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. The
fossil record indicates that
birds are modern
feathered dinosaurs, having evolved from
theropod ancestors during the Jurassic Period.
[2] Birds were the only dinosaurs to survive the extinction event that occurred 66 million years ago.
[3]
Dinosaurs are a varied group of animals from
taxonomic,
morphological and
ecological standpoints. Birds, at over 10,000 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 1,000 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 modern 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 over 40 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 means "terrible lizard", the name is somewhat misleading, as dinosaurs are not
lizards. Instead, they represent a separate group of
reptiles that, like many extinct forms, did not exhibit characteristics traditionally seen as reptilian, such as a
sprawling limb posture or
ectothermy. Additionally, many prehistoric animals, including
mosasaurs,
ichthyosaurs,
pterosaurs,
plesiosaurs, and
Dimetrodon, are popularly conceived of as dinosaurs, but 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 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.
