Isolated freshwater pliosaurid teeth from the Middle Jurassic Xintiangou Formation of Chongqing, China

Abundant vertebrate remains have been reported from the Middle Jurassic Xintiangou Formation in Yunyang County, Chongqing, China. In this study, we report newly discovered isolated plesiosaurian teeth of various sizes excavated from Yunyang County. The specimens are classified into two types of pliosauroid: one type, has a tooth crown with a circular cross-section and apicobasal ridges that fully cover the enamel surface, extending to the apex. This conical-toothed morphology is typical of plesiosauroids in the Middle Jurassic. The other type exhibits much closer-spaced apicobasal ridges and carinae on the enamel surface. This may represent a transitional taxon between the Middle Jurassic and Late Jurassic pliosaurids. However, due to their fragmentary nature, it is not possible to determine whether the Yunyang specimens belong to a known taxon or represent a new species of Plesiosauria. These specimens suggest a multispecies plesiosaurian assemblage in Yunyang County and further demonstrate that the freshwater plesiosaurians are common in the Jurassic deposits of the Sichuan Basin.