Stegosaurus << `stehg` uh SAWR uhs >> was a large, plant-eating dinosaur that lived about 150 million years ago in what is now the western United States. Stegosaurus had two rows of bony plates shaped like huge arrowheads sticking out of its back and tail.
Stegosaurus grew up to 30 feet (9 meters) long and had a heavy, bulky body. The dinosaur’s small, narrow head had a beak at the front and teeth farther back. The front legs were less than half as long as the back legs, and so the body sloped down from the hips. At the end of the tail, the animal had pairs of bony spikes, each about 3 feet (0.9 meter) long. Stegosaurus could have defended itself by swinging its heavy tail and hitting an attacker with its sharp spikes. Among paleontologists, the spiked tail of Stegosaurus is referred to as the “thagomizer.” The term comes from a popular comic by the American cartoonist Gary Larson.
No one really knows why Stegosaurus had its bony plates. They may have discouraged attackers or attracted mates. The plates may also have helped Stegosaurus warm up and cool down. It could warm up by turning the broad, flat sides of the plates toward the sun to soak up heat. If it became too warm, air moving along its back could have cooled blood flowing through the thin plates.
The first Stegosaurus fossils were discovered in Colorado by fossil hunters working for the American paleontologist Othniel Charles Marsh in 1876. Marsh published a description of the fossils and named Stegosaurus in 1877. At first, Marsh believed the fossils were of a giant prehistoric turtle. Later, he realized the fossils were remains of a dinosaur. Colorado designated Stegosaurus as the official state fossil in 1982.
