| The Plesiosauria is a group of marine reptiles that originated at the base of the Jurassic and went extinct in the end-Cretaceous mass extinction. Plesiosaur fossils have been known since 1822, and were prominent players in the inception of vertebrate paleontology in the Victorian age. Two basic plesiosaur morphotypes have been recognized since the last century; the pliosauromorph morphotype possesses a large head and short neck, and the plesiosauromorph morphotype possesses a small head and long neck. I investigate these morphotypes quantitatively using multivariate statistics. This analysis demonstrates that the pliosauromorph/plesiosauromorph dichotomy is an overgeneralization, and that other morphotypes also existed. A novel model of locomotion is used to advance the hypothesis the Plesiosauria is divisible into cruising and acceleration specialists. Pliosauromorphs tend to be acceleration specialists, while plesiosauromorphs tend to be cruising specialists. However, adaptations of the head and neck are not always correlated with these specializations. A cladistic analysis is used to establish the relationships within the group. Characters of the palate and basicranium are important in the classification of plesiosaurs, as well as characters of the skull roof. The phylogeny demonstrates that the pliosauromorph body type evolved three different times, and from different regions of morphospace. The last aspect of my thesis is an investigation of the behavior of characters linked to convergent body form in the cladistic analysis. I develop two methods for investigating dependence among cladistic characters. Both are based on the concept of character compatibility, and utilize similarity coefficients computed from data on shared compatibility in all pairwise character comparisons. The method of compatibility ordination is useful for studying a priori hypotheses of character correlation. The method of partition analysis is useful for identifying suites of correlated characters without a prior hypothesis. Use of both methods on the plesiosaur data set allows refinement of the phylogeny, and additional hypotheses concerning functional correlation. |
