A simplified phylogenetic tree illustrating the evolution of the various components of the ornithischian hip from the Triassic Period (bottom) to the Cretaceous Period (top; app: anterior pubic process; hs: hip socket or acetabulum; il: ilium; is: ischium; ps: pubic shaft). The hypothetical ornithischian ancestor (full skeleton, bottom right) lacks an anterior pubic process, which is a projection of the pubis bone pointing towards the head. As Radermacher et al. demonstrate, this structure was present in Heterodontosaurus, an early ornithischian which walked on two legs. During evolution, the process grew bigger, and is thought to have helped anchor a muscle responsible for pelvic bellows, a new breathing mechanism which could have persisted even when ornithischians started to move on four legs (like, for example, Stegosaurus). All hips are oriented in the same direction and are not to scale. Estimated dates given in millions of years (Ma) and taken from Cohen et al., 2013.
Image credit: Christopher Brochu created the skeletons of Heterodontosaurus and Stegosaurus.