A stegosaurus was a dinosaur that lived about 150 million years ago, during a period scientists call the Jurassic. Picture a creature the size of a school bus, with a tiny head no bigger than a dog's, a huge rounded body, and a double row of enormous bony plates standing upright along its back like a spiky mohawk. Its tail bristled with four sharp spikes, each as long as a baseball bat, which it could swing as a weapon against predators.

The name comes from Greek words meaning “roofed lizard” because scientists first thought those back plates lay flat like roof tiles. We now know they stood upright, possibly helping the stegosaurus control its body temperature by absorbing warmth from the sun or releasing heat, like built-in radiators. Despite its imposing size (some weighed as much as an elephant), the stegosaurus was a plant-eater, using its beak-like mouth to strip leaves from low-growing plants.

Scientists once joked that the stegosaurus had a brain the size of a walnut, and some mistakenly claimed it had a second brain near its tail. Neither is true, though its brain was unusually small for its body size. Still, the stegosaurus survived for millions of years, which suggests it was well-adapted to its world.