Evolution and functional morphology of the axial skeleton in the Synapsida

The overarching purpose of this study is to use vertebral morphology to test a hypothesis about the origin of a novel locomotor pattern within the Synapsida. As presented in the first chapter, along the way I found I could also address the issue of how the axial skeletons of basal cynodonts Thrinaxodon and Galesaurus are distinguishable from one another. The evidence from a series of specimens at the Bernard Price Institute indicates that ribs are useful for specimen identification, and highlights the fact that important information can be derived even from incomplete specimens. In the second chapter, I established a basis for later tests by investigating which criterion for dividing mammal trunk vertebrae into two distinct morphological categories is most appropriate in the context of questions about regional variation in muscle structure and joint mechanics. The zygapophyseal-based division into pre- and postdiaphragmatic regions is shown to be preferable to the traditional, rib-based thoracic/lumbar division.; In the remaining four chapters, I use vertebral morphology to test the hypothesis that asymmetrical gaits originated among synapsids prior to the origin of therian mammals. In Chapter 3, I demonstrate which vertebral structures that may directly influence vertebral joint mechanics or reveal muscle function have different character states in extant therians, which use asymmetrical gaits, and basal synapsids, which used symmetrical gaits only. In Chapter 4 I demonstrate that most of the derived features can be found among non-therian therapsids, thereby supporting trackway evidence that asymmetrical gaits pre-date the origin of Theria. In Chapter 5 I consider in detail the biomechanical evidence that certain character states indicate use of asymmetrical gaits. Finally, in Chapter 6 I integrate the vertebral evidence with other studies of synapsid locomotor evolution to propose a scenario for the origin of the novel locomotor pattern of asymmetrical gaits.