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Dinosaurs are a diverse group of animals of the clade Dinosauria. They first appeared during the Triassic period, 231.4 million years ago, and were the dominant terrestrial vertebrates for 135 million years, from the start of the Jurassic (about 200 million years ago) until the end of the Cretaceous (66 million years ago),[1] when the Cretaceous–Paleogene extinction event led to the extinction of most dinosaur groups at the end of the Mesozoic Era.

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 theropod ancestors during the Jurassic Period,[2] and are now termed "avian dinosaurs". As such, birds were the only dinosaur lineage to survive the mass extinction event.[3] 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 the extinct non-avian dinosaurs.

Dinosaurs are a varied group of animals from taxonomic, morphological and ecological standpoints. Birds, at over 10000 living species,[4] are the most diverse group of vertebrates besides perciform fish.[5] Using fossil evidence, paleontologists have identified over 500 distinct genera[6] and more than 1000 different species of non-avian dinosaurs.[7]

Dinosaurs are represented on every continent by both extant species and fossil remains.[8] 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)[9] and heights of 18 meters (59 feet)[10][11] 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 somewhat an etymological misnomer and misleading, being that even though dinosaurs represent a separate group of reptiles, they are not lizards, nor are they descended from the lizard group of reptiles. Instead, dinosaurs, like many extinct forms of reptile sub-groups, 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.