F. Robin O'Keefe

Academic paper

Journal of Vertebrate Paleontology

19

Supplement to N

67A

The Plesiosauria is a clade of marine reptiles which evolved from basal sauropterygians near the Triassic-Jurassic boundary and survived until the end- Cretaceous mass extinction. Traditionally Plesiosaurs have been divided into two clades thought to be monophyletic: the long-necked plesiosauromorphs and the short-necked pliosauromorphs. I tested the monophyly of both types by conducting the first comprehensive cladistic analysis of the Plesiosauria. Data were collected from 18 institutions in the United States and Europe. The data matrix contained 26 genera scored for 163 characters. Five most-parsimonious trees were found, with a tree length of 388 and a Cl of.523. The strict consensus tree yields several novel results, including the polyphyly of the Pliosauria via the inclusion of the Polycotylidae, and the recognition of a new clade containing the Elasmosauridae, Cryptoclididae, and Polycotylidae. This clade, here named the Euplesiosauria, has strong bootstrap support and is diagnosed by synapomorphies of the skull roof and pectoral girdle. A clade containing Tricleidus, Morturneria, and the Polycotylidae is diagnosed by features of the braincase, palate, and atlas-axis complex. The traditional classification of Jurassic pliosaurs is generally supported by this analysis. However, material from Germany indicates that the pliosauromorph body type evolved at least three times from the plesiomorphic condition. The pliosauromorph body type therefore evolved independently a minimum of four times. This striking convergence in body type is documented morphometrically, based on another data set of measurements take from complete Plesiosaur specimens. The combination of these data sets allows the study of how character dependence in different data set partitions affects phylogenetic analysis.

English