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Modern birds inhabit most available habitats, from terrestrial to marine, and there is evidence that some non-avian dinosaurs (such as Microraptor) could fly or at least glide, and others, such as spinosaurids, had semiaquatic habits.

While recent discoveries have made it more difficult to present a universally agreed-upon list of dinosaurs' distinguishing features, nearly all dinosaurs discovered so far share certain modifications to the ancestral archosaurian skeleton, or are clear descendants of older dinosaurs showing these modifications.

In traditional taxonomy, birds were considered a separate class that had evolved from dinosaurs, a distinct superorder.

However, a majority of contemporary paleontologists concerned with dinosaurs reject the traditional style of classification in favor of phylogenetic taxonomy; this approach requires that, for a group to be natural, all descendants of members of the group must be included in the group as well.

Dinosauria itself was re-defined as the last common ancestor of Triceratops horridus, Passer domesticus, Diplodocus carnegii, and all of its descendants, to ensure that sauropods and kin remain included as dinosaurs.

Many prehistoric animal groups are popularly conceived of as dinosaurs, such as ichthyosaurs, mosasaurs, plesiosaurs, pterosaurs, and pelycosaurs (especially Dimetrodon), but are not classified scientifically as dinosaurs, and none had the erect hind limb posture characteristic of true dinosaurs.

Both definitions result in the same set of animals being defined as dinosaurs: "Dinosauria = Ornithischia Saurischia", encompassing ankylosaurians (armored herbivorous quadrupeds), stegosaurians (plated herbivorous quadrupeds), ceratopsians (herbivorous quadrupeds with horns and frills), ornithopods (bipedal or quadrupedal herbivores including "duck-bills"), theropods (mostly bipedal carnivores and birds), and sauropodomorphs (mostly large herbivorous quadrupeds with long necks and tails).

Birds are now recognized as being the sole surviving lineage of theropod dinosaurs.

Some are herbivorous, others carnivorous, including seed-eaters, fish-eaters, insectivores, and omnivores.

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.

Although known for large size, many Mesozoic dinosaurs were human-sized or smaller, and modern birds are generally small in size.

Dinosaurs today inhabit every continent, and fossils show that they had achieved global distribution by at least the early Jurassic period.

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