Claim: Birds are a dinosaur lineage.

Dinosaurs are a diverse group of reptiles of the clade Dinosauria that first appeared during 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 between 231 and 243 million years ago . 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 and ended when the Cretaceous -- Paleogene extinction event led to the extinction of most dinosaur groups 66 million years ago .   Until the late 20th century , all groups of dinosaurs were believed to be extinct . The fossil record , however , indicates that birds , which are now termed `` avian dinosaurs , '' are the modern descendants of feathered dinosaurs , having evolved from theropod ancestors during the Jurassic Period . 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 the combined group of avian dinosaurs -LRB- birds -RRB- and non-avian dinosaurs ; at other times it is used to refer to the non-avian dinosaurs specifically , while 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 10,000 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 1,000 different species of non-avian dinosaurs . Dinosaurs are represented on every continent by both extant species -LRB- birds -RRB- and fossil remains . 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 . Some are herbivorous , others carnivorous . Evidence suggests that egg laying and nest building are additional traits shared by all dinosaurs .   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 . While the dinosaurs ' modern-day surviving avian lineage -LRB- birds -RRB- are generally small due to the constraints of flight , many prehistoric dinosaurs -LRB- non-avian and avian -RRB- were large-bodied -- the largest sauropod dinosaurs are estimated to have reached lengths of 39.7 m and heights of 18 m 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 long .   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 .