Pachycephalosaurus is a genus of herbivorous dinosaur that lived during the Late Cretaceous period, approximately 70 million years ago. It belongs to the family Pachycephalosauridae, which includes other dinosaurs with thick and domed skulls.

The most distinguishing feature of Pachycephalosaurus is its thick skull, which had a flat and dome-shaped profile with bony knobs and nodes. The skull bone is incredibly thick, up to 9 inches in some specimens, and it is thought that this adaptation allowed for head-butting behaviors during intraspecific combat or mating rituals.

Pachycephalosaurus had a long and slender body, with short forelimbs and powerful hind limbs adapted for bipedal locomotion. It walked on two legs and likely had a fast and agile gait. Its long tail balanced its body while running.

This dinosaur was an herbivore, feeding on low-growing vegetation. Its small, leaf-shaped teeth suggest a diet of ferns, leaves, and other plant materials. Pachycephalosaurus was estimated to be around 15 to 20 feet in length, with an average weight of 1,000 to 2,000 pounds.

Fossils of Pachycephalosaurus have been primarily found in the western region of North America, including Montana, Wyoming, and South Dakota. These fossils have provided important insights into the anatomy, behavior, and evolution of this dinosaur.

The study of Pachycephalosaurus and other pachycephalosaurs continues to shed light on dinosaur paleobiology and the evolutionary history of complex cranial structures. Ongoing research aims to understand the ecological role and behavior of Pachycephalosaurus within Late Cretaceous ecosystems.