Reidentification of Wayaosaurus bellus and the conservative trunk and tail shape of Thalattosauria

Thalattosauria is an enigmatic marine reptile clade that from the Middle to Late Triassic. The taxonomy and relationships within this clade remain controversial. Here, we redescribed the holotype of Wayaosaurus bellus, GZDB 0005, from the Upper Triassic rocks of Guanling, Guizhou Province, China. The specimen was originally and previously misidentified as a pachypleurosaur, but we hereby emend its diagnosis and discussed its taxonomy and systematic affinity. Miodentosaurus brevis shares diagnostic characters with GZDB 0005, suggesting that M. brevis may be a junior synonym of W. bellus. We also investigated evolutionary trends in thalattosaur morphology. Compared to Ichthyosauriformes and Sauropterygia, the trunk size and tail length/trunk length ratio showed a reversed evolutionary trend in Thalattosauria. Categories within the morphospaces of thalattosaur rostrum types are isolated, while their functional significance still remains unclear. Our results suggest that the diverse morphotypes of Ichthyosauriformes and Sauropterygia were related to ecological specialisation and adaptation to oceanic environment and pelagic environments in the Middle-Late Triassic, while Thalattosauria may have been restricted in the nearshore. Thalattosauria may have been outcompeted by Ichthyosauriformes and Sauropterygia in the oceanic environment due to their conservative trunk and tail shape, and their diversity declined after the Carnian.