Stegosaurs: The "Roofed Lizards"

One of the more famous species of dinosaurs is Stegosaurus; it has been featured in books and movies since its first discovery, even in this year's Jurassic Park III.

The subOrder Stegosauria (meaning "roofed reptile") are a group of ornithischian ("bird-hipped") dinosaurs that arose during the Mid-Jurassic Period (195 to 140 million years ago) and reached the peak of their diversity toward the end of the Jurassic Period. They spread across North America, Western Europe, East Asia and East Africa before beginning to decline during the Cretaceous Period (140 to 65 million years ago).

The Stegosauria were herbivorous (plant-eating) dinosaurs with heads that were relatively small for their body size. Like other ornithischian dinosaurs, the Stegosauria probably lived in herds for protection against predators. Unlike the Hadrosaurs, Iguanodonts and Pachychepahlosaurs; however, the Stegosauria were not agile enough to run away from predators on two legs. The obligate quadruped (meaning that they always walked on four legs) Stegosaurs would have probably held their ground if attacked, relying instead on the bony plates along their backs for protection and the large spikes on their tails to deter predators such as Allosaurus.

How important were the broad bony plates that ran down either sides of the backbones of the Stegosauria? Over two feet (60 cm) high in Stegosaurus, the plates were embedded in the animals' skin, and contained many blood vessels. This has led many paleontologists to believe that the plates were used to exchange heat between the animal and its environment. Basking could help to raise body temperature, while shunting large amounts of blood to the plates on a hot day could help to cool off the Stegosauria. This is similar to the sort of heat exchange that many paleontologists believed Dimetrodon used to regulate its body temperature.

The most famous member of the subOrder Stegosauria is Stegosaurus, named and described in 1877 by O. C. Marsh. Stegosaurus is the state fossil of Colorado and is one of the most famous of all dinosaurs. Stegosaurus reached 20 feet (six meters) in length and weighed up to 4,000 pounds (two tonnes). Its skull was only about 16 inches (40 cm) in length, with a toothless beak for cropping plant material at the front of its mouth and small, weak cheek teeth. It has often been said that Stegosaurus was a particularly dim-witted dinosaur with a brain only the size of a walnut; in fact, its brain was more than two times the size of a walnut, thank you very much!

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