A second specimen of Manchurochelys manchoukuoensis Endo & Shikama, 1942 (Testudines: Eucryptodira) from the Early Cretaceous Yixian Formation of western Liaoning, China

摘要

Within the Early Cretaceous Jehol Biota of China, the turtle Manchurochelys manchoukuoensis Endo & Shikama, 1942 was one of the first discovered tetrapod fossils, but no additional information on this enigmatic taxon has become available during the past half century since its discovery. Here, a new turtle skeleton from the Yixian Formation of western Liaoning Province is identified as referable to M. manchoukuoensis on the basis of an elongate oval shell, long and narrow second to fourth vertebral scutes, and two suprapygals, of which the second is much larger than the first. This specimen therefore represents the second specimen of M. manchoukuoensis, the holotype of which was probably lost during World War II. This discovery not only provides essential material that allows the validity of M. manchoukuoensis to be tested, but also substantially expands understanding of its bony anatomy, especially with regard to cranial morphology. Among other features, M. manchoukuoensis is characterized by a postorbital that does not contact the quadrate/squamosal and a deep temporal emargination, which support a close relationship with Sinemys spp. and distinguish it from the co-existing Ordosemys liaoxiensis. A cladistic analysis that includes M. manchoukuoensis further supports the hypothesis that Sinemydidae is a monophyletic group that includes Sinemys spp. and Dracochelys bicuspis.