Yet another of the "classic" dinosaurs--including Allosaurus and Triceratops--that were discovered in the American west during the late 19th-century Bone Wars, Stegosaurus was also one of the most distinctive. In fact, this dinosaur had such a characteristic appearance that any fossils vaguely attributable to it wound up being assigned as separate Stegosaurus species, a confusing (though not unusual) situation that took decades to sort out!
First things first, though. The "type fossil" of Stegosaurus, discovered in Colorado's stretch of the Morrison Formation, was named in 1877 by the famous paleontologist Othniel C. Marsh. Marsh was originally under the impression that he was dealing with a gigantic prehistoric turtle, and he thought the scattered plates of his "roof lizard" lay flat along its back. Over the next few years, though, as more and more Stegosaurus fossils were discovered, Marsh realized his mistake, and properly assigned Stegosaurus as a late Jurassic dinosaur.
The March of the Stegosaurus Species
A low-slung, small-brained dinosaur with characteristic triangular plates and sharp spikes protruding from its tail: this general description of Stegosaurus was broad enough for Marsh (and other paleontologists) to include numerous species under its umbrella, some of which later turned out to be dubious or deserving assignment to their own genera. Here's a list of the most important Stegosaurus species:
Stegosaurus armatus ("armored roof lizard") was the species originally named by Marsh when he coined the genus Stegosaurus. This dinosaur measured about 30 feet from head to tail, possessed relatively small plates, and had four horizontal spikes jutting out from its tail.
Stegosaurus ungulatus ("hoofed roof lizard") was named by Marsh in 1879; oddly enough, given the reference to hooves (which dinosaurs definitely did not possess!), this species is known only from a few vertebrae and armored plates. Given the lack of additional material, it may well have been a juvenile S. armatus.
Stegosaurus stenops ("narrow-faced roof lizard") was identified by Marsh 10 years after he had named Stegosaurus armatus. This species was only three-quarters as long as its predecessor, and its plates were also correspondingly smaller--but it's based on far more abundant fossil remains, including at least one fully articulated specimen.
Stegosaurus sulcatus ("furrowed roof lizard") was also named by Marsh in 1887. Paleontologists now believe that this was the same dinosaur as S. armatus, though at least one study maintains that it's a valid species. S. sulcatus is best known for the fact that one of its "tail" spikes may actually have been located on its shoulder.
Stegosaurus duplex ("two-plexus roof lizard"), also named by Marsh in 1887, is notorious as the Stegosaurus that had a brain in its butt. Marsh hypothesized that the enlarged neural cavity in this dinosaur's hip bone contained a second brain, to make up for the unusually small one in its skull. This may also have been the same dinosaur as S. armatus.
Stegosaurus longispinus ("long-spined roof lizard") was about the same size as S. stenops, but was named by Charles W. Gilmore rather than Othniel C. Marsh. Not one of the better attested Stegosaurus species, this may actually have been a specimen of the closely related stegosaur Kentrosaurus.
The teeth of Stegosaurus madagascariensis ("Madagascar roof lizard") were discovered on the island of Madagascar in 1926. Since, as far as we know, the genus Stegosaurus was restricted to late Jurassic North America and Europe, these teeth may well have belonged to a hadrosaur, a theropod, or even a prehistoric crocodile.
Stegosaurus marshi (which was named in honor of Othniel C. Marsh in 1901) was reassigned a year later to a genus of ankylosaur, Hoplitosaurus, while Stegosaurus priscus, discovered in 1911, was later reassigned to Lexovisaurus (and later became the type specimen of an entirely new stegosaur genus, Loricatosaurus.)
The Reconstruction of Stegosaurus
Stegosaurus was so strange, compared to other dinosaurs discovered during the Bone Wars, that 19th-century paleontologists had a difficult time reconstructing what this plant-eater looked like. As mentioned above, Othniel C. Marsh originally thought that he was dealing with a prehistoric turtle--and he also suggested that Stegosaurus walked on two legs and had a supplementary brain in its butt! The earliest illustrations of Stegosaurus, based on the knowledge available at the time, are virtually unrecognizable--a good reason to take the reconstructions of any newly discovered dinosaurs with a big grain of Jurassic salt.
By far the most puzzling thing about Stegosaurus, which is still being discussed by modern paleontologists, is the function and arrangement of this dinosaur's famous plates. Lately, the consensus is that these 17 plates were arranged in alternating rows down the middle of Stegosaurus' back, though occasionally there are other suggestions out of left field (for example, Robert Bakker hypothesizes that Stegosaurus' plates were only loosely attached to its back, and could be flopped back and forth to deter predators). For further discussion of this issue, see this article, Why Did Stegosaurus Have Plates?