Mounted skeletons of Tyrannosaurus (left) and Apatosaurus (right) at the AMNH.

Dinosaurs are vertebrate animals that dominated terrestrial environments for over 160 million years, first appearing approximately 230 million years ago. At the end of the Cretaceous Period, approximately 65 million years ago, a catastrophic extinction event ended the dinosaurs' dominance on land, making them extinct. However, taxonomists consider modern birds to be direct descendants of theropod dinosaurs, making them the only "dinosaurs" alive today. The term dinosaur is sometimes used informally to describe other prehistoric reptiles, such as the pelycosaur Dimetrodon, the winged pterosaurs, and the aquatic ichthyosaurs, plesiosaurs and mosasaurs, although technically none of these were dinosaurs.

Before their extinction, dinosaurs were the dominant group of lifeforms on the planet. They were extremely varied, abundant and found in most areas of the earth. Some were herbivorous, others carnivorous. Some dinosaurs were bipeds, some were quadrupeds, and others, such as Ammosaurus and Iguanodon, could walk just as easily on two or four legs. Regardless of body type, nearly all known dinosaurs were well-adapted for a predominantly terrestrial, rather than aquatic or aerial, habitat.

Since the first dinosaur fossils were recognized in the nineteenth century, mounted dinosaur skeletons have become major attractions at museums around the world and have amassed a large fan base among children and adults alike. They have been featured in best-selling books and films such as Jurassic Park, and new discoveries are regularly covered by the media.