Middle Jurassic turtles from the Sichuan Basin, China: a review

The turtle fauna of the Middle Jurassic Xiashaximiao Formation in the Sichuan Basin and the type series of Chengyuchelys baenoides Young & Chow, 1953 are revised. By the absence of a mesoplastron and other shell characters, both the holotype and paratype of Chengyuchelys baenoides belong to the family Xinjiangchelyidae and come probably from the Upper Jurassic Shangshaximiao Formation. The Middle Jurassic turtle assemblage of the Sichuan Basin is composed of two entities: the Bashuchelyidae fam. nov. (Bashuchelys gen. nov., Chuannanchelys gen. nov.) and Protoxinjiangchelys gen. nov. on the one hand, and Sichuanchelys on the other hand, with the former as the dominant group. Bashuchelyids and xinjiangchelyids are closely related to one another, while Sichuanchelys is more primitive and has no shared apomorphic features with bashuchelyids. The whole assemblage appears to be endemic to the Sichuan Basin at genus level and distinct from the Late Jurassic turtle fauna of the same basin in its relict nature and absence of the Polycryptodira.