Stegosaurus was a slow moving plant eating dinosaur. The main feature of Stegosaurus are the series of bony plates running along the top of the dinosaur's back. The purpose of the plates is not known, but may have been for display to attract other dinosaurs, or to scare attackers away. It is possible that skin covered the plates and their purpose was to control the dinosaur's body temperature.

The main form of defence were a set of spikes on the tail. While the plates on the back were staggered, the four spikes were arranged in pairs. The spikes could be up a meter in length.

The head of the Stegosaurus was very small compared to the size of its body. It also had a tiny brain compared to it's size. Possibly one of the smallest brains of any dinosaur. But that may not have mattered to a creature that spent all of its time grazing. All of the dinosaur's teeth were at the back of the jaw and were used for grinding the vegetation is feed on. At the front of the mouth was a horny beak used for cutting the plants.

This dinosaur was first described by Othniel Charles Marsh, but because the bones were scattererd the proper arrangement of the bony plates was not initially understood. The first thoughts were that the plates lay flat on the back of the creature and this is why the name 'roof lizard' was given. It was later found from more examples found that the plates stood vertical on the back.