The skeletal anatomy of Manchurochelys manchoukuoensis (Pan-Cryptodira: Sinemydidae) from the Lower Cretaceous Yixian Formation

Manchurochelys manchoukuoensis is a sinemydid stem-cryptodire turtle known by fossils from the Lower Cretaceous beds exposed in western Liaoning and Inner Mongolia of China. This fossil taxon is important for understanding the origin and evolution of Cryptodira (crown-group hidden-neck turtles). The holotype of M. manchoukuoensis was presumably lost during the Second World War and several aspects of the osteology of the species remains unknown. We here describe a near-complete fossil skeleton coming from the Lower Cretaceous Yixian Formation, western Liaoning, China, 50 km away from the type locality in the same formation. PKUP V1071 represents the most completely preserved specimen of the species and includes a well-preserved plastron, which was otherwise only known partially in the lost holotype. We provide a detailed osteological description of M. manchoukuoensis including data from micro-CT and X-ray computed laminography scanning of PKUP V1071. Of particular significance is the anchor-shaped entoplastron transversely extending to completely separate the small and triangular epiplastra from the hyoplastra; this peculiar morphology is otherwise only present in Sinemys spp. among turtles. Additional novel insights into plastron and cranial anatomy further support a close relationship between Sinemys and Manchurochelys.