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While dinosaurs were ancestrally bipedal, many extinct groups included quadrupedal species, and some were able to shift between these stances.Elaborate display structures such as horns or crests are common to all dinosaur groups, and some extinct groups developed skeletal modifications such as bony armor and spines.Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical for Dinosauria; the earliest dinosaurs had them and passed them on to their descendants.Such modifications, originating in the most recent common ancestor of a certain taxonomic group, are called the synapomorphies of such a group.Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic, especially the Jurassic and Cretaceous periods.Other groups of animals were restricted in size and niches; mammals, for example, rarely exceeded the size of a domestic cat, and were generally rodent-sized carnivores of small prey.Birds are thus considered to be dinosaurs and dinosaurs are, therefore, not extinct. Barrett in 2017 suggested a radical revision of dinosaurian systematics. recovered the Ornithischia as being closer to the Theropoda than the Sauropodomorpha, as opposed to the traditional union of theropods with sauropodomorphs.
Dinosaurs today inhabit every continent, and fossils show that they had achieved global distribution by at least the early Jurassic period. Clockwise from top left: Microraptor gui (a winged theropod), Apatosaurus louisae (a giant sauropod), Edmontosaurus regalis (a duck-billed ornithopod), Triceratops horridus (a horned ceratopsian), Stegosaurus stenops (a plated stegosaur), Pinacosaurus grangeri (an armored ankylosaur) They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event 201 million years ago; their dominance continued through the Jurassic and Cretaceous periods.The fossil record indicates that birds are modern feathered dinosaurs, Dinosaurs can therefore be divided into avian dinosaurs, or birds; and non-avian dinosaurs, which are all dinosaurs other than birds.Dinosaurs have always been an extremely varied group of animals; according to a 2006 study, over 500 non-avian dinosaur genera have been identified with certainty so far, and the total number of genera preserved in the fossil record has been estimated at around 1850, nearly 75% of which remain to be discovered.Some are herbivorous, others carnivorous, including seed-eaters, fish-eaters, insectivores, and omnivores.
In traditional taxonomy, birds were considered a separate class that had evolved from dinosaurs, a distinct superorder.