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SVP 76TH ANNUAL MEETING Abstracts part 2 (1 Viewer)

Fred Ruhe

Well-known member

CARNEY, Ryan M., Brown University, Providence, RI, United States of America 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 the unresolved question of active flight in 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 the Archaeopteryx flight apparatus was then created, through 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 precise orientations of the scapula, glenoid, and wing, along with the range of motion and articular topology of the glenoid. Next, in order to inform and constrain the reconstruction using extant phylogenetic bracketing, I investigated the in vivo glenohumeral kinematics and muscle activity in alligators (Alligator mississippiensis) during high and low walk, using marker-based Xray Reconstruction of Moving Morphology (XROMM) and electromyography. Findings confirm the presence of an active, protractor muscle-based stabilization mechanism, which has evolutionary implications for the ancestral archosaurian shoulder. Data were also compared to published XROMM kinematics from a flapping bird (Alectoris chukar) during ascending flight and wing-assisted inclined running. Results confirm the hypothesis that the glenohumeral motions of these disparate archosaurs are fundamentally similar, despite moving against very different media. The aforementioned 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 glenohumeral 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 performing an active flight stroke.
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