Stegosaurus is a genus of armored dinosaurs darting back to the Late Jurassic. They were heavy, herbivorous quadrupeds with rounded backs, short fore limbs, long hind limbs, broad, upright plated along the spinal region, and spiked tails. Although the function of the plates and spikes has been the subject of much speculation among paleontologists, the spikes were most likely used for protection against predators and the plates may have been used for display and termoregulatory functions. The plates numbered from 17 to 22 and were modified bony=cored scales. They were not directly connected to the skeleton, rising instead from the skin. Its small neck and small head probably inferred that they ate low-lying bushes and shrubs. While an average adult grew to a length of about 21 feet (6.5 m) with some growing to lengths of 30 feet (9 m) with a weight of about 4 tons, its brain was the size of that of a golden retriever. Similar to other plant-eating dinosaurs, Stegosaurus swallowed small rocks known as gastroliths to mash up tough vegetable matter in an enormous stomach.