Adam Stuart Smith

MSc Thesis

University of Bristol,

MSc

An extensive cladistic analysis comprising 41 plesiosaur taxa and 171 characters is performed and a phylogenetic hypothesis is presented. Definitions and diagnoses of all plesiosaur clades are revised and numerous controversial issues are addressed and resolved. Of significance are the identification of Plesiosauridae and Cryptoclididae as paraphyletic assemblages, the confident allocation of �Stretosaurus� macromerus to the genus Liopleurodon, and the confirmation of the systematic position of many previously controversial taxa. Kaiwhekea is transferred from Cryptoclididae and united with the confirmed cimoliasaurid Aristonectes (=Morturneria) within the Cimoliasauridae, the significance of which receives special attention. Elasmosauridae is recognised as a long ranging but rather plesiomorphic group, in terms of their divergence at a basal position in the plesiosauroid phylogenetic tree and in their subsequent acquisition of only a few synapomorphies. The taxon Eretmosaurus is recognised as problematic and prompts an assessment of the relative importance of cranial versus postcranial characters in plesiosaur systematics. Statistical, and stratophenetic approaches are employed to engage important temporal aspects, typically overlooked by cladistic methods and to investigate other possible tree topologies. A convincing fit is illustrated between plesiosaur stratigraphy and phylogeny. A unique method of combining phylogenetic hypotheses and stratigraphy is developed which overcomes the problem of losing character change information and allows the calculation of relative rates of character change among various clades.

English