Convergences and Trends in the Evolution of the Archosaur Pelvis

The pelvic structure in non-avian archosaurs plays a key role in understanding the evolution of terrestrial locomotor patterns because the pelvis contains major attachment sites for proximal hind limb musculature. In order to investigate patterns of pelvic evolution in archosaurs, this study compiled three pelvic indices, as well as femoral head orientation, for 92 archosaur taxa. With the metrics and a reconstructed supertree, we examined the correlated evolution of the pelvis and femur, the correlation among pelvic components, and temporal trends in the evolution of the pelvis. The result shows that archosaurs with medially directed femoral heads have more cranially shifted iliac centroids and more posteriorly rotated pubes than taxa with anteromedially directed femoral heads. The craniad shift of the iliac centroid might be correlated to the posterior rotation of pubis. The pelvic structures of pterosaurs, ornithischians, sauropods, and avetheropods occupy a different morphospace from basal archosaurs, pseudosuchians, basal dinosauromorphs, basal theropods, and basal sauropodomorphs in having more cranially expanded ilia, more posteriorly rotated pubes, and medially deflected femoral heads. This may imply that pterosaurs and those derived dinosaurs independently underwent similar shifts in thigh muscles and locomotion. The evolutionary model fitting supports the early-burst model for iliac and pubic metrics in more inclusive archosaur clades, indicating that larger changes of archosaur pelves occurred in early times of the clade's history