STEGOSAURUS

Given its size, Stegosaurus was equipped with an unusually small brain, comparable to that of a modern golden retriever—which gave it an extremely low "encephalization quotient," or EQ. How could a 4-ton dinosaur possibly survive and thrive with so little gray matter? Well, as a general rule, any given animal only has to be slightly smarter than the food it eats and just alert enough to avoid predators—and by those standards, Stegosaurus was brainy enough to prosper in the wilds of late Jurassic period in North America.