Meeting Abstract
Birds walk using a habitually flexed hip posture and a locomotor cycle proximally driven by knee flexion and femoral axial rotation. This unique locomotor repertoire originated in a diverse assemblage of theropod dinosaurs. Theropods are characterized by wide disparities in body size, locomotor posture, and hip joint morphology. However, the origin of the modern avian hip joint is poorly understood. This study reconstructs the soft tissue anatomy of theropod hip joints using osteological correlates, infers trends in character transitions, and tests the integration between femoral and acetabular anatomy. Femora and pelves of 96 theropods and outgroup taxa were digitized using 3D imaging techniques. Key transitions were estimated using maximum likelihood ancestral state reconstruction. The femora of basal theropods possessed expanded fibrocartilage sleeves on the metaphysis, which surrounded the hyaline cartilage cores. The acetabulae of basal theropods permitted mostly parasagittal femoral movements, due to bony constraints imposed by the rostrolaterally ossified joint capsule. In contrast, avian-like articular cartilage, which consists of a composite fibro-hyaline structure, originated within Maniraptora. Reduction of joint capsule in Avetheropoda allowed the femur to undergo axial rotation and coupled protraction-abduction. Multiple maniraptoran lineages independently expanded the bony antitrochanter, suggesting bird-like hip postures evolved independently in Therizinosauria, Oviraptorosauria, Deinonychosauria, and Avialae. In particular, the femoral and the acetabular cartilages evolved as distinct modules in the avian stem-lineage, likely associated with the correlated evolution with joint loading, growth strategies, and body size.
