Pachycephalosaurus was a dinosaur that lived during the late Cretaceous period, around 70 million years ago. It is known for its thick, dome-shaped skull, which could be up to 10 inches (25 cm) thick. This unique feature suggests it may have used its head for head-butting or other social behaviors. This dinosaur was a herbivore, primarily feeding on low-lying plants. Pachycephalosaurus was part of a group called the ornithischians, which included other plant-eating dinosaurs. Fossils have been found mainly in North America, providing valuable insights into its habitat and lifestyle.