Glenn W. Storrs

Conference Abstracts Volume

SVP

Rigorous phylogenetic analysis of basal sauropterygian reptiles, traditionally known as 'nothosaurs', confirms that Nothosauria is a paraphyletic taxon and must be abandoned. The probable origin of the Sauropterygia from primitive diapsid (younginiform?) stock has provided a basis for outgroup comparison with which known sauropterygians ('nothosaurs' and plesiosaurs) were analyzed using a computerized cladistic algorithm (PAUP). Inclusion of the enigmatic placodonts suggests that they also form part of a monophyletic Sauropterygia. A basal dichotomy within the Sauropterygia is indicated between a monophyletic Pachypleurosauria, which consists of generally plesiomorphic genera, and nothosauriforms (Placodontia + 'nothosaurids' + Plesiosauria). The perhaps unexpected position of placodonts as the sister group of traditional 'nothosaurids' + plesiosaurs is supported by several synapomorphies, while their obvious distinctive characteristics can be considered autapomorphies arising subsequent to divergence of the lineage. Corosaurus, a generally conservative �nothosauridf from the Alcova Limestone of Wyoming, also possesses several uniquely derived features which have long confused its systematic position. It now appears that Corosaurus represents the branch of least recent common descent of all known 'nothosaurids'. Under this hypothesis the plesiosaurs remain a highly derived, monophyletic clade with sisters amongst the nothosauriform �nothosaurs'.

English