Biodiversity and Sequence of the Middle Triassic Panxian Marine Reptile Fauna,Guizhou Province,China

The Middle Triassic Panxian fauna is a physical marker and representative record of the rapid recovery of the Triassic marine ecosystem following the Early Triassic stagnant stage after the end-Permian mass extinction.Ten marine reptile taxa have been found from the 1.82-2.10 m-thick fossiliferous level in the Upper Member of the Guanling Formation,which can be subdivided into three marine reptile beds through the analysis on the stratigraphic distributions of fossil reptiles.The Lower Reptile Bed yields the sauropterygians Placodus inexpectatus Jiang et al.,2008 and Lariosaurus hongguoensis Jiang et al.,2006,the ichthyopterygians Xinminosaurus catactes Jiang et al.,2008 and Phalarodon cf.Phalarodon fraasi Merriam,1910,associated with Mixosaurus panxianensis Jiang et al., 2006,representing a stage of predominance of durophagous taxa.In this bed,the large complete skeletons may reach up to 2.3 m in length,and lithofacies and chemostratigraphic analyses indicate a relatively deep carbonate platform with an oxic water environment near the bottom,as well as a rising sea level.The Middle Reptile Bed yields the sauropterygian Nothosaurus yangjuanensis Jiang et al., 2006 and the archosaur Qianosuchus mixtus Li et al.,2006,associated with Mixosaurus panxianensis Jiang et al.,2006.The fossils in this bed are characterized by its pincering dentition and large overall body size,with the largest possibly exceeding 3 m in length.This bed might represent a time of deepest basin with relatively anoxic condition near the bottom.The Upper Reptile Bed yields the sauropterygians Wumengosaurus delicatomandibularis Jiang et al.,2008,Keichousaurus sp.,the protorosaur Dinocephalosaurus orientalis Li,2003,and the ichthyopterygian Mixosaurus panxianensis Jiang et al.,2006.In this bed,reptilian taxa characterized by suction feeding appeared,and most are less than 1 m long.This bed corresponds to a period of decreasing water depth.