Later , Brown revised the phylogeny of Corythosaurus , and found that it was closely related , and possibly ancestral to Hypacrosaurus . The only differences he found between them was the development of the vertebrae , and the proportions of the limbs . During a study of dinosaurian ilia in the 1920s , Alfred Sherwood Romer proposed that the two orders of dinosaurs might have evolved separately , and that birds , based on the shape and proportions of their ilia might truly be specialized ornithischians . He used both Tyrannosaurus and Corythosaurus as a base model to analyze which theory is more likely true . He found that even though birds are thought of as saurischians , it is very plausible for them to have evolved their specific pelvic musculature and anatomy if they evolved from ornithschians like Corythosaurus . However , even though Corythosaurus do resemble modern birds in pelvic structure more than saurischians , birds are now thought to have descended from the latter .