The Saurischia are one of the two basic divisions of dinosaurs. Saurischian dinosaurs are traditionally distinguished from ornithischian dinosaurs by their three-pronged pelvic structure, with the pubis pointed forward. The ornithischian pelvis is arranged with the pubis rotated backward, parallel with the ischium, often also with a forward-pointing process, giving a four-pronged structure. The oldest known dinosaurs, from the middle Triassic of South America, were saurischians.

The saurischians form two major sub-groups: the Sauropoda and the Theropoda. The Sauropoda were large herbivores such as Apatosaurus and Diplodocus. The Theropoda were bipedal carnivores (meat eaters), ranging from the chicken-sized Compsognathus and the fearsome Deinonychus and Velociraptor to the crested Dilophosaurus and the gigantic Tyrannosaurus. Living birds had common ancestors on the theropod lineage.