Stegosaurus means "roofed reptile." Its "roof" is made up of the large plates along the ridge of the animal's back. What were they for? Originally, paleontologists speculated that the plates represented a type of armor, used for protection. But the surfaces of the plates are crisscrossed with grooves for blood vessels, indicating that they were covered with skin when the animal was alive. This led later paleontologists to speculate that the plates were used for controlling body temperature -- like solar collectors or heat radiators. While that is an interesting hypothesis, it cannot be tested scientifically, since all stegosaurs are extinct. This is one of the many cases in which scientists do not have conclusive answers. Extinct dinosaurs present us with many such mysteries.

Stegosaurus offers more mysteries, such as how it could function with its tiny brain. The image of dinosaurs as massive beasts with walnut-sized brains is not true of many dinosaurs, but it is true of Stegosaurus. It has even been suggested that Stegosaurus had a second brain to control its hindquarters, since the one in its head seems to be too small to manage the entire beast. But it was, and it did. Stegosaurus had only one, small brain.

Still another mystery involves Stegosaurus's front legs. As a baby fossil, the front legs are straight up and down. As an adult fossil, however, they are splayed out to the side. Which is correct? We're not sure, since they seem to fit both ways.

Only Stegosaurus, silent and still for 140 million years, contains the answers to these and the many other questions that surround it.