct anterior notch (Fig. 4A-C).

DISCUSSION

FIGURE I . Exterior view of three examples of dinosaur eggshell from the Cloverly Formation. A, irregular interlocking ridges , varying degree of pitting. No pores visible. B, robust eggshell with a mildly undulating surface, pitting penetrates deep into the shell. No pores visible. C, random, low, circular to oblong nodes on a faintly ridged surface. No pores visible. Scale bar equals 5 mm.

The lithology of the locality is a grey mudstone, typical of the overbank deposits that constitute most of Unit VII of the Cloverly Formation. The mudstone has been weathered to a lumpy powder, to a depth of approximately 150 mm, with a hard surface crust. The microsite is most likely a washed-in accumulation, considering the combination of eggshell and the diverse aquatic and terrestrial taxa represented, but there are no sedimentological structures in the surrounding overbank deposits to indicate current direction. Further study of the sedimentology of the site and surrounding area is required to determine the conditions oftransportation and deposition. Two ornithopods, T. tilJetti and the hypsilophodontid Zephyrosaurus schaffi are known from the Cloverly Formation of Montana (Ostrom, 1970; Sues, 1980), and Galton and Jensen (1979) identified Hypsilophodon from the laterally equivalent Lakota Sandstone of South Dakota (Young,


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B

FIGURE 3.

Neonate dinosaur, left metatarsal III. A, dorsal surface. B, distal articular surface. Scale bar equals 10 mm.

1970) . Galton and Jensen (1979) proposed a new species, H. wielandi, on the basis of a left femur, but this is considered a nomen dubium (Sues and Norman, 1990). The tibia described here (MOR 722-1, 722-2) is similar to that of H. foxii but unlike that of T. tilletti. The tibia of Z. schaffi is not known. There is also a similarity between MOR 722-1 and the proximal tibia of the hypsilophodontid O. makelai from the Upper Cretaceous Judith River Formation. This scant evidence suggests that the tibia material collected at the MOR CL-121 locality most likely represents a neonate hypsilophodontid. The metatarsal III (MOR 722-3) is unlike that of H. foxii, O. makelai, or T . tilletti; the metatarsals of Z. schaffi are not known. The affinities of the animal cannot be determined from such a small, incomplete bone. This is the first published account of dinosaurian eggshell from the Cloverly Formation. Several indistinct fragments,

confirmed as eggshell by thin sectioning, were collected in 1986 from Middle Dome, Montana, by the Museum of the Rockies. The examples reported here show much better preservation and finely detailed surface ornamentation. There is also no prior record of neonate dinosaurian remains from the Early Cretaceous. This discovery of eggshell and neonate remains illustrates that the Cloverly Formation, despite the "extreme rarity of most vertebrate taxa" (Ostrom, 1970:51), is worthy of continued intensive study and may yet yield information on reproductive behavior in hypsilophodontids and other ornithopods. Acknowledgments.- We thank John Taylor and the Montana Bureau of Land Management for bringing the site to our attention. The photographs were prepared by Bruce Selyem. COllection, preparation, and research of all specimens were supported by the Museum ofthe Rockies. We also thank C. A. Forster and J . H . Ostrom for their helpful reviews.

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A

B

c


--- ~~5

FIGURE 4. Left metatarsal III of juvenile specimens of A, Ten ont osaurus (MOR 678), B , HypsiJoph odon (BMNH R5830), and C, Orodromeus (MOR 623). Scale bars equal 10 mm.

LITERATURE CITED DOff, J . A., Jr. 1985. Newfound Early Cretaceous dinosaurs and other fossils in southeastern Idaho and westernmost Wyoming. Contributions, Museum of Paleontology, University of Michigan 27 :73-85. Galton, P. M., and J . A. Jensen . 1979. Remains of ornithopod dinosaurs from the Lower Cretaceous of N00h America. Brigham Young University Geology Studies 25:1-10. Jensen, J. A. 1970 . Fossil eggs in the Lower Cretaceous of Utah. Brigham Young University Geology Studies 17: 51-65. Kurzanov, S. M. , and K. E. Mikhailov. 1989 . Dinosaur eggshells from the Lower Cretaceous of Mongolia; pp. 109-113 in D. D. Gillette and M. G. Lockley (eds.) , Dinosaur Tracks and Traces. Cambridge University Press, Cambridge, New York, Melbourne. Moberly, R., Jr. 1960. Morrison, Cloverly, and Sykes Mountain formations, northern Bighorn Basin, Wyo-

ming and Montana. Bulletin of the Geological Society of America 71:1137-1176. Ostrom, J. H . 1970. Stratigraphy and paleontology of the Cloverly Formation (Lower Cretaceous) of the Bighorn Basin Area, Wyoming and Montana. Bulletin, Peabody Museum of Natural History 35:1-234. Sues , H.-D. 1980. Anatomy and relationship ofa new hypsilophodontid dinosaur from the Lower Cretaceous of North America. Palaeontographica, A, 169:51-72. - - - and D. B. Norman. 1990. Hypsilophodontidae, Tenontosaurus, Dryosauridae; pp . 498-509 in D. B. Weishampel, P. Dodson, and H . Osmolska (eds .), The Dinosaura. Berk eley: University of California Press. Young, R. G . 1970 . Lower Cretaceous of Wyoming and the southern Rockies; pp. 147-160 in R. L. Enyert (ed .), Wyoming Geological Association Guidebook, 22nd Annual Field Conference.

Recei ved 20 A ugust 1992; accepted 25 March 1993 .