Meeting Abstract

45.7  Sunday, Jan. 5 11:45  Archosaur hip joint and its significance in body size and locomotor evolution TSAI, H.P.*; MIDDLETON, K.M.; HOLLIDAY, C.M.; Univ. of Missouri; Univ. of Missouri; Univ. of Missouri hptkr7@mail.missouri.edu

Reconstructing joint anatomy and function is critical to understanding posture, locomotor behavior, ecology, and evolution of vertebrates. Archosaurs evolved a wide diversity of hip joint morphology and locomotor postures. Among archosaurs, the very largest dinosaurs tend to exhibit terminal ends of long bones that differ in shape and size, suggesting the presence of large volumes of soft tissue. This study tests the association between hip joint dimensions, morphological characters, body mass, and locomotor postures of archosaurs. Femora and pelves of 72 taxa were digitized using 3D imaging techniques. Discrete and continuous characters were analyzed using phylogenetically corrected correlation to reveal trends in body size and postural evolution. Among saurischians, large theropods (e.g., Tyrannosaurus) and sauropods (e.g., Apatosaurus) convergently evolved incongruent hip joints, medially deflected femoral heads, reduced supraacetabular crests, and cranioventrally oriented antitrochanters. Sauropods retained the rugose proximal femur plesiomorphic for stem archosaurs, suggesting a reliance on femoral epiphyseal cartilage to increase joint congruence. In contrast, even the largest theropods exhibit smooth subchondral surfaces, suggesting the presence of thinner femoral epiphyseal cartilage and reliance on acetabular soft tissue to maintain congruence. Retention of thick femoral cartilage likely constrained sauropods to largely columnar hind limb postures, whereas the extensive modification of theropod acetabular tissues may be associated with greater degrees of hip joint movement (e.g. abduction and long-axis rotation) during locomotion.