Stegosaurus is an Ornithischian (bird hipped) Dinosaur classified in the family Stegosauridae, though Stegosaurus is the most famous and well known of any Stegosaurs, as well as the largest known subspecies. Stegosaurus is said to have lived 150-145 million years ago in the late Jurassic. Though Creationist researchers and other Creationists obviously do not believe in such ancient dates. Stegosaurus, like it's kin, were Herbivores, meaning they only ate plant material as opposed to eating meat of any kind, other than the ocassional unlucky invertebrates on the plants. Stegosaurus had a small, horse-like head with a tiny brain about the size of a Golf ball. It's body was large with a curved back leading to a fairly long tail studded with large spikes. The animal supported it's weight with four large Elephantine, or perhaps even Tortise-like legs with 3 clawed feet.
The name Stegosaurus is derived from the old Greek words "Stegos" meaning "roof", and "Sauros" meaning "lizard". This comes from the fact that the back is lined with large plates made of bone. These plates will be discussed shortly.
Stegosaurus normally measured 30 feet long, though some fossil specimens have measured 39 feet. It stood 14 feet tall thanks to it's plates lining it's back. This makes Stegosaurus larger than modern African Elephants, the largest land Mammal still believed to be living. It is widely believed that Stegosuaurs was a browser since it had a beak instead of front teeth.
The first fossil remains of Stegosaurus were discovered in 1877 and since then, most Stegosaurus fossils have been of adult specimens. This may mean that Stegosaurus reproduced slowly, leading to fewer juveniles. Though this is merely speculation and unproveable at this point.
The two most noteable features of Stegosaurus are it's bony plates extending from it's back and it's tail spikes. Originally the bony plates that line it's back were thought to lie flat against the back like a turtles shell. The function of the plates has been widely disputed and many have suggested they are for defense, although many now believe they were used for thermoregulation. It may be a combination of both however it may be unlikely given the shallow base of the large plates. The tail spikes, named Thagomizers extend upward near the end of the tail and were most likely used for defense. Stegosaurus had four Thagomizers, each 2-3 feet long. It is believed by most researchers that these were used for defense since there is little reason to believe they were used for anything else.
Stegosaurus is certainly one of the most famous Dinosaurs and is used in virtually every single piece of media that discusses Dinosaurs. And while Stegosaurus is one of the most popular Dinosaurs, it is one of the least understood in terms of Evolution. The Evolutionary history of Stegosaurs is so lacking that virtually nothing can be said about their Evolution other than "they evolved". However according to the Genesis model, Stegosaurs, along with all other Dinosaur types never had any preceding forms, but have always been fully formed creatures with built in variability.