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Pachycephalosaurus lived about 70 million years ago. It was a medium-sized dinosaur, about 15 feet long. That’s about the size of a minivan. It probably weighed about 950 pounds. Its skull was two feet long. It was a strange-looking dinosaur with an odd dome-shaped head. Its skull was very thick, with about eight inches of bone on top and spiky bones around the edges. This is how it got its name, which means “thick-headed lizard”. Underneath all that thick bone, however, was a very small brain. This dinosaur was not very intelligent. Paleontologists still don’t really know what these creatures did with their hard heads. At first they believed that Pachycephalosaurs used their heads to butt each other, the way big-horned sheep do when they are trying to attract a mate.

But after studying the skull carefully, paleontologists know that it was not solid bone. It was not even very strong. If two Pachycephalosaurs tried to butt heads, they would almost certainly hurt each other badly. Now we believe that theyrammed into other dinosaurs’ sides. They probably rammed hunting predators when there was nowhere left to run. Pachycephalosaurus was a bipedal dinosaur, meaning that it walked on two legs. It was not a very fast runner. It probably held its heavy head close to the ground. It may have walked on all four legs while it was looking for food. Pachycephalosaurus was an herbivore, eating soft plants, fruit, and seeds. It had triangular teeth that were serrated like the edge of a saw. This was very useful for biting through tough plants.

Pachycephalosaurus had large holes in its skull for eyes, so it could see very well, possibly even at night. It might have also had a good sense of smell. It had short arms and a stiff tail that stood out straight behind it when it ran or walked. Pachycephalosaurus moved around in small groups. It probably lived on the coasts, near the ocean. Its bones were found in what is now Montana, in the western United States. Other fossils have been found in what is now Alberta, Canada, Mongolia and Madagascar.

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