SULLIVAN, C; Harvard University: Three-dimensional hindlimb kinematics of Alligator mississippiensis

In contrast to birds and large, cursorial mammals, living amphibians and reptiles do not restrict their limbs to movement in a parasagittal plane during normal walking. The three-dimensional nature of limb mechanics in amphibians and reptiles implies that movement at a given joint cannot be fully described in terms of changes in a single angle. Rather, it is necessary to measure possible rotation around each of three distinct and potentially independent axes. In a study of hindlimb kinematics in juvenile American alligators (Alligator mississippiensis) walking on a treadmill, I obtained this three-dimensional information using simultaneous radiographic and light videos taken from mutually perpendicular angles. Analysis using the animation program Maya shows that, in the stance phase of each stride, the femur rotates about its long axis by approximately 90 degrees as it retracts. Significant axial rotation also takes place within the knee joint. These movements allow the foot to remain approximately parallel to the direction of motion until late in the stance phase, even though retraction of the femur in a non-parasagittal plane would otherwise rotate the foot laterally on the substrate. Maintenance of the orientation of the foot in turn permits the calcaneal ‘heel’ of the alligator to provide effective leverage during plantarflexion of the ankle. Many Triassic archosaurs are comparable to living crocodilians in the general structure of the ankle and in the asymmetric nature of the distal end of the femur, resemblances which suggest that hindlimb kinematics in these extinct forms may have been broadly similar to those of Alligator.