Meeting Abstract
Reconstructing joint anatomy and function is critical to understanding locomotor behavior, ecology, and evolution of vertebrates. Saurischian dinosaurs evolved a wide diversity of hip joint morphology and locomotor postures, as well as seven orders of magnitude in body size. Sauropods and theropods independently, iteratively evolved large body size, and used large volumes of soft tissues to maintain hip joint articulation. This study tested the relationships among hip joint morphological characters, body mass, and locomotor postures of saurischians. Femora and pelves of 120 taxa were digitized using 3D imaging techniques. Discrete and continuous characters were analyzed using phylogenetically corrected correlation to reveal trends in body size evolution. Theropods and sauropods decreased bony hip joint congruence by reducing supraacetabular ossifications and medially deflecting the femoral head, such that contact between the thick femoral chondroepiphysis and acetabular pads maintains articulation. However, both characters preceded phylogenetic body size increases, suggesting changes in soft tissue morphology occurred in small bodied basal taxa. Moreover, sauropods used thick femoral cartilage for maintaining joint congruence, whereas theropods relied on acetabular soft tissues such as ligaments and articular pads. Differential distribution of fibro- and hyaline cartilages suggests that the sauropod hip joint is built to sustain heavy compressive loads, whereas those of theropods experienced both compression and shear forces. These data indicate that the archosaur hip joint underwent divergent transformations in soft tissue morphology reflective of body size, locomotor posture, and joint loading.
