Illustration of Stegosaurus armatus. Stegosaurus is a genus of stegosaurid armored dinosaur from the Late Jurassic period in what is now western North America. Due to its distinctive tail spikes and plates, Stegosaurus is one of the most recognizable dinosaurs, along with Tyrannosaurus, Triceratops, and Apatosaurus. They lived some 155 to 145 million years ago. A large, heavily built, herbivorous quadruped, Stegosaurus had a distinctive and unusual posture, with a heavily arched back, short forelimbs, head held low to the ground and a stiffened tail held high in the air. Its array of plates and spikes has been the subject of much speculation. The spikes were most likely used for defense, while the plates have been proposed as a defensive mechanism, as well as having display and thermoregulatory (heat control) functions.