Two New Specimens Of Middle Triassic Mixosaurid Ichthyosaurs From Yunnan And Guizhou,China

The Mixosauridae was a clade of ichthyosaurs that existed in the Middle Triassic for a short time but evolved very successfully.It was widely distributed in the epicontinental seas and carbonate platforms along the ancient Tethyan and Panthalassic terranes.In recent years,a large number of mixosaurid materials have been found in the Middle Triassic strata in southwest China,which has promoted the research on the morphology,systematics,biodiversity and functional morphology of mixosaurs.Even so,the phylogeny of mixosaurid ichthyosaurs still needs further investigation.Here I describe two new specimens of mixosaurs from the upper part of the Middle Triassic Guanling Formation in Guizhou and Yunnan provinces.The specimen from Panxian in Guizhou Province can be diagnosed as Mixosaurus panxianensis because the presence of following characters: premaxillary dentition showing clearly thecodont implantation;lacrimal separated by the maxilla and not participating in the formation of the external naris;supraorbital crest composed of prefrontal and postfrontal;ulna larger than radius;femur strongly constricted medially,forming a slender shaft region,proximal width remarkably larger than medial width.The specimen from Luoping in Yunnan Province is tentatively described as Mixosaurus cf.panxianensis because the pubis obturator foramen is open,while in M.panxianensis the foramen is closed.Finally,I coded the two specimens as two OUTs in an existing data matrix of Triassic ichthyosaurs consisting of 47 taxa and 218 characters.The result confirms the monophyly of the Mixosauridae,and Mixosauridae is more advanced in the phylogenetic tree compared with Cymbospondylidae,but the ingroup relationships of Mixosauridae need further investigation.The Mixosauridae can be diagnosed by the following characters: the dorsal process of premaxilla are short;supraorbital crest on prefrontal and postfrontal;sagittal eminence is present and large,involving the parietal,frontal and nasal;parietal ridge is present;parietal supratemporal process is short;anterior terrace of utf is present and large;interclavicle posterior process is triangular;humerus distal proximal width ratio is distally wider;manual pisiform is present;pubis and ischium median symphysis present;pubis twice as large as ischium;anterior dorsal neural spine is narrow.