Meeting Abstract
Several terrestrial vertebrate lineages include members that have evolved nearly exclusive use of aquatic habitats. Such transitions are often accompanied by the evolution of flattened limbs that are used to swim via dorsoventral flapping. Such changes in shape might be facilitated by changes in bone loading during limb use in novel aquatic environments. Recent studies on limb bone loading during walking and swimming in turtles have found that torsion (twisting) is high relative to bending loads on land, but torsion is greatly reduced compared to bending during aquatic rowing (anteroposterior limb cycles). Release from torsion during swimming could have facilitated the evolution of hydrodynamically advantageous flattened limbs in aquatic species. Because aquatic rowing is considered to be an intermediate locomotor stage between walking and flapping, we predicted that rowing species might show intermediate limb bone flattening compared to terrestrial walkers and marine flappers. To test this possibility, we measured the humerus and femur from museum specimens representing three functionally divergent turtle clades: sea turtles (marine flappers), softshells (freshwater rowers), and tortoises (terrestrial walkers). Analyses showed no difference in scaling patterns across these clades for bone length versus diameter, or either measurement versus body mass. However significant differences in overall limb bone shape are present. For a given mass, softshells show greater lengths and diameters for both the humerus and the femur than either tortoises or sea turtles. Thus, other factors may supersede the influence of locomotor loading on turtle limb design. In particular, the robust limb bones of softshells may provide weight that compensates for a reduced shell and helps them maintain their typical benthic position in water.
