Period: Late Jurassic
Order, Suborder, Family: Ornithischia, Thyreophora, Stegosauridae
Location: North America
Length: 20-24 feet (6-7 meters)
No dinosaur has been the subject of as much controversy as Stegosaurus, the armored dinosaur of the Late Jurassic. For more than a hundred years, this strange plant-eater has baffled paleontologists and captured the imagination of the public. Nothing interesting about this dinosaur is very typical.
For example, few ornithischian dinosaurs were quadrupedal (walked on four legs), fewer were armored, and no dinosaur besides Stegosaurus and its relatives had huge plates of bone arranged in rows along their backs. Pointed spines on its tail gave the rear end of Stegosaurus more protection than its front end. Its name comes from this armor; Stegosaurus means "covered reptile."
Stegosaurus weighed more than two tons. This plant-eater had few competitors in the Jurassic. Stegosaurus preferred food that was near the ground. It was not an agile animal, so it could not compete with other herbivores for leaves and twigs higher off the ground. Perhaps Stegosaurus was hidden by low plants, hiding in the cy-cads and tree ferns from the giant predators Allosaurus and Ceratosaurus.
Stegosaurus was tallest at the hips, which were about ten feet high. The largest bony plates were just behind the hips and added another three feet to its height. This exaggerated its profile, which curved steeply downward both in the front and back. From the side, a predator would have a hard time deciding which end was the head and which was the tail.
The head was small, with weak jaws. The fronts of the jaws were toothless; it had a beak that chopped vegetation. On the sides of its jaws, Stegosaurus had dozens of leaf-shaped teeth with grooves for crushing food.
Some writers thought Stegosaurus was stupid because it had a small head and brain. This idea seems to be the beginning of the belief that all dinosaurs were dimwitted, slow, and stupid. Because Stegosaurus had a small brain, some scientists believed it had a secondary brain in its hip region. The brain of Stegosaurus was just the right size for its body and lifestyle. After all, stegosaurs lived for millions of years. The enlargement in the hip region was not brain tissue, but a complex nerve center called the "sacral plexus." It was a secondary control center for the spinal cord. Stegosaurus may have lacked intelligence, but not all dinosaurs were stupid. Most others had larger brains and were possibly capable of more complex mental processing.
The front legs of Stegosaurus were only half as long as the heavy rear legs, but they were stout and well suited for carrying the weight of the front of the body. The feet were short and stubby, with four blunt toes on the front feet and three toes on the rear feet. The difference in size between the front legs and rear legs shows the bipedal (two-legged) ancestry of Stegosaurus. Because of the evolutionary development of its heavy armor, it could no longer walk on two legs.
A double row of flat triangular plates of bone extended from its neck to its tail, which at the tip was armed with two to four pairs of pointed spikes. The plates were several inches thick at the base where they attached to the body, but they were thin and narrow at the tips. Also, smaller knobs and plates in the skin strengthened and protected the flanks and legs.
Scientists have often wondered why these animals had bony plates. The plates were covered with skin that had many blood vessels in it. The blood would have released heat if the animal was too warm and taken heat from the sun to warm the animal if it was cold. Others argue that the plates were for protection from predators or from coarse vegetation, much as the bony armor of armadillos protects them from predators and plants that would pierce their skin. Some scientists think that the plates were for display in combat or when faced with an enemy. These plates made them look twice as large as they were. Perhaps all these ideas are correct.
Another old argument is how the plates were arranged on the animal's body. The plates of several skeletons seem to be staggered and project upward from the backbone; it is possible that this is because of changes that happened after the animal died. So some paleontologists have restored Stegosaurus with the plates covering the sides of the body for protection from predators such as Allosaurus, but this leaves the lower parts of the body unprotected. The staggered rather than paired plates is also difficult to explain, and it was recently proposed that the plates of Stegosaurus were arranged in a single row along its back. But no complete animal has yet been discovered to resolve these different interpretations.
Stegosaurus has been found only in western North America, but close relatives such as Kentrosaurus from Tanzania and Tuojiangosaurus from China show a worldwide distribution of this family. Stegosaurs became extinct in North America at the end of the Jurassic, but they survived in other places until late in the Cretaceous Period.