8musrose

  Dinosaurs were a successful group of animals that emerged between 240 million and 230 million years ago and came to rule the world until about 66 million years ago, when a giant asteroid slammed into   Earth . During that time, dinosaurs evolved from a group of mostly   dog - and   horse -size creatures into the most enormous beasts that ever existed on land. Some meat-eating  dinosaurs shrank over time  and evolved into birds. So, in that sense, only the non-avian dinosaurs went extinct. (For the purposes of this article, "dinosaurs" will refer to non-avian dinosaurs, unless otherwise stated.) During the roughly 174 million years that dinosaurs existed, the world changed greatly. When dinosaurs first appeared in the  Triassic period  (252 million to 201 million years ago), they roamed the supercontinent of  Pangea . But by the time the asteroid hit at the end of the  Cretaceous period  (145 million to 66 million years ago), the continents were in approximately the same pla