A new basal hupehsuchian from the Early Triassic of South China and its implication to the phylogenetic relationships of Ichthyosauromorpha (Reptilia: Diapsida)

Abstract

ABSTRACT The endemic Hupehsuchia from the Early Triassic of South China is generally accepted as the sister group of ichthyosauriforms and represents the only ichthyosauromorph that lived mainly in the shallow aquatic environment. Therefore, the detailed morphological study of basal hupehsuchians is important to elucidate the relationships of Ichthyosauromorpha with other reptiles. However, Nanchangosaurus, the most basal hupehsuchian taxon, is represented by only two skeletons, and the morphology of its palate and most of the appendicular skeleton remains unknown, limiting its role in resolving the phylogenetic relationships of Ichthyosauromorpha. Here we report a new hupehsuchian specimen from the Early Triassic of South China, which we have identified as a new morphotype of Nanchangosaurus. The new skeleton shows for the first time the morphology of the palate, zeugopodium and autopodium in Nanchangosaurus. Phylogenetic analyses including new morphological information from this skeleton support the recent recovery of an aquatic clade that includes the three major Triassic marine reptile groups. A clade comprising Helveticosaurus and Sauropterygomorpha forms the sister group to Ichthyosauromorpha, and Thalattosauria occupies the most basal position in this large aquatic clade. Archosauromorpha is recovered as a sister group to this aquatic clade, and together they form part of a monophyletic Archelosauria.