When did the first dinosaurs appear?

Dinosaurs first emerged around 230 million years ago in the Triassic period, evolving from small, agile reptiles. Early dinosaurs like Eoraptor and Herrerasaurus were relatively small — often no larger than a dog or a person — and were not yet the dominant land animals. They rose to dominance after a mass extinction at the end of the Triassic, about 201 million years ago, wiped out many of their competitors. The dinosaurs then ruled the land for roughly 165 million years across the Jurassic and Cretaceous periods, until the asteroid impact 66 million years ago ended the era of the non-bird dinosaurs.