A small, exquisitely preserved specimen of Mosasaurus missouriensis (Squamata, Mosasauridae) from the upper Campanian of the Bearpaw Formation, western Canada, and the first stomach contents for the genus

ABSTRACT

A new, exquisitely preserved specimen of a small mosasaur, referable to Mosasaurus missouriensis, is reported from the Bearpaw Formation (ca. 75 Ma, upper Campanian) of southern Alberta, Canada. Many calcified cartilaginous elements, including tracheal rings and the sternum, are preserved. The sternum most closely resembles that of Clidastes propython, bearing five shallow sternal rib facets on each side. Our comparative study of the new material with the holotype, referred material, and the genotype M. hoffmannii is congruent with the preexisting hypothesis that M. missouriensis and M. hoffmannii are phylogenetically more closely related to each other than to the other congeners, in spite of a temporal gap of nearly 10 million years between them. Also preserved with the mosasaur, inside its ribcage and around the specimen, are well-preserved aulopiform fish bones, including a skull. The fish skull is punctured and its centra truncated, suggesting macrophagy was employed by M. missouriensis despite the apparent lack of tooth wear. A sympatric specimen of Prognathodon overtoni is known to have consumed a sea turtle as well as fishes, and consistently exhibits apical wear across marginal teeth. We hypothesize that coexistence of these apex predators in the Bearpaw Sea was possible because of niche partitioning. Finally, the mosasaur carcass was likely scavenged by at least three lamniform sharks, based on their shed teeth and a series of truncated transverse processes on the mosasaur tail.

ACKNOWLEDGMENTS

The authors thank the mining company, Korite International, for their initial discovery of the mosasaur specimen and promptly accommodating the field work by the Royal Tyrrell Museum to allow for its immediate collection. Additional field assistance was also provided by Korite International during the excavation, which was greatly appreciated. Study of TMP 2003.400.0001 was possible because of the recognition of its scientific importance by the Blood Tribe (Kainai), Chief, and Council, which provides the specimens on long-term loan to the Royal Tyrrell Museum. We are grateful for the assistance of R. Tailfeathers, who was instrumental in acquiring the specimen. We also thank M. Mitchell (TMP) for the careful and skillful preparation of TMP 2008.036.0001, including discovery and careful preparation of delicate calcified cartilaginous structures and of the fish material. A. Fotheringham (TMP) was responsible for the highly detailed preparation of all fish material, and B. Sanchez for the additional preparation of TMP 2003.400.0001. D. Sloan was instrumental in preparing the scientific illustrations for TMP 2008.036.0001 shown in Figures 2, 3, and 7. The careful and constructive comments provided by J. Lindgren, A. LeBlanc, and an anonymous reviewer improved the manuscript. T. Cook kindly provided comments on the shark portion of the manuscript, and H. Street generously provided T.K. with photographic images of KU 1034 and USNM 4910. Funding for this project was provided to T.K. by Government of Alberta Japanese-Canadian Initiative Postdoctoral Fellowship, to M.G.N. by Royal Tyrrell Museum Cooperating Society to support postdoctoral research as well as three travels in support of this research, and to M.W.C. by NSERC Discovery Grant no. 238458, NSERC Accelerator Grant, and Chairs Research Allowance.

Handling editor: Jack Conrad