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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.
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.
The taxon Dinosauria was formally named in 1842 by paleontologist Sir Richard Owen, who used it to refer to the "distinct tribe or sub-order of Saurian Reptiles" that were then being recognized in England and around the world.
Birds are classified as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs. They resurrected the clade Ornithoscelida to refer to the group containing Ornithischia and Theropoda.
The large sizes of some dinosaur groups, as well as their seemingly monstrous and fantastic nature, have ensured dinosaurs' regular appearance in best-selling books and films, such as Jurassic Park.
Persistent public enthusiasm for the animals has resulted in significant funding for dinosaur science, and new discoveries are regularly covered by the media.
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.