Pachycephalosaurs were a group of herbivorous dinosaurs known for their thick, domed skulls. They lived during the late Cretaceous period, around 75 to 65 million years ago. These dinosaurs were bipedal, meaning they walked on two legs, and were part of the larger group called ornithischians, which included other plant-eating dinosaurs. The most famous member of this group is Pachycephalosaurus, which had a dome-shaped head that could reach up to 10 inches thick. Scientists believe these thick skulls may have been used in head-butting behavior, similar to modern-day bighorn sheep. Pachycephalosaurs were relatively small compared to other dinosaurs, typically measuring around 10 to 15 feet long.