SULLIVAN, C.; Harvard University: Varanus, Alligator, and the diversity of non-parasagittal locomotion
The American alligator (Alligator mississippiensis) and the savannah monitor (Varanus exanthematicus) both hold the femur in an abducted, “sprawling” posture during terrestrial locomotion. However, despite sharing this non-parasagittal mode of walking, the two taxa differ substantially in their hindlimb kinematics. Analysis of the 3-D movements of the limb bones, using light and cineradiographic video footage, shows that Alligator is capable of greater femoral adduction than Varanus. Furthermore, even fully abducted strides performed by Alligator are in many ways kinematically closer to adducted Alligator strides than to strides by Varanus. Regardless of femoral adduction, Alligator exhibits relatively little lateral deflection (yaw) of the pelvis, and the extensor surface of the crus remains directed almost ventrally until very late in the stance phase. In Varanus, by contrast, yaw often exceeds 25° and the crus has a strong tendency to rotate laterally (in an absolute frame of reference) as the femur retracts. The difference in crural rotation is related to the ankle structure of each taxon, since Alligator has a posteriorly aligned calcaneal tuber whereas the calcaneum of Varanus bears a posterolateral flange. Crural rotation in Varanus brings the flange close to a parasagittal plane, and helps the flange to increase the moment arm of M. peroneus longus as this muscle resists the flexor moment produced about the ankle by the ground reaction force. Particularly when extinct forms are considered, documented differences in the hindlimb osteology of various non-parasagittal taxa imply a high level of kinematic diversity. The ancestral archosaur ankle, from which the crocodylian pattern ultimately arose, probably bore a closer functional resemblance to Varanus than to Alligator, based on the presence of lateral or posterolateral calcaneal flange.
