Stegosaurus is an ornithiscian, Stegosaurid plant-eater that lived approximately 152-141 million years ago in the United States.
Overview
http://www.dinosaursinart.com/stegosaurus/IMAG0009.JPG
Stegosaurus is an interesting animal, to say the least. It's body design is unique: it has a bulky, quadrupedal design, medium-length tail, and small head. But Stegosaurus' most interesting feature is the double rows of large, bony plates running down it's spine. These back plates are what makes Stegosaurus so well-known, though their pupose is a mystery. Stegosaurus was a well-defended animal: a cluster of armor-like scutes covered it's most delicate region: the throat. A thagomizer formation of spikes adorned the tip of it's tail, making an effective defense for attacking carnivores on the attack.
Environment
Stegosaurus lived in the late Jurassic period, in the western and central United States. This puts it in a wet, humid, plains and forest habitat. Stegosaurus lived alongside other herbivores like the sauropods, Barosaurus, Diplodocus, and Apatosaurus, and other ornithicsians like Camptosaurus and Drinker. It may have been antagonized by carnivores like Allosaurus, but some say that Stegosaurus was too dangerous to worry about threat from predators.
Anatomy
One of the most enigmatic features of Stegosaurus is the dorsal plates aligning it's back. The purpose of these is unknown, but there are many theories. One such thoery suggests that they were used to regulate temperature. Here, the spines would be filled with blood vessels, meaning that if the creature was cold, it could lean to the side, allowing sunlight to hit the plates like a solar panel, sending warmth through the bloodstream. Another theory says that they were used as a warning against carnivores, pumping with blood so as to flare up with color, bright reds and pinks, creating an intimidating display. Another feature is the spiked tail, also known as a thagomizer. The end of the tail has three foot long conical spines pointing out to the side. This would have been useful protection. The tail itself was enough to break bones, and with the spikes, Stegosaurus could inflict serious or fatal wounds. Stegosaurus had a small head, containing an even smaller brain, barely the size of two walnuts. There is a famous statement that Stegosaurus had two brains, one in its hips. This is probobly because the discoverer of Stegosaurus, Onethiel Marsh, said he found a "big nerve cluster" in its hips. Brains basically nerve clusters, so people thought that there was a brain in there! But in the 1990's paleontologist Emily Buchholtz relizied all vertabrates, including us, have the same space in their hips called a ganglion. This is filled with fatty tisse! We know now that Stegosaurus, and all other dinosaurus have only one brain.