Dinosaurs are land reptiles that include some of the biggest terrestrial life forms ever on Earth. We know dinosaurs from fossilized bones, teeth, footprints, eggs, and occasionally even soft tissues, although complete skeletons are rare. The earliest known dinosaurs are from the late Triassic (about 230 million years ago), when they were still overshadowed by other animals, such as synapsids (related to the ancestors of mammals). By the end of the Triassic, dinosaurs had begun their remarkable diversification, with several major lineages present. The Jurassic and Cretaceous periods saw dinosaurs dominate many land environments, reach enormous sizes, and diversify into thousands of species (including the first birds). Paleontologists have named more than 1200 species of dinosaurs, and that’s probably just a fraction of all the dinosaur species that existed. Most dinosaurs went extinct abruptly 66 million years ago (end of the Cretaceous), along with many other species. A period of severe environmental stress, punctuated by an asteroid impact, was likely responsible for this mass extinction.