When Did Dinosaurs First Appear?

Dinosaurs first emerged during the Triassic period, roughly 230 to 240 million years ago, evolving from a group of reptiles called archosaurs. The earliest dinosaurs were relatively small, two-legged creatures. At that time, all continents were joined in the supercontinent Pangaea. Dinosaurs rose to dominance after a mass extinction at the end of the Triassic (about 201 million years ago) cleared out many competitors. They then ruled the land throughout the Jurassic and Cretaceous periods, evolving into an incredible diversity of forms — from the gigantic long-necked sauropods like Brachiosaurus (among the largest land animals ever) to fearsome predators like Tyrannosaurus rex and Velociraptor. Dinosaurs dominated terrestrial ecosystems for about 165 million years, an almost unimaginably long span. Their reign ended abruptly around 66 million years ago when a massive asteroid impact (and possibly volcanic activity) triggered a mass extinction that killed all non-bird dinosaurs. Modern birds are the direct descendants of small feathered theropod dinosaurs, so in a real sense, dinosaurs never fully went extinct.