Dinosaurs are divided into two groups, Saurischians and Ornithischians, based on their pelvic structure. Saurischians included carnivorous theropods and herbivorous sauropodomorphs, while Ornithischians included various herbivores. Both groups evolved around 230-225 million years ago and only became prominent after the Permian-Triassic extinction event. All non-avian dinosaurs went extinct 65 million years ago, leaving no living descendants.
Saurischians and Ornithischians are the two groups of dinosaurs, defined in terms of their pelvic structure. Saurischians, whose name means “with lizard-like hips,” had a pelvic structure more similar to that of modern lizards, while ornithischians (“bird-like hips”) had a pelvic structure more like that of modern birds. Confusingly, many scientists believe that modern birds actually evolved from a group of lizard-hipped dinosaurs called coelurosaurs, which evolved bird hips in an example of parallel evolution. All non-avian dinosaurs went extinct about 65 million years ago in an event called the Cretaceous-Tertiary extinction. Therefore, the saurischians left no living descendants.
Saurischians consisted primarily 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 major lineages of herbivorous dinosaurs. Ornithischians included large numbers of herbivores, some of which traveled in herds, including ceratopsians such as triceratops, armored and club-tailed ankylosaurs, duck-billed hadrosaurs, and stegosaurs, with prominent plates and spines along the back. Ornithischians were preyed upon by theropods and were generally smaller than the titanic sauropods.
The two groups branched off from each other in the Late Triassic, between 230 and 225 million years ago. One of the earliest and possibly closely related common ancestor of all dinosaurs, the fossil species Eoraptor, had lizard pelvis, while the earliest known ornithiscian dinosaur, Pisanosaurus mertii, is dated several million years later. Both were about a meter long. A sauropod-like lineage, called prosauropods, also evolved around this time and quickly reached sizes of 10 m (33 ft), unusually large for this period in Earth’s history.
Both groups of dinosaurs only came to prominence 20-30 million years after the Permian-Triassic extinction event, which wiped out most life and created many open niches for dinosaurs to fill. The evolutionary situation at the time was analogous to what happened just 65 million years ago, when dinosaurs became extinct and left many niches open for mammals.
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