This large, plant-eating dinosaur roamed the Earth during the late Jurassic period, between 155 and 145 million years ago, alongside dinosaurs such as Apatosaurus, Diplodocus, Brachiosaurus, and Allosaurus.

The name is derived from the Greek stegos ("roof") and sauros ("lizard") due to the early but mistaken belief that this dinosaur's armored plates lay flat over its back, overlapping like the shingles or tiles on a roof.

When Othniel Charles Marsh initially described Stegosaurus in 1877, he thought the remains were from an aquatic turtle-like animal.

While some paleontologists argue that its spiked tail was meant for display only, most believe the tail was used as a weapon. A recent study found a high incidence of trauma-related damage to Stegosaurus tail spikes, lending more weight to the position that they were indeed used for combat.

The term paleontologists use for the spiked tail was inspired by one of Gary Larson's beloved "Far Side" cartoons, in which a group of cavemen are clustered around a picture of Stegosaurus. One of them points to the sharp spikes and says, "Now this end is called the thagomizer ... after the late Thag Simmons."

The myth of a second brain started in the 1870s, when Marsh first described and named Stegosaurus. He speculated that an extra-large space for the spinal cord in the dinosaur's hips may have functioned as a second brain that helped coordinate the stride of its long back legs with its short front legs. Marsh quickly abandoned this hypothesis because no other features of the hip bones confirmed the theory, but popular media had already latched onto the idea, and many ill-informed enthusiasts continue to spread it to this day.

Relative to its body size, Stegosaurus had the smallest brain of any known dinosaur. Its body was the size of a van, but its brain was the size of a walnut (about 2.8 oz).

It had short fore limbs in relation to its hind limbs, suggesting that it couldn't move very fast, as the stride of the back legs would have overtaken the front legs at fast speeds. It probably topped out at about 7 km/h (4.3 mph).

By the early 1960s, two rows of alternating plates had become the dominant view, mainly because of the discovery of some fossils in which the plates were still partially articulated in this manner.

These plates may have helped Stegosaurus to control its body temperature, in a similar way to modern elephant and rabbit ears. The plates had blood vessels running through grooves and air flowing around the plates would have helped cool the blood.