11.2 Wednesday, Jan. 4 Pelvic anatomy of Alligator mississippiensis and its significance for interpreting limb function in fossil archosaurs TSAI, Henry P.*; WARD, Carol V.; HOLLIDAY, Casey M.; University of Missouri email@example.com
Reconstructing joint anatomy and function of extinct vertebrates is critical to understanding their posture, locomotor behavior, ecology, and evolution. Major changes occurred in hip joint morphology during archosaur evolution. However, the lack of soft tissues, such as cartilage, ligaments, and tendons in fossil taxa makes accurate inferences of joint function difficult. We test the hypothesis that hard-tissue manipulation alone is insufficient to accurately predict in-vivo range of motion in hip joint of the American alligator. We also describe the anatomy of extant archosaur appendicular joints by providing the first description of the acetabular anatomy of the American alligator, coupled with comparative data from other sauropsids. Hip joints of ten specimens ranging from hatchling to adult individuals were subjected to dissection, iodine-enhanced 3D imaging, and histology. Our results show that alligator hips possess two capsular ligaments, as well as a ligamentum teres capitus. The lunate surface has two distinct cranial and caudal regions of articular cartilage, bounded caudally by menisci and dorsally by the supraacetabular labrum. During femoral abduction, the ligamentum teres capitis limits movement of the proximal femur, allowing it to slide dorsoventrally within the acetabulum. Posture-specific, reconstructed CT data show that the femoral center of rotation is localized inside the metaphysis, a condition different from epiphysis-centric COR in mammals, and previously overlooked for archosaurs. This study provides new insight into soft tissue structures and their osteological correlates in the archosaur hip joint. Continuing identification and testing of these osteological correlates in fossil archosaurs will greatly enhance our understanding on evolution of locomotor capabilities such as bipedality and posture of fossil archosaurs and other reptiles.