Stegosaurus: Bony Plates & Tiny Brain

Stegosaurus was a large, plant-eating dinosaur that lived during the late Jurassic Period, about 155 million to 150 million years ago, primarily in western North America. It was about the size of a bus but carried around two rows of bony plates along its back that made it appear even bigger.

Artwork by Scott Hartman reveals the bone structure of Stegosaurus.
Artwork by Scott Hartman reveals the bone structure of Stegosaurus.
Credit: © Scott Hartman / All rights reserved

Stegosaurus is a bit of media darling because there is so much material to help scientists reconstruct its distinctive appearance. It has been depicted on television and in movies, most notably chasing Faye Wray in "King Kong" and appearing in the second and third installments of the "Jurassic Park" films. A newspaper cartoon even helped name one of its body parts.

Stegosaurus also has a reputation for having a small brain. Its brain was about the size of a walnut, about the equivalent of a dog's brain, making it among the dinosaurs with the lowest brain-to-body ratio. At one point scientists theorized that it had an auxiliary "brain" — not an actual brain but a bundle of nerves — above its rear legs to help control its movements because its brain was so small. However, that theory has since been rejected by many paleontologists.

Size of Stegosaurus

Stegosaurus was the largest and most well-known of the stegosaurids, a family of armored dinosaurs. One species, S. armatus, was about 30 feet (9 meters) long. The two other recognized species, S. stenops and S. longispinus, were 23 long (7 m), about 9 feet tall (2.75 m), and weighed about 6,800 pounds (3,100 kg).

The name means "roofed lizard," which was derived from the belief by 19th-century paleontologists that the plates lay flat along its back like shingles on a roof. Most evidence supports the idea that the plates alternated in two rows pointy side up from the dinosaur's neck down to its rear. Its 17 plates, called scutes, were made of a bony material but were not solid; they had lattice-like structures and blood vessels throughout. The biggest plate was about 25 feet (76 cm) tall and long.

It is uncertain what purpose the plates served, but the blood vessels suggest temperature regulation was likely one function, but that theory has been questioned. They were also possibly used for mating and defense. However, since they did not protect any of the dinosaurs' major organs, they were mostly likely just for show and possibly moved up and down to intimidate potential predators.

Stegosaurus also had spikes at the end of its flexible tail, which dragged on the ground. These spikes, which pointed outward from the sides of the tail, were up to 4 feet (1.2 m) long and were used for protection from predators. Scientists began calling the spikes thagomizers after a pop culture reference in 1982, when a "Far Side" cartoon showed a group of cavemen calling the sharp spikes thagomizers "after the late Thag Simmons."

Its long skull was pointed and narrow. It had an unusual posture, carrying its head only about 3 feet (1 m) from the ground. Its back was rounded and it leaned forward on its short forelimbs and its stiff tail shot high into the air.

Stegosaurus did have very short forelimbs in relation to its hind legs, so it could only move about 4 mph to 5 mph (6 kph to 7 kph) — as his back legs would have overtaken the front legs if he tried to go too fast.

What did Stegosaurus eat?

Stegosaurus was an herbivore, as its toothless beak and small teeth in its cheeks were not designed to eat flesh and its jaw was not very flexible. Its cheeks were an unusual feature, providing Stegosaurus with room to chew and pre-digest its food and store more food than the majority of dinosaurs that lacked cheeks.

As a result of its short neck and small head, it most likely ate low-lying bushes and shrubs and other vegetation, including ferns, mosses, cycads, fruits, conifers, horsetails and even fallen fruit. It is also thought possible that Stegosaurus could have risen on its hind legs to reach some taller plants.

Stegosaurus lived in the late Jurassic Period.
Credit: Ozja |

Fossil discoveries

Fossils suggest that Stegosauruses traveled in multi-age herds.

The first Stegosaurus fossils were discovered in 1876 in Colorado by M.P. Felch and were named by Othniel C. Marsh in 1877.

Fossils from about 80 individuals were discovered in the Morrison Formation, which is centered in Wyoming and Colorado. Stegosaurus is even the state fossil of Colorado in tribute to the number of dinosaur skeletons found in the state. The formation also reaches into Montana, North Dakota, South Dakota, Nebraska, Kansas, the panhandles of Oklahoma and Texas, New Mexico, Arizona, Utah, and Idaho.

The most complete Stegosaurus fossil was discovered in Colorado in 1992 by Bryan Small, Tim Seeber and Kenneth Carpenter. Although most of the fossils discovered have been adults, a juvenile specimen was discovered in 1994 in Wyoming.

Fossils have also been discovered in Western Europe, southern India, China and southern Africa.

— Kim Ann Zimmermann, LiveScience Contributor

Learn about the plates, bones, habitat and other secrets of Stegosaurus.
Learn about the plates, bones, habitat and other secrets of Stegosaurus.
Credit: Ross Toro, Livescience contributor


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