The term dinosaur refers to a nearly extinct group of animals that were alive for 175 million years. They first appeared during the Triassic period, approximately 230 million years ago, and were dominant for 135 million years, from the beginning of the Jurassic (about 201 million years ago) until the end of the Cretaceous (66 million years ago), when an 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 by many paleontologists. Some birds survived the extinction event that occurred 66 million years ago, and their descendants continue the dinosaur lineage to the present day.