Modern therian mammals are an active, diverse group characterized in part by upright limbs operating in a parasagittal plane. By bringing ground reaction forces closer to the center of mass, parasagittalism decouples fore- and hindlimb function and enables classically mammalian behaviors such as the selection of asymmetrical gaits and the use of forelimbs in food prehension. Though the adaptive advantages conferred by parasagittalism are clear, the exact route taken by the forelimb to arrive at its present mammalian condition is less well resolved. Fossil evidence from the non-mammalian synapsid lineage suggests that forelimb parasagittalism was preceded by that of the hindlimb by some 50 million years; later therapsids and cynodonts fall within this interval, and are reconstructed with reptile-like sprawling forelimbs and mammal-like parasagittal hindlimbs. By combining detailed musculoskeletal models of key fossil non-mammalian synapsid species, with ex-vivo range of motion and in-vivo locomotor dynamics from an extant phylogenetic bracket, we aim to gain insight into the morphological underpinnings of forelimb function in transitional forms, and ultimately shed light on the acquisition of full parasagittalism on the path to mammals.
The Triassic cynodont Thrinaxodon liorhinus (Jenkins 1971)