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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.