The systematic position ofSilvestrosaurus and a classification of Triassic sauropterygians (Neodiapsida)

The Triassic marine reptiles at the base of the extinct Order Sauropterygia, traditionally known as nothosaurs, represent an unnatural assemblage of superficially similar forms. The traditional grouping of these animals has generally relied on the flawed basis of “primitive” characteristics whereas two distinct nothosaurian lineages can now be identified through the use of cladistic methodology. The Pachypleurosauria are the most plesiomorphic, but not the earliest, sauropterygians known. The Nothosauriformes include not only ‘nothosaurs’ but also the monophyletic group Plesiosauria and seemingly also the Placodontia. Rigorous coding of characters for the recently described nothosaurian genusSilvestrosaurus allows expansion of the phylogenetic data base for the Sauropterygia. The revised analysis reaffirms recent studies of relationship for the sauropterygians and demonstrates the affinity ofSilvestrosaurus with the Nothosauriformes. The explicit hierarchical classification scheme presented incorporates the hypothesized phylogenetic relationships of these long-misunderstood taxa. The traditional Suborder Nothosauria, now seen to be paraphyletic, must be abandoned.