David B. Baier, Brigid M. Garrity, Sabine Moritz, and Ryan M. Carney
Crocodilians have played a significant role in evolutionary studies of archosaurs. Given that several major shifts in forelimb function occur within Archosauria, forelimb morphologies of living crocodilians are of particular importance in assessing locomotor evolutionary scenarios. A previous X-ray investigation of walking alligators revealed substantial movement of the shoulder girdle, but as 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 sternocoracoid joint; however, rotations of the sternum could also produce similar displacement of the scapulocoracoids. 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 precise measurement of the vertebral axis, sternum and coracoid in walking alligators. We found that movements of the sternum and sternocoracoid joint both contribute to shoulder girdle mobility and stride length, and that the sternocoracoid contribution was less than previously estimated. On average, the joint contributions to stride length (measured with reference to a point on the distal radius, thus excluding wrist motion) are as follows: thoracic vertebral rotation 6.2±3.7%, sternal rotation 11.1±2.5%, sternocoracoid joint 10.1±5.2%, glenohumeral joint 40.1±7.8% and elbow 31.1±4.2%. To our knowledge, this is the first evidence of sternal movement relative to the vertebral column (presumably via rib joints) contributing to stride length in tetrapods.