More than one way to be a giant: convergence and disparity in the hip joints of saurischian dinosaurs

Saurischian dinosaurs evolved seven orders of magnitude in body mass, as well as a wide diversity of hip joint morphology and locomotor postures. The very largest saurischians possess incongruent bony hip joints, suggesting that large volumes of soft tissues mediated hip articulation. To understand the evolutionary trends and functional relationships between body size and hip anatomy of saurischians, we tested the relationships among discrete and continuous morphological characters using phylogenetically corrected regression. Giant theropods and sauropods convergently evolved highly cartilaginous hip joints by reducing supraacetabular ossifications, a condition unlike that in early dinosauromorphs. However, transitions in femoral and acetabular soft tissues indicate that large sauropods and theropods built their hip joints in fundamentally different ways. In sauropods, the femoral head possesses irregularly rugose subchondral surfaces for thick hyaline cartilage. Hip articulation was achieved primarily using the highly cartilaginous femoral head and the supraacetabular labrum on the acetabular ceiling. In contrast, theropods covered their femoral head and neck with thinner hyaline cartilage and maintained extensive articulation between the fibrocartilaginous femoral neck and the antitrochanter. These findings suggest that the hip joints of giant sauropods were built to sustain large compressive loads whereas those of giant theropods experienced compression and shear forces.

Methods
Discrete characters: The bony hip joints of saurischian-line archosaurs were examined for discrete characters, including osteological correlates of hip joint articular soft tissues. We identified 14 osteological characters based on osteological correlates of homologous articular soft tissues among extant diapsids (Tsai and Holliday, 2015)

Continuous characters: Linear dimensions of fossils were measured using a SPI 31-518-4 dial caliper and a tape measure on physical specimens, as well as from reconstructed 3D surface models of hip joints using the measure distance function of Geomagic (V11 see Appendix S1 of manuscript). Surface area dimensions were measured from reconstructed 3D surface models of hip joints by highlighting relevant osteological correlates and using the select area function of Geomagic (V11 see Appendix S1 of manuscript).

Phylogenetic trees: Composite phylogenetic trees (Fig. 4) were constructed using Mesquite (V2.73) based on published studies (See citations in manuscript), with branch lengths based on hypothesized divergence date between sister clades and sister taxa.