Pachycephalosaurus

dinosaurs · thick skull · Late Cretaceous · North America

Mega thick skull. Pachycephalosaurus had one of the thickest skulls of any dinosaur, made of solid bone that could be 10 inches thick. Scientists think it probably used this helmet-like dome to headbutt rivals, like modern bighorn sheep do.

Western North America home. This dinosaur lived in what is now Montana, South Dakota, Wyoming, and Alberta during the very last part of the dinosaur age, about 66 million years ago. It was one of the last dinosaurs to walk Earth before the great extinction.

Bus-sized but hefty. Pachycephalosaurus was about 15 feet long, which sounds big until you remember it weighed as much as 5 cows. That thick, heavy skull made up a huge portion of its weight.

Mystery fossil puzzle. Scientists mostly know Pachycephalosaurus from just one skull and a few thick bone pieces, so they had to guess a lot about what the rest of its body looked like. More complete skeletons were found later, helping fill in the picture.

Head bashing champion. With that incredible dome, Pachycephalosaurus probably fought others of its kind by ramming their heads together, a behavior called intraspecific combat. The thick bone would protect its brain from the impact.