[Photo: Dinosaurs] Dinosaurs were the dominant vertebrate animals of terrestrial ecosystems for over 160 million years, from the late Triassic period (about 230 million years ago) until the end of the Cretaceous period (65 million years ago), when most of them became extinct in the Cretaceous–Tertiary extinction event. The 10,000 living species of birds have been classified as dinosaurs. The 1861 discovery of the primitive bird Archaeopteryx first suggested a close relationship between dinosaurs and birds. Aside from the presence of fossilized feather impressions, Archaeopteryx was very similar to the contemporary small predatory dinosaur Compsognathus. Research has since identified theropod dinosaurs as the most likely direct ancestors of birds; most paleontologists today regard bir...more >>ds as the only surviving dinosaurs, and some suggest that dinosaurs and birds should be grouped into one biological class. Aside from birds, crocodilians are the only other close relatives of dinosaurs to have survived until the present day. Like dinosaurs and birds, crocodilians are members of Archosauria, a group of reptiles that first appeared in the very late Permian and came to predominate in the mid-Triassic.