Dinosaur

Dinosaurs are a diverse group of Terran animals of the clade Dinosauria. They first appeared during Terra's Triassic period, 231.4 million years ago, and were the dominant vertebrates for 135 million years, from the end of the Triassic (about 231 million years ago) until the end of the Cretaceous(66 million years ago), when the Cretaceous–Paleogene extinction event led to the extinction of most dinosaur groups (and some three-quarters of plant and animal species on Earth) at the close of the Mesozoic Era. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic Period and, consequently, they are considered to be modern feathered dinosaurs. Some birds survived the extinction event that occurred 65 million years ago, and their descendants continue the dinosaur lineage to the present day.

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 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 modern dinosaurs (birds) are generally small due to the constraints of flight, many prehistoric dinosaurs were large-bodied—the largest sauropod dinosaurs may have achieved lengths of 58 meters and heights of 9.25 meters. Still, the idea that non-avian dinosaurs were uniformly gigantic is a misconception based 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.

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 are popularly conceived of as dinosaurs, but are not taxonomically classified as dinosaurs. Through the first half of the 20th century CE, 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 CE, however, has indicated that all dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction. Persistent public enthusiasm for the animals has resulted in significant funding for dinosaur science, and new discoveries are regularly covered by the media.

