A new pachypleurosaur (Reptilia: Sauropterygia) from the Middle Triassic of southwestern China and its phylogenetic and biogeographic implications

After the devastating Permo-Triassic Mass Extinction, several new groups of large predators invaded the sea in the early part of the Triassic, including sauropterygians, ichthyosauromorphs and thalattosaurs. Among these predators, sauropterygians are the most abundant group in terms of the generic/species diversity. Here we report a new species of Pachypleurosauria (Sauropterygia: Eosauropterygia) from a recently discovered Lagerstätten in the Upper Member of Anisian Guanling Formation. The only known specimen of the new species was collected from Muta village, Luxi County, Yunnan Province, South China. Our new phylogenetic analysis based on a novel data matrix recovered the new taxon as a sister group to Dianmeisaurus. The new phylogenetic analysis also collapsed the monophyly of traditionally recgonized Eusauropterygia. Pistosauridea, Majiashanosaurus, and Hanosaurus comprise the consecutive sister groups to a new clade including Pachypleurosauria and Nothosauroidea. A monophyletic Pachypleurosauria, of which the clade consisting of Dianmeisaurus and Panzhousaurus occupy the basal-most position, is recovered by this study. The clade consisting of Dawazisaurus and Dianopachysaurus forms the sister group to remaining pachypleurosaurs included in this study. Since Dianmeisaurus, Panzhousaurus, Dawazisaurus and Dianopachysaurus are all exclusively known from South China, we suggest that pachypleurosaurs had a paleobiogeographic origin in the eastern Tethys.