New digital anatomical data of <i>Keichousaurus hui</i> (Reptilia: Sauropterygia) and its phylogenetic implication

Three complete skulls of <i>Keichousaurus hui</i> from the Middle Triassic Xingyi Fauna of southwestern China were examined using high-resolution computed tomography (CT) scanning. The CT images allow a few refinements and supplements in cranial anatomy. Some previously ambiguous anatomical characters were identified, including the presence of an L-shaped ectopterygoid that extends from the lateral side of the pterygoid and bends ventrally, the wedge-shaped posterolateral process of the frontal, the trapezoidal pterygoid for articulating with the palatine, and a rodlike basioccipital tuber that extends posterolaterally. These new features provide new detailed anatomical information for taxonomy. The new phylogenetic analysis of Sauropterygiformes places <i>Keichousaurus</i> as an eosauropterygian that is more basal than the monophyly which includes Nothosauridae and Pistosauroidea. Moreover, the result also suggests that <i>Keichousaurus</i> is more closely related to Chinese pachypleurosaurs-like eosauropterygians than to European pachypleurosaurs and more derived than other Chinese pachypleurosaurs-like forms.