Dinosaurs are a diverse group of animals of the clade Dinosauria. They first appeared during the Triassic period, approximately 230 million years ago, and were the dominant terrestrial vertebrates for 135 million years, from the beginning of the Jurassic (about 200 million years ago) until the end of the Cretaceous (65.5 million years ago), when the Cretaceous–Paleogene extinction event led to the extinction of most dinosaur groups 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 a subgroup of dinosaurs in modern classification systems.[1][2] Some birds survived the extinction event that occurred 65 million years ago, and their descendants continue the dinosaur lineage to the present day.

Dinosaurs are a varied group of animals from taxonomic, morphological and ecological standpoints. Birds, at over 9,000 living species, are the most diverse group of vertebrates besides perciform fish.[3] Using fossil evidence, paleontologists have identified over 500 distinct genera[4] and more than 1,000 different species of non-avian dinosaurs.[5] Dinosaurs are represented on every continent by both extant species and fossil remains.[6] Some are herbivorous, others carnivorous. Most dinosaurs have been bipedal, though many extinct groups included quadrupedal species, and some were able to shift between these body postures. Many species possess elaborate display structures such as horns or crests, and some prehistoric groups developed skeletal modifications such as bony armor and spines. Birds have been the planet's dominant flying vertebrate since the extinction of the pterosaurs, and evidence suggests that egg laying and nest building is a trait shared by all dinosaurs. While many prehistoric dinosaurs were large animals—the largest sauropods could reach lengths of almost 60 meters (200 feet) and were several stories tall—the idea that non-avian dinosaurs were uniformly gigantic is a misconception; many ancient species were nearly as small as birds are today.

Although the word dinosaur means "terrible lizard," the name is somewhat misleading, as dinosaurs are not lizards. Rather, they represent a separate group of reptiles with a distinct upright posture not found in lizards, and many extinct forms did not exhibit traditional reptilian characteristics. Through the first half of the 20th century, before birds were recognized to be dinosaurs, most of the scientific community believed dinosaurs to be sluggish and cold-blooded. Most research conducted since the 1970s, however, has indicated that ancient dinosaurs, particularly the carnivorous groups, 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.

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