The stegosaur is one of the most singular and most familiar representatives of the group in the Jurassic.
Any of several large herbivorous ornithischian dinosaurs of the group Stegosauria of the Jurassic and early Cretaceous Periods. The largest genus, Stegosaurus, had a tail with two horizontal spikes for defense, and an arched back with an alternating double row of large, triangular, upright bony plates. Stegosaurs grew over 6 m (20 ft) long, but had extremely small heads with brains the size of a walnut. The hindquarters were controlled by a neural ganglion in the hip region that was larger than the brain.