Abstract
A new species of ichthyosauriform is recognized based on 20 specimens, including nearly complete skeletons, and named Chaohusaurus brevifemoralis. A part of the specimens was previously identified as Chaohusaurus chaoxianensis and is herein reassigned to the new species. The new species differs from existing species of Chaohusaurus in a suite of features, such as the bifurcation of the caudal peak neural spine and a short femur relative to trunk length. The specimens include both complete and partially disarticulated skulls, allowing rigorous scrutiny of cranial sutures. For example, the squamosal does not participate in the margin of the upper temporal fenestra despite previous interpretations. Also, the frontal unequivocally forms a part of the anterior margin of the upper temporal fenestra, forming the most medial part of the anterior terrace. The skull of the holotype largely retains three-dimensionality with the scleral rings approximately in situ, revealing that the eyeball was uncovered in two different directions, that is, laterally and slightly dorsally through the main part of the orbit, and dorsally through the medial extension of the orbit into the skull roof. This skull construction is likely a basal feature of Ichthyosauromorpha. Phylogenetic analyses place the new species as a sister taxon of Chaohusaurus chaoxianensis.
Highlights
The marine reptile clade Ichthyosauromorpha comprises two groups, namely Ichthyosauriformes that lasted for about 160 million years and spread worldwide (McGowan, 1991; Motani et al, 2015a) and Hupehsuchia, which is a small group of heavily-built reptiles known only from the Spathian (Lower Triassic) of Hubei Province, China (Young, 1972; Chen et al, 2014a)
The purpose of this paper is to describe Type B as a new species of Chaohusaurus, and clarify its diagnostic differences with Chaohusaurus chaoxianensis
This tendency for curled posture is shared with Chaohusaurus chaoxianensis (Fig. 1), and probably with other species of the genus
Summary
The marine reptile clade Ichthyosauromorpha comprises two groups, namely Ichthyosauriformes that lasted for about 160 million years and spread worldwide (McGowan, 1991; Motani et al, 2015a) and Hupehsuchia, which is a small group of heavily-built reptiles known only from the Spathian (Lower Triassic) of Hubei Province, China (Young, 1972; Chen et al, 2014a). Ichthyosauriforms from that time period had been known since 1929 based on Grippia longirostris from Spitsbergen (Wiman, 1929, 1933), and three additional species were reported within the same century, namely Chaohusaurus geishanensis from China (Young & Dong, 1972), Utatsusaurus hataii from Japan (Shikama, Kamei & Murata, 1978), and Parvinatator wapitiensis from Canada (Nicholls & Brinkman, 1995). Thaisaurus chonglakmanii from Thailand (Mazin et al, 1991) may belong to the list but further study is needed to establish the exact stratigraphy
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