Meeting Abstract
Archosaurs are a remarkable group of animals that exhibit a diverse locomotor repertoire at the shoulder (glenohumeral) joint, from quadrupedal alligators and dinosaurs to flying pterosaurs and birds. The origin of avian flight, despite a multitude of exciting new fossils, remains both controversial and inextricably linked to Archaeopteryx. Here I address this question through an integration of theoretical, anatomical, experimental, and comparative approaches. First, I established a standardized, joint-based approach for analyzing skeletal anatomy and motion (kinematics), which served as a comparative framework throughout. A high-resolution 3D reconstruction of Archaeopteryx was then created via multiplanar X-ray microtomosynthesis of the Thermopolis specimen (WDC-CSG-100). Results provide resolution to controversial aspects of Archaeopteryx anatomy that are critical for assessing flying ability, such as the orientations and articulations of the scapula, glenoid, and wing. Next, in order to inform and constrain the reconstruction using extant phylogenetic bracketing, I investigated the in vivo glenohumeral kinematics of walking alligators and flapping chukars, recorded via marker-based X-ray Reconstruction of Moving Morphology. Results confirm the hypothesis that the glenohumeral motions of these disparate archosaurs are fundamentally similar, despite moving against very different media. The joint-based approach also provided a framework for “scientific motion transfer.” This tested whether the in vivo motions are consistent with the range of motion in Archaeopteryx, and provided empirical, phylogenetic constraints for reconstructing motion. Findings reveal that the Archaeopteryx glenoid permits most humeral excursions from both extant archosaurs, and also indicates a more avian motion path. Ultimately, these anatomical and experimental lines of evidence demonstrate that Archaeopteryx was kinematically capable of active flight.
