Phylogeny of Archosauria (including Aves)

        Although the term is often used for any large, extinct " reptilian" animal, including swimming (Phytosaurs) and flying (Pterosaurs) forms, true Dinosauria comprises only two entirely terrestrial clades, the Ornithischia ("bird-hipped") and Saurischia ("lizard-hipped") dinosaurs, as well as Aves (Birds). Hip structures in Aves and Ornithischia are analogous, not homologous*. Modern avian birds are more closely related to saurischian dinosaurs (such as quadrupedal Apatosaurus and bipedal Tyrannosaurus) than to ornithischian  dinosaurs (such as quadrupedal Triceratops) & bipedal Pachycephalosaurus).

        *The famous paleontologist Alfred Sherwood Romer reminds us, "Student: You said that before. Romer: Yes, but you had forgotten it. Student: Not a bit of it. Romer: Very well, let's continue."