30 feet long
Late Jurassic, 65 million years ago
Western North America
Scientists are not sure what use Stegosaurus made of the big plates on its back. Perhaps they helped cool the animal on hot days. Or maybe they were colored in displays to attract mates. Stegosaurus has a brain the size of a ping-pong ball. In the past, scientists thought Stegosaurus had a second brain in its butt. But no animal does. But Stegosaurus did have a lump of nerves in its rear.