A new species of Cymbospondylus (Diapsida, Ichthyosauria) from the Middle Triassic of Nevada and a re-evaluation of the skull osteology of the genus

Abstract

A new species of Cymbospondylus from the Favret Formation in the Augusta Mountains, Nevada, is described based on one specimen (FMNH PR2251), of which large parts of the skull as well as postcranial material are preserved in three dimensions. A number of species have been ascribed to the genus Cymbospondylus since its first description, but so far only two species are well known from almost complete skeletons, rather than from fragmentary material: C. petrinus from the Humboldt Range, Nevada, and C. buchseri from the Monte San Giorgio, Switzerland. The assignment of the new specimen to the genus Cymbospondylus is supported by a number of distinctive characters, such as the general pattern of the skull bones, the concave basioccipital, the pattern of rib articulation and the form of clavicle and coracoid. The new specimen helps to clarify the controversial osteology of the skull of Cymbospondylus, but raises new questions as well. The reinterpretation of the skull anatomy suggests that an additional skull bone posterior to the parietal is present in all members of the genus, which represents either a neomorphic bone or a postparietal. The loss of the postparietal has been viewed as a synapomorphy of the Ichthyosauria and its presence would have extensive consequences for the phylogeny and the origin of this group. © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society, 2006, 147, 515–538.