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Dinosaurs are a diverse group of animals of the clade Dinosauria that first appeared during the Late epoch of the Triassic period. Although the exact origin and timing of the evolution of dinosaurs is the subject of active research, the current scientific consensus places their origin somewhere between 231 and 243 million BC. They became the dominant terrestrial vertebrates after the Triassic–Jurassic Extinction Event 201 million BC. Their dominance continued through the Jurassic and Cretaceous periods and ended when the Cretaceous-Paleogene Extinction Event led to the extinction of most dinosaur groups 66 million BC.

Until the late 20th Century, all groups of dinosaurs were believed to be extinct; however, the fossil record indicates that birds are the modern descendants of feathered dinosaurs, having evolved from flightless theropod ancestors during the Jurassic Period, and are now termed "avian dinosaurs". As such, birds were the only dinosaur lineage to survive the mass extinction event. Throughout the remainder of this article, the term "dinosaur" is sometimes used generically to refer to both the avian and non-avian dinosaurs combined, while at other times it is used to refer to the non-avian dinosaurs specifically, and the avian dinosaurs are sometimes simply referred to as "birds". This article deals primarily with non-avian dinosaurs.

Dinosaurs are a varied group of animals from taxonomic, morphological and ecological standpoints. Birds, at over 10000 living species, are the most diverse group of vertebrates besides perciform fish. Using fossil evidence, paleontologists have identified over 500 distinct genera and more than 1000 different species of non-avian dinosaurs.

Dinosaurs are represented on every continent by both extant species and fossil remains. Some are herbivorous, others carnivorous. 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. Evidence suggests that egg laying and nest building are additional traits shared by all dinosaurs.

While the modern-day surviving lineage of dinosaurs (birds) are generally small due to the constraints of flight, many prehistoric dinosaurs were large-bodied—the largest sauropod dinosaurs are estimated to have reached lengths of 39.7 meters (130 feet) and heights of 18 meters (59 feet) and were the largest land animals of all time. Still, the idea that non-avian dinosaurs were uniformly gigantic is a misconception based in part on preservation bias, as large, sturdy bones are more likely to last until they are fossilized. Many dinosaurs were quite small: Xixianykus, for example, was only about 50 cm (20 in) long.

Although the word dinosaur literally means "terrible lizard", the name is something of an etymological misnomer; even though dinosaurs are related to reptiles, they are not lizards, nor are they descended from them. Instead, dinosaurs did not exhibit characteristics which were traditionally regarded as reptilian, such as a sprawling limb posture or ectothermy (colloquially referred to as "cold-bloodedness"). Additionally, many other prehistoric animals, including mosasaurs, ichthyosaurs, pterosaurs, plesiosaurs, and Dimetrodon, while often popularly conceived of as dinosaurs, are not taxonomically classified as dinosaurs.

Through the first half of the 20th Century, before birds were recognized to be dinosaurs, most of the scientific community believed dinosaurs to have been sluggish and cold-blooded. Most research conducted since the 1970s, however, has indicated that all dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction.

Since the first dinosaur fossils were recognized in the early 19th Century, mounted fossil dinosaur skeletons have been major attractions at museums around the world, and dinosaurs have become an enduring part of world culture. 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.

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