STEGOSAURUS

genus of armoured dinosaurs of the Late Jurassic Period (163 to 144 million years ago). Stegosaurus reached a length of about 6.5 m (20 feet). The skull and brain were very small for such a large animal. The forelimbs were much shorter than the hind limbs, giving the back a characteristically arched appearance. Some authorities believe that the hind limbs were so long because Stegosaurus originally evolved from a bipedal form and then reverted to a quadrupedal posture. The feet were short and broad. Perhaps the most distinctive feature of Stegosaurus was a series of large, triangular, horn-covered bony plates along its back and tail. There exist various theories as to their arrangement and use. Paleontologists had long thought that Stegosaurus had two parallel rows of plates, either staggered or in pairs with each other, and that these served a defensive purpose, affording protection to the animal's backbone and spinal cord. Reexamination of Stegosaurus specimens during the 1970s, however, suggested to some investigators that the alternating placement of these highly vascularized back plates was consistent with a temperature-regulating function. It was proposed that they acted as radiators, releasing body heat to a cooler ambient environment by way of the warm blood-filled structures. No two plates from the same animal have exactly the same shape or size; because of this, it is believed that there were double rows of alternating plates rather than a single midline row. Two or three pairs of long, pointed bony spikes were present on the end of the tail of Stegosaurus. These are presumed to have served as defensive weapons, but they may have been ornamental. The spinal cord in the region of the sacrum was enlarged and actually larger than the brain, giving rise to the misconception that Stegosaurus possessed two brains. Stegosaurus was a plant eater, probably feeding on soft vegetation.