Meeting Abstract
Transitions from terrestrial to aquatic habitats by vertebrates are often accompanied by the evolution of flattened limbs for swimming via dorsoventral flapping. Changes in bone loading during limb use in swimming might facilitate such changes in shape. Studies of turtles found that torsion is high relative to bending on land, but low compared to bending during aquatic rowing. Release from torsion could have facilitated the evolution of flattened limbs that later evolved in flapping aquatic species. Rowing has been regarded as an intermediate behavior between walking and flapping; thus, rowing species might show limb bone flattening intermediate between the tubular bones of terrestrial walkers and the flat bones of marine flappers. We measured the humerus and femur of museum specimens from four functionally divergent turtle clades: sea turtles (flappers), softshells (specialized rowers), emydids (generalist rowers), and tortoises (terrestrial). We compared patterns of limb bone scaling with body mass across lineages using phylogenetic comparative methods. Rowing taxa did not show intermediate scaling patterns between tortoises and sea turtles, but our data provide other functional insights. For example, flattening of sea turtle limb bones relates to positive allometry in the limb bone diameter perpendicular to flexion-extension, rather than negative allometry of the flexion-extension diameter. Moreover, softshells show positive allometry of femoral diameters that may provide additional weight to compensate for a reduced shell, helping to maintain a benthic position. Tortoise limb bones showed positive allometry in diameter, as well as relatively long humeri, potentially helping to resist high strains associated with digging. Thus, scaling patterns of some turtle lineages may correlate to their distinctive locomotor habits.
