The relationship between form and function can have profound impacts on the evolution and ecology of a lineage. Because of this relationship, variation in the morphology of a lineage has often been linked to adaptive radiations. However, form-function relationships are not linear, and variation in morphology does not necessarily predict variation in function due to the pervasive presence of mechanical equivalence in physiological systems. This possibility is often investigated through the lens of biomechanics, which uses physical principles to create a framework for comparing different systems with similar mechanical behaviors. Turtles represent an excellent system for studying how variation in structure might impact function. All extant turtles have descended from an aquatic common ancestor, and can be differentiated into two clades: cryptodires and pleurodires. These two clades can be distinguished by their pelvic girdle morphology. Cryptodires have an ancestral pelvic girdle morphology where the pelvis articulates with the sacral vertebrae at a joint, whereas pleurodires possess a derived morphology in which the pelvic girdle has been fused to the shell.

My dissertation investigates the functional role of pelvic girdle fusion in pleurodire turtles by studying functional differences in the musculoskeletal system between pleurodires and cryptodires, and then by investigating how these functional differences might impact performance in water and on land. First, I evaluate differences in girdle movements between cryptodire and pleurodire turtles using X-ray Reconstruction of Moving Morphology. Next, I examine how pelvic girdle fusion impacts muscle function and use during walking and swimming. Third, I studied the potential for this novel structure to influence swimming performance. Finally, I compare the bone loading regimes of pleurodires with cryptodires during terrestrial locomotion. Data from these studies provide insight into the functional importance of novel structures and how they can impact the ecological and evolutionary history of lineages.