Stegosaurus is the classic plated dinosaur most people picture: large back plates, a small head, and a spiked tail used for defense. It lived late in the Jurassic, especially in what is now the western United States, and is best known from the Morrison Formation. This is the famous, fully recognizable stegosaur of popular imagination — the polished, later form of a branch that had already been evolving for quite some time.