Meeting Abstract
Alligators have played a significant role in evolutionary studies of archosaurs. Given that several major shifts in forelimb function (including 2 of the 3 origins of vertebrate flight) occur within this group, living crocodilian forelimb movements are of particular importance in assessing archosaur locomotor transitions. Previous X-ray investigations of walking alligators revealed substantial movement of the shoulder girdle, but since the sternal cartilages do not show up in X-ray, the source of the mobility could not be conclusively determined. Scapulocoracoid movement was interpreted to indicate independent sliding of each coracoid at the coracosternal joint; however, rotations of the entire shoulder girdle (both sternal cartilages + interclavicle) could also produce a similar outcome. Here, we present new data employing marker-based XROMM (X-ray Reconstruction of Moving Morphology) wherein simultaneous biplanar X-ray video and surgically implanted radio-opaque markers permit clear measurement of the vertebral axis, sternum and coracoid in walking alligators. We found that rotations of the sternum and coracosternal joint movement both contribute to shoulder girdle mobility. The few studies on coracosternal movement relevant to archosaurs were limited to either single X-ray views or standard light cameras and revealed conflicting data in lepidosaurs. Thus, mobility of the shoulder girdle in ancestral archosaurs remains unresolved. However, the loss of the clavicle in Crocodylomorpha may explain the increased mobility in the alligator shoulder girdle, suggesting that the degree of mobility may be unique to this group.
