3 Dinosaur

Dinosaurs are a diverse group of reptiles[note 1] of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago,[1][2] although the exact origin and timing of the evolution of dinosaurs is the subject of active research.[3] 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. Reverse genetic engineering[4] and the fossil record both demonstrate that birds are modern feathered dinosaurs,[5] having evolved from earlier theropods during the late Jurassic Period.[6] As such, birds were the only dinosaur lineage to survive the Cretaceous–Paleogene extinction event 66 million years ago.[7] Dinosaurs can therefore be divided into avian dinosaurs, or birds; and non-avian dinosaurs, which are all dinosaurs other than 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,[8] are the most diverse group of vertebrates besides perciform fish.[9] Using fossil evidence, paleontologists have identified over 500 distinct genera[10] and more than 1,000 different species of non-avian dinosaurs.[11] Dinosaurs are represented on every continent by both extant species (birds) and fossil remains.[12] 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 were herbivorous, others carnivorous. Evidence suggests that egg-laying and nest-building are additional traits shared by all dinosaurs, avian and non-avian alike.

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 (birds) are generally small due to the constraints of flight, many prehistoric dinosaurs (non-avian and avian) were large-bodied—the largest sauropod dinosaurs are estimated to have reached lengths of 39.7 meters (130 feet)[13] and heights of 18 meters (59 feet)[14] 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.

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.

Origins and early evolution

Dinosaurs diverged from their archosaur ancestors during the middle to late Triassic period, roughly 20 million years after the Permian–Triassic extinction event wiped out an estimated 95% of all life on Earth.[43][44] Radiometric dating of the rock formation that contained fossils from the early dinosaur genus Eoraptor at 231.4 million years old establishes its presence in the fossil record at this time.[45] Paleontologists think that Eoraptor resembles the common ancestor of all dinosaurs;[46] if this is true, its traits suggest that the first dinosaurs were small, bipedal predators.[47] The discovery of primitive, dinosaur-like ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators. Dinosaurs may have appeared as early as 243 million years ago, as evidenced by remains of the genus Nyasasaurus from that period, though known fossils of these animals are too fragmentary to tell if they are dinosaurs or very close dinosaurian relatives.[48] Recently, it has been determined that Staurikosaurus from the Santa Maria Formation dates to 233.23 Ma, making it older in geologic age than Eoraptor.[1]

When dinosaurs appeared, they were not the dominant terrestrial animals. The terrestrial habitats were occupied by various types of archosauromorphs and therapsids, like cynodonts and rhynchosaurs. Their main competitors were the pseudosuchia, such as aetosaurs, ornithosuchids and rauisuchians, which were more successful than the dinosaurs.[49] Most of these other animals became extinct in the Triassic, in one of two events. First, at about 215 million years ago, a variety of basal archosauromorphs, including the protorosaurs, became extinct. This was followed by the Triassic–Jurassic extinction event (about 200 million years ago), that saw the end of most of the other groups of early archosaurs, like aetosaurs, ornithosuchids, phytosaurs, and rauisuchians. Rhynchosaurs and dicynodonts survived (at least in some areas) at least as late as early-mid Norian and late Norian or earliest Rhaetian, respectively,[50][51] and the exact date of their extinction is uncertain. These losses left behind a land fauna of crocodylomorphs, dinosaurs, mammals, pterosaurians, and turtles.[19] The first few lines of early dinosaurs diversified through the Carnian and Norian stages of the Triassic, possibly by occupying the niches of the groups that became extinct.[21] Also notably, there was a heightened rate of extinction during the Carnian Pluvial Event.[52]