AbstractsEarth & Environmental Science

The origin of birds is one of the major evolutionary events in vertebrate history. The transition from non-avian to avian theropod dinosaurs encompass the origin of powered flight, a suite of musculoskeletal adaptations for powered flight, and early radiation of one of the most taxonomically and ecologically diverse groups of vertebrates today. In this thesis I will examine four separate but interrelated aspects of the non-avian to avian theropod transition and comment on how my findings shape our view of the origin of birds and powered flight in vertebrates and the tempo and mode of a macroevolutionary transition.The first chapter of my thesis is a detailed examination of character change in the forelimb and pectoral girdle of Theropoda including the non-avian to avian transition. This work focuses on both the placement and magnitude of character change along the phylogenetic backbone from early theropods through to birds. Across these phylogenetic permutations, three nodes repeatedly showed significant increased levels of evolutionary change: Tetanurae, Paraves, and Ornithothoraces. Notably Aves itself did not have above average evolutionary change and in most permutations had little to no character change in the forelimb at this node. This punctuated signal and the lack of forelimb skeletal evolution at Aves supports the hypothesis that the basic composition of the avian wing was in place before the origin of birds, and that other factors had a significant role in the transition from non-volant to volant locomotion. Chapter two is a re-description and re-analysis of a small feathered maniraptoran for the Jehol Biota Yixianosaurus longimanus. The phylogenetic analysis suggests that Yixianosaurus shares a basal position within Maniraptora, near Coelurus and Therizinosauria. This re-description was incorporated into a larger study of the proposed ecological niches selection of Jehol theropods based on the linear bone measurements and morphology of the forelimb skeleton. This work adds to our understanding of the evolution of Maniraptora by further refining the evolutionary characters and trends at its origins and suggests more accurate functional characters and paleoecolgical reconstructions for some advanced maniraptorans. The third section of my thesis is a test of the arboreal origin of birds theory and an examination of the paleoecological setting for the origin of the avian flight stroke. This work presents the traditional "tree's down" versus "ground up" debate on the origin of flight in a testable framework. This work demonstrates that all non-avian theropods and Archaeopteryx group closer to terrestrial cursors than any climbing lineage. It also highlights the differences between post-Archaeopteryx basal birds, which are generally accepted to have had the ability to actively fly, and non-avian theropods in regards to arboreal adaptations. Basal birds cluster closer to modern perching birds whereas all non-avian theropods cluster with strictly terrestrial avians, such as ratites. This work strongly suggests…