As dinosaurs go, stegosaurs are relatively easy to describe: these quadrupedal, small-to-medium-sized, small-brained herbivores were characterized by the double rows of plates and spikes along their backs and the sharp spikes on the ends of their tails. The most famous stegosaur (and the one that has lent its name to entire group) is, of course, Stegosaurus, but there are at least a dozen other related genera, most of which are no less important from a historical perspective. (See a gallery of stegosaur pictures.)
Evolutionarily speaking, stegosaurs are classified as ornithischian ("bird-hipped") dinosaurs. Their closest relatives were the armored dinosaurs known as ankylosaurs, and they were more distantly related to other four-footed herbivores like hadrosaurs and ornithopods. In a crucial way, though, stegosaurs were less successful than these other dinosaurs: they only flourished toward the end of the Jurassic period (about 160 to 150 million years ago), with only a handful of species managing to survive into the early Cretaceous.
Types of Stegosaurs
Because they constituted such a small group, it's relatively easy to distinguish among the various types of stegosaurs. The earlier, smaller stegosaurs of the middle to late Jurassic period are known as "huayangosaurids," typified by Huayangosaurus (naturally) and less well-known genera like Regnosaurus. The better-known "stegosaurids" were larger, with more elaborate spikes and plates, and are best represented by Stegosaurus.
As far as paleontologists can tell, the stegosaur family tree took root with the huayangosaurids of Asia, and grew bigger and more ornate by the time Stegosaurus planted itself in North America. There are still some mysteries, though: for example, the tantalizingly named Gigantspinosaurus had two huge spikes protruding from its shoulders, making its exact classification within the stegosaur line (if it even belongs there) a matter of controversy. The last stegosaur to appear in the fossil record was the mid-Cretaceous Wuerhosaurus, though it's possible that some as-yet-undiscovered species may have survived to the brink of the K/T Extinction 65 million years ago.
Why Did Stegosaurs Have Plates?
The most enduring mystery about stegosaurs is why they had those characteristic double rows of plates and spikes along their backs, and how these plates and spikes were arranged. To date, no stegosaur has been unearthed with its plates still attached to its body, leading some experts to conclude that these scutes (as they're technically called) lay flat along the dinosaur's back, like the thick armor of ankylosaurs. However, most paleontologists still believe these plates were arranged semi-vertically, as in popular reconstructions of Stegosaurus.
This leads naturally to the question: did these plates have a biological function, or were they strictly ornamental? Because scutes pack a large surface area into a small volume, some paleontologists believe they helped dissipate or collect heat, and thus regulated their owner's metabolism. Other experts claim that plates evolved to deter predators, or to help differentiate males from females. The trouble with these latter two explanations is that a) it's hard to see how an upright array of blunt plates could possibly have intimidated a hungry Allosaurus, and b) there has been very little evidence to date of sexual dimorphism among stegosaurs.
The prevailing theory is a bit less exciting: it's possible that the plates and spikes of stegosaurs evolved as a way of differentiating individuals within the herd, along the same lines as the slightly varying black-and-white stripes of zebras (because they were well supplied with blood, these scutes may also have changed color with the seasons). No such controversy attaches to the sharp spikes at the end of most stegosaurs' tails, which were doubtless used for defensive purposes (and are often called thagomizers in tribute to a famous "Far Side" cartoon by Gary Larson).
Here's a list of the most notable stegosaurs; just click on the links for more information.
Chialingosaurus One of the earliest Asian stegosaurs.
Chungkingosaurus This early stegosaur had some primitive characteristics.
Dacentrurus The first stegosaur ever to be described.
Dravidosaurus This "stegosaur" may actually have been a marine reptile.
Gigantspinosaurus This may or may not have been a true stegosaur.
Hesperosaurus The oldest stegosaur yet discovered in North America.
Huayangosaurus Could this have been the ancestor of all the stegosaurs?
Kentrosaurus A smaller, African cousin of Stegosaurus.
Lexovisaurus One of the oldest European stegosaurs.
Miragaia This stegosaur had an unusually long neck.
Monkonosaurus The first dinosaur ever to be discovered in modern-day Tibet.
Paranthodon This stegosaur was discovered over 150 years ago.
Regnosaurus This stegosaur lived in what is now modern-day England.
Stegosaurus The small-brained, spike-tailed plant eater.
Tuojiangosaurus One of the most well-known Chinese stegosaurs.
Wuerhosaurus Could this have been the last of the stegosaurs?