COMPARING DEGREE OF MORPHOLOGICAL CHANGE ACROSS MARINE MAMMAL AND REPTILE GROUPS: INVESTIGATING THE INFLUENCE OF ANCESTRAL TERRESTRIAL ANATOMY ON THE LAND-TO-SEA TRANSITION

The secondarily aquatic, or land-to-sea transition is a major transition in vertebrate evolution. Numerous terrestrial amniote clades with differing terrestrial functional anatomy entered aquatic environments, but whether ancestral terrestrial posture and locomotion, which are key functional differences in terrestrial animals especially between mammals and “reptiles,” may have facilitated or constrained certain aquatic locomotory types for animals that transitioned from land to sea has not been addressed. To explore this question, I selected transition-spanning taxa and terrestrial outgroups from extinct and extant secondarily aquatic amniote groups: mosasaurs, cetaceans, sauropterygians, pinnipeds, sea turtles, and sirenians. Using 2D geometric morphometrics and linear measurements of the shoulder and pelvic girdles, limb bones, and axial column proportions and regional vertebral differences, these posture and locomotion related anatomical regions were compared across chosen groups; to explore if any clades have comparatively less or greater change between early transitioning and derived members relative to other clades, and if the degree of morphological change aligns with ancestral terrestrial posture and locomotion and derived aquatic swimming style. By comparing degree of morphological change within regions of anatomy that are relevant to posture and locomotion across multiple secondary aquatic clades, which have different ancestral terrestrial posture styles, I can assess if this morphological change is perhaps driven by these functional differences. Such patterns would demonstrate that there are existing functional controls which can control the trajectories of major evolutionary transitions.