Saurischians and ornithischians are the two groups of dinosaur, defined in terms of pelvic structure. Saurischians, whose name means "lizard-hipped," had a pelvic structure more similar to that of modern lizards, while the ornithischians ("bird-hipped") had a pelvic structure more like modern birds. Confusingly, modern birds actually evolved from a group of lizard-hipped dinosaurs called coelurosaurs, which evolved the bird hip in an example of parallel evolution. All non-avian dinosaurs went extinct 65.5 million years ago in an event called the Cretaceous-Tertiary extinction. Thus, saurischians left no living descendants.

Saurischians mostly consisted of theropods, which included all carnivorous dinosaurs including T. rex, and sauropodomorphs, which included the sauropods, by far the largest group of dinosaurs and one of the two great dinosaur herbivore lineages. Ornithischians included a large number of herbivores, some of which traveled in herds, which include ceratopians such as Triceratops, the armored and club-tailed ankylosaurs, the duck-billed hadrosaurs, and stegosaurs, with prominent plates and spines along their back. Ornithischians were preyed on by theropods and were generally smaller than the titanic sauropods.

The two groups branched off from each other in the late Triassic period, between 230 and 225 million years ago. One of the earliest and possibly closely related to the common ancestor of all dinosaurs, the fossil species Eoraptor, was lizard-hipped, while the earliest known ornithiscian dinosaur, Pisanosaurus mertii, is dated to several million years later. Both were about a meter in length. A sauropod-like lineage, called prosauropods, also evolved around this time, and quickly reached sizes as large as 10 m (33 ft), unusually large for this time in Earth's history.

Both groups of dinosaur came to prominence just 20-30 million years after the Permian-Triassic extinction, which wiped out the majority of all life and created many open niches for dinosaurs to occupy. The evolutionary situation at the time was analogous to what happened just 65 million years ago — when dinosaurs died off and left many niches open for mammals.