Phylogeny of Archosauria (including Aves)

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