Stegosaurus When: Late Jurassic Where: North America Length: Up to 30 ft. (9 m) Weight: 4 tons Besides the huge 3-foot-long tail spikes, Stegosaurus had two rows of bony plates that armored its spine. Some scientists believe the plates may have also been heat regulators. Its brain was so small, about the size of a walnut, that it needed a second brain that was housed in its hindquarters near its spinal cord. Like other plant eaters, it probably lived in herds.
