On Xinpusaurus (Reptilia : Thalattosauria)

摘要

Xinpusaurus is one of two thalattosaur genera known from the early Carnian (Upper Triassic) of Guizhou Province, P.R. China (Yin in Yin et al., 2000; Liu and Rieppel, 2001; Luo and Yu, 2002; Jiang et al. 2004). The genotypical species is Xinpusaurus suni, originally based on three specimens figured by Yin, in Yin et al. (2000:pl. VII, figs. 1-3). An isolated skull, referred to Xinpusarus cf. X. suni, was described by Liu and Rieppel (2001), whereas the description of part of the postcranial skeleton was based on a fifth specimen (Liu, 2001). The cranial anatomy of Xinpusaurus suni was the subject of further comments by Luo and Yu (2002) on the basis of yet another well-preserved, isolated skull. The second species, Xinpusaurus bamaolinensis, was described by Cheng (2003) on the basis of a single specimen. The species differs from Xinpusaurus suni in size and skull morphology, with Xinpusaurus bamaolinensis being nearly twice as large as Xinpusaurus suni. Differences in skull morphology include the striking overbite of the premaxillary rostrum in Xinpusaurus bamolinensis, where the rostrum is longer than the rest of the skull, and the premaxilla extends anteriorly far beyond the dentary. Jiang et al. (2004) commented on the skull anatomy of Xinpusaurus suni once again, and described a new species, Xinpusailrus kolii, which, however, is a subjective junior synonym of Xinpusaurus bamaolinensis Cheng (2003). Their phylogenetic analysis showed Xinpusaurus to be the sister taxon of Nectosaurus from the Upper Triassic of California (Merriam, 1905, 1908; Nicholls, 1999). This relationship of Xinpusaurus, previously reported by Liu and Rieppel (2001), now seems highly questionable in the light of ongoing research on the rostrum morphology in Nectosaurus and other thalattosaurs (Rieppel, Muller, and Liu, in press). Here, we will not comment further on analyses of the phylogenetic relationships of thalattosaurs, but rather describe a new skull of Xinpusaurus suni (Figs. 1, 2). which is the best preserved in its genus and thus helps to clarify a number of previously problematic issues of cranial anatomy. Indeed, it turns out that the motivation for the repeated revisions of the skull anatomy of Xinpusaurus suni were uncertainties and ambiguities that resulted from the partially problematic preservation of previously available specimens. The new skull described here (Figs. 1, 2) is kept at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences (IVPP V14372). Like the skulls of Xinpusaurus suni described by Liu and Rieppel (2001) and Luo and Yu (2002), the new specimen was completely removed from the surrounding matrix and mechanically prepared from both sides. As are all the other specimens, this skull is from the early Carnian Wayao member of the Falang Formation (L. Wang et al., 2001; the Falang Formation is also known as the Xiaowa Formation: X. Wang et al., 2002, 2003), and it is somewhat smaller that the specimen described by Liu and Rieppel (2001).