JASINOSKI, S.J.; Univ. of Calgary: Transformation of the theropod scapulocoracoid using thin-plate splines

Non-avian theropods were bipedal saurischian dinosaurs, closely related to modern birds. Bipedalism decoupled the theropod shoulder girdle from its weight-bearing role, allowing the forelimbs to engage in functions other than locomotion. Within the theropod lineage, several morphological changes occurred in the shoulder apparatus that may correlate with changes in forelimb function, leading to the evolution of powered flight. In this study, morphological differences in the scapulocoracoid of fourteen taxa of non-avian and avian theropods are examined quantitatively using thin-plate splines. This shape analysis allows the magnitude and direction of change of the landmarks in comparative taxa to be visualized as deformations of a grid, thereby allowing localized variations in form to be detected and their significance to be quantified. The areas of the scapulocoracoid shown to have undergone the most change, such as the posterior coracoid process, appear to coincide with modifications in the sizes and actions of muscles that arise in those areas. The morphological differences revealed by the thin-plate spline analysis are assessed within a robust phylogenetic framework of the Theropoda. Using squared-change parsimony, the landmark configurations of the hypothetical ancestors at the internal nodes of the cladogram are estimated. These are used to estimate shape changes that occurred along branches of the cladogram, and to infer the relative magnitude of change between successive hypothetical ancestral nodes. These estimates of shape change are then compared with previous hypotheses of morphological transformation of the scapulocoracoid, which only considered changes between the terminal taxa. This comparison will indicate whether previous hypothesized transformations between known taxa are feasible.