Comparative kinematics of the forelimb during swimming in red-eared slider(Trachemys scripta) and spiny softshell (Apalone spinifera) turtles

Softshell turtles (Family Trionychidae) possess extensive webbing between t he digits of the manus, suggesting that the forelimb may serve as an effect ive thrust generator during aquatic locomotion. However, the hindlimb has p reviously been viewed as the dominant propulsive organ in swimming freshwat er turtles. To evaluate the potential role of the forelimb in thrust produc tion during swimming in freshwater turtles, we compared the forelimb morpho logy and three-dimensional forelimb kinematics of a highly aquatic trionych id turtle, the spiny softshell Apalone spinifera, and a morphologically gen eralized emydid turtle, the red-eared slider Trachemys scripta. Spiny softs hells possess nearly twice as much forelimb surface area as sliders for gen erating drag-based thrust. In addition, although both species use drag-base d propulsion, several aspects of forelimb kinematics differ significantly b etween these species. During the thrust phase of the forelimb cycle, spiny softshells hold the elbow and wrist joints significantly straighter than sl iders, thereby further increasing the surface area of the limb that can mov e water posteriorly and increasing the velocity of the distal portion of th e forelimb. These aspects of swimming kinematics in softshells should incre ase forelimb thrust production and suggest that the forelimbs make more sub stantial contributions to forward thrust in softshell turtles than in slide rs. Spiny softshells also restrict forelimb movements to a much narrower do rsoventral and anteroposterior range than sliders throughout the stroke, th ereby helping to minimize limb movements potentially extraneous to forward thrust production. These comparisons demonstrate considerable diversity in the forelimb kinematics of turtles that swim using rowing motions of the li mbs and suggest that the evolution of turtle forelimb mechanics produced a variety of contrasting solutions for aquatic specialization.