Stegosaurus

Stegosaurus was a large plant-eating dinosaur that lived about 150 million years ago. That was during a time called the Late Jurassic period, long before humans existed. Stegosaurus fossils have been found in the western United States, mostly in Colorado, Wyoming, and Utah. The name Stegosaurus means "roof lizard" in Greek.

A full-grown Stegosaurus was about 30 feet long, roughly the length of a school bus. It weighed around five tons, about as much as a small elephant. It walked on four thick legs. Its back legs were much longer than its front legs, which gave it a humped, tilted shape. Its small head hung low to the ground, perfect for munching on ferns, mosses, and other low plants.

The most famous thing about Stegosaurus is the row of bony plates along its back. The biggest plates were more than two feet tall and two feet wide. Scientists have argued about these plates for more than 140 years. Some think the plates were used for defense, like armor. Others think they helped the dinosaur warm up or cool down, like a car radiator. Still others think the plates were mostly for show, used to attract mates or warn off rivals. Scientists have not settled the question.

What scientists agree on is the tail. Stegosaurus had four long, sharp spikes at the end of its tail, each up to three feet long. Paleontologists call this spiky weapon a "thagomizer." The name comes from a cartoon, but the word stuck and is now used in real science books. Fossils show that Stegosaurus could swing its tail hard enough to punch holes through the bones of attackers. Allosaurus, a big meat-eater that lived at the same time, sometimes came away wounded from these fights.

Stegosaurus had a surprisingly tiny brain. It was about the size of a lime, even though the whole animal was as long as a bus. For a long time, people believed Stegosaurus had a second brain in its hip to help control its back legs and tail. Scientists no longer think this is true. The space in its hip probably held nerves or a bundle of tissue that stored energy, not a separate brain.

Stegosaurus died out about 145 million years ago, tens of millions of years before the famous dinosaurs like T. rex. So even a T. rex would have thought of Stegosaurus as an ancient creature from the deep past.