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Stegosaurus

STEG - oh - SAW - rus

Stegosauria/Stegosauridae
Copyright 2007

Field Notes

Name Means: "Roofed lizard"
Length: 29 feet 6 inches (9 m)
Diet: Herbivore (Plant-Eater)
Time: Late Jurassic
Location: Colorado, Utah, Wyoming, USA

One of the most famous dinosaurs of all, Stegosaurus is striking, first of all for its great size. Even more remarkable, though, are the bizarre plates and spikes that stand up, almost vertically like battlements, along each side if its backbone, from the neck right down to the middle of the tail. These plates have fascinated both paleontologists and the general public since the first specimen was described by Othniel Charles Marsh in 1877.

Marsh originally thought that these plates lay flat on the back, forming a kind of roof-hence the animals name. Later finds showed the the plates stood upright, and that the name, which nevertheless has endured, is a misnomer. There has been ongoing debate about whether the two rows were paired, mirror images of each other, or whether they were staggered and alternating. More recent finds confirm that the plates were staggered.

But what function did these plates serve? They could have been for protection, making it difficult for an attacker to bite into its back. Channels in the plates that held blood vessels suggest thst they may have helped to regulate the animal's body temperature. By turning its plates broadside to the sun, Stegosaurus could have warmed its blood; by standing with them edge-on to the sun, it could have cooled the blood down. Yet another possibility is that the plates were a display feature. Perhaps, when flushed with blood, they could change color, to impress a potential mate or scare off a predator. It is possible that the plates in fact performed all of the above functions.

Displayed at the American Natural History Museum in New York. This well preserved fossil clearly shows the fine vein-like passages where the blood vessels once ran. These vessels covered the surface of each plate on the animals back, and were likely to be used for display, or to intimidate its rivals during courtship season.

The spikes at the end of the tail-which could be more than 3 feet (1 m) long-would have been a very effective weapon. Swinging on the end of the long, flexible tail, they could strike a lethal blow to a potential predator. Most species of Stegosaurus had four tail spikes, but one species may have had eight. Unlike the staggered plates, the spikes were arranged in matched pairs. The bulk of Stegosaurus's weight was carried by the heavily built hind legs. These were almost twice as long as the front legs. The shorter front legs meant that Stegosaurus walked with its shoulders, neck, and head close to the ground.

Stegosaurus had a small head with a long snout. Its teeth were small, but wear that it evident on fossil remains indicates that the animal ground the upper and lower teeth against each other to cut and slice food. All Stegosaurus's teeth were at the rear of its mouth. At the front it had a horny beak that could cut through plants as effectively as a pair of shears. Recent research suggests that Stegosaurus had cheek pouches in which it could hold food that was waiting to be chewed.

Not surprisingly, given the interest it has aroused, Stegosaurus has generated a number of dinosaur myths. One of them was the "two brain" theory. A prominent cavity in Stegosaurus's hips suggested to some researchers that this "pocket" housed an auxiliary brain that controlled the rear end of the animal. We now understand that this area housed a bundle of nerves (a ganglion) that acted as a relay center that passed on messages from the brain. It has also been suggested that the animal could move its plates-that it could "wag" them as part of a display to mates or predators. But there are no scars on the skeletons that would mark the places where the huge muscles required to move the plates could have been anchored. Some paleotologists have suggested that Stegosaurus could stand up on its hind legs, either to reach higher food or to intimidate rivals and attackers. This now seems unlikely.

As with most dinosaurs, ideas about Stegosaurus have been revised in recent decades. We once thought, for example, that it dragged its tail along the ground. Now we think that it held this tail high.

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