摘要：

Pachypleurosaur material from the Middle Triassic `Grenzbitumen'-horizon (Anis-Ladin boundary) of Monte San Giorgio, Kanton Tessin, Switzerland, is described as a new genus and species, Serpianosaurus mirigiolensis. A detailed morphological description is followed by a quantitative analysis. The taxon differs from other pachypleurosaurids primarily by its relatively large skull and its straight lower jaw. Rib pachyostosis is absent. In most characters the taxon remains plesiomorphous with respect to other pachypleurosaurs from the Middle Triassic deposits of Monte San Giorgio, which accords well with its early stratigraphic occurrence at that locality. Sexual dimorphism is expressed by the size and shape of the humerus. A cladistic analysis shows the Pachypleurosauridae to constitute the sistergroup of all other Sauropterygia. The Sauropterygia and the Placodontia together form a monophyletic group, the Euryapsida, which is subordinated to the Diapsida and to the Neodiapsida, but which is classifed outside the archosauromorph-lepidosauromorph dichotomy within the Neodiapsida. A concluding paragraph discusses the status of the pachypleurosaurs within the Sauropterygia. Skull structure, and jaw mechanics in particular, suggest a relatively plesiomorph position of pachypleurosaurs within the Sauropterygia. Their adaptation to an aquatic mode of life was not carried to the degree observed in other sauropterygians, and the ability for rapid and deep dives seems to have been limited by middle-ear structure.

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