Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period (about 230 million years ago) until the end of the Cretaceous (about 65 million years ago), when the Cretaceous–Paleogene extinction event led to the extinction of all non-avian dinosaurs at the close of the Mesozoic era. The fossil record indicates that birds evolved within theropod dinosaurs during the Jurassic period. Some of them survived the Cretaceous–Paleogene extinction event, including the ancestors of all modern birds. Consequently, in modern classification systems, birds are considered a type of dinosaur—the only group which survived to the present day.

Dinosaurs are a varied group of animals. Birds, at over 9,000 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 and fossil remains. Some are herbivorous, others carnivorous. Many dinosaurs have been bipedal, and many extinct groups were also quadrupedal, 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 even developed skeletal modifications such as bony armor and spines. Avian dinosaurs have been the planet's dominant flying vertebrate since the extinction of the pterosaurs, and evidence suggests that all ancient dinosaurs built nests and laid eggs much as avian species do today. Dinosaurs varied widely in size and weight; the smallest adult theropods were less than 100 centimeters (40 inches) long, while the largest sauropods could reach lengths of almost 50 meters (165 feet) and were several stories tall.