A LARGE SHELL OF THE CRETACEOUS TURTLE BASILEMYS FROM THE GLENROCK EXPOSURE WITH POSSIBLE EVIDENCE OF A TYRANNOSAURUS BITE

GPM 322 is an articulated carapace and plastron of the trionychoid turtle Basilemys, recovered from low in the Glenrock Exposure of the Lance Formation (Upper Maastrichtian), WY. The plastron of GPM 322 is complete, and the carapace is largely complete with only an anterior section having been lost to erosion, prior to excavation. GPM 322 is ascribed to B. sinuosa based on the characteristic form median sulcus of the plastron. At just over 200 cm in total circumference and with a plastron 63.8 cm long, GPM 322 is roughly 18% smaller than the type specimen. Based on the strongly convex form of the plastron, GPM 322 is identified as male. The left lateral margin of the carapace bears a conspicuous elliptical hole (roughly 16 mm in width and 29 mm in length). This hole is not positioned on a portion of the shell that was exposed prior to excavation, and the surface of the shell around the hole shows evidence of localized abrasion, such that the outer osteological shell surface has been scraped away. This hole appears to be a single tooth puncture mark. Abundant shed teeth and osteoderms belonging to the crocodiliamorphs Brachychampsa and Borealosuchus are known from the Glenrock Exposure. However, none of the recovered specimens confirm the presence of a crocodiliamorph sufficient in size to have created the GPM 322 bite mark, and crocodiliamorph teeth are circular in cross-section, not elliptical. The shape and size of the hole are consistent with a medium-large sized tyrannosaur tooth. Tyrannosaur teeth, and herbivorous dinosaur bones with comparable bite marks attributed to tyrannosaurs, are also locally abundant.