Meeting Abstract
Many tetrapods can increase the length of hind limb strides by bending the body axis, and/or by rotating the pelvis in conjunction with each step. In turtles, however, the vertebrae are fused to the shell, immobilizing the body axis. To what extent can pelvic movements contribute to stride length in turtles? The two extant lineages of turtles differ in pelvic structure: pleurodires show a derived fusion of the pelvis to the shell, whereas cryptodires have unfused contacts between the pelvis and shell that may retain the capacity for movement. However, with the pelvis surrounded by the bony shell in turtles, visualization and comparison of such movements across taxa have been impeded. We used marker based X-Ray Reconstruction of Moving Morphology (XROMM) to test whether the differences in pelvic girdle morphology between pleurodires and cryptodires are associated with differences in girdle mobility and contributions to locomotor strides. We filmed representative cryptodire (Pseudemys concinna) and pleurodire (Emydura subglobosa) species while walking and swimming. We found that the pleurodire pelvis did not move in either environment, but the cryptodire pelvis yawed, pitched and rolled in both environments, with more movement during walking than swimming. These rotations increased cryptodire femoral excursion by up to 20 degrees. In addition, the ilio-sacral joint in P. concinna translated antero-posteriorly in conjunction with pelvic rotation. These differences in pelvic mobility may contribute to observed differences in aquatic locomotor stability between these lineages, with the derived reduction in pelvic mobility in pleurodires facilitating more stable swimming.
