Stegosaurus (steg-oh-sore-us) means "Roofed Lizard." Stegosaurus got its name because, when it was discovered in the 1800s, scientists thought that its famous back plates laid down flat like roof shingles, for protection. Stegosaurus also had a small head and a very small brain, which was roughly the size of a walnut. Two rows of large bony plates ran down the back and tail of Stegosaurus, and were most likely used for blood regulation. Also, the 4 spikes on its tail were used for defense.