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 Ornithischia. Their unique skull structure may have been used for head-butting or display purposes. Fossils of pachycephalosaurs have been found primarily in North America, with notable genera including Pachycephalosaurus and Stegoceras. They varied in size, with some species reaching lengths of up to 15 feet. Their distinctive features and adaptations make them a fascinating subject of study in paleontology.