Vertebrate Diversity of the Middle Triassic Xingyi Fauna

Abstract

The Xingyi Fauna yields abundant and well-articulated skeletons of Ladinian (Middle Triassic, ab. 240 Ma) marine reptiles, associated with fishes, conodonts, crinoids, ammonoids, bivalves, arthropods, and other fossils including nannofossils and coprolites. It represents a new marine ecosystem fully developed after the end-Permian Mass Extinction, and characterized by the appearance of a diversity of large marine reptiles with large ichthyosaurs as the apex predators. Twenty marine reptile and 17 fish species have been reported. The sequence of the Xingyi Fauna records the transition from a marine ecosystem dominated by air-breathing tetrapods extending across the shallow platform to the deep ocean, as indicated by large marine reptiles with a capability for long-distance cruising into the outer sea. The faunal composition of the Lower Assemblage of the Xingyi Fauna, dominated by small- to medium-sized pachypleurosaurids and nothosaurids, is similar to that of the older Anisian Panxian Fauna and the western Tethyan Monte San Giorgio Fauna, but the faunal composition of the Upper Assemblage, with large ichthyopterygians, pistosauroid sauropterygians, and flying fishes, is similar to that of the younger Carnian Guanling Biota as well as the Raibl and Polzberg Faunas in the Alps and California. Therefore, the Xingyi Fauna can be considered a hub of paleobiogeological exchange connecting the western Tethys and the eastern Pathalassa.