Platecarpus tympaniticus (Squamata, Mosasauridae): osteology of an exceptionally preserved specimen and its insights into the acquisition of a streamlined body shape in mosasaurs

ABSTRACT

LACM 128319, which was collected in western Kansas, U.S.A., and is assignable to Platecarpus tympaniticus (Mosasauridae, Plioplatecarpinae), represents arguably one of the most exquisite mosasaur specimens known to date. Measuring 5.67 m from the tip of the snout to the end of the tail, it comprises an exceptionally well-articulated skeleton, accompanied by soft-tissue remains, such as skin impressions and tracheal cartilage. P. tympaniticus is one of the most numerously collected mosasaur taxa in North America, but as most specimens are fragmentary or reconstructed to various degrees, LACM 128319 provides a unique opportunity to document the taxon's osteology from a single skeleton. In this study, we first present a detailed osteological description of LACM 128319. Following this, we present an analysis of the evolution of a streamlined body shape in P. tympaniticus, specifically by comparing the length distribution of the dorsal ribs in relevant anguimorphan taxa. We conclude that both an anterior migration of the rib cage and an increasing regionalization within the dorsal vertebral series are key features contributing to formation of a streamlined body profile in P. tympaniticus, and probably in many other hydropedal members of mosasaurs.

ACKNOWLEDGMENTS

We would like to thank first the staff at the Natural History Museum of Los Angeles County for their extensive support at the museum in the course of this study: P. Johnson, K. Urhausen, and D. Goodreau. We are especially grateful for S. Abramowicz's photographic work, which formed a basis for our description of the beautiful mosasaur specimen. Comments provided by A. Schulp and M. Everhart substantially improved the original manuscript of this paper. This research was fully or partially funded by the following grants: Alberta Ingenuity Fund Ph.D. Student Scholarship to T.K.; the Swedish Research Council, the Crafoord Foundation, and the Royal Swedish Academy of Sciences to J.L.; and an NSERC Discovery Grant (no. 238458-01) and Chair's Research Allowance to M.W.C.

Handling editor: Johannes Müller