Stegosaurs - The Spiked, Plated Dinosaurs

The Evolution and Behavior of Stegosaur Dinosaurs

Stegosaurus, the classic Jurassic stegosaur that gave this breed its name (Senckenberg Museum).

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. By far the most famous stegosaur (and the one that has lent its name to this entire family) is, of course, Stegosaurus, but there are at least a dozen other closely related genera, most of which are no less important from a historical perspective. (See a gallery of stegosaur pictures and profiles and Why Did Stegosaurus Have Plates on its Back?)

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 plant-eaters like hadrosaurs (aka duck-billed dinosaurs) 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 ensuing Cretaceous.

Types of Stegosaurs

Because they constituted such a small family of dinosaurs, 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, you guessed it, Huayangosaurus and less well-known genera like the European Regnosaurus. The better-known "stegosaurids" were larger, with more elaborate spikes and plates, and are best represented by the classic body plan of Stegosaurus.

As far as paleontologists can tell, the stegosaur family tree took root with the huayangosaurids of Asia, and grew larger 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 is the mid-Cretaceous Wuerhosaurus, though it's possible that some as-yet-undiscovered genus 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 possessed those characteristic double rows of plates and spikes along their backs, and how these plates and spikes were arranged. To date, no stegosaur fossil has been unearthed with the plates still attached to its skeleton, leading some paleontologists 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 researchers still believe that 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, it's possible that they helped to dissipate heat during the night and absorb it by day, and thus regulated their owner's presumably cold-blooded metabolism. But it's also possible that these 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: the bulk of opinion today is 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).