Pachycephalosaur

Pachycephalosaur is a common name for a group of small to medium-sized ornithischian dinosaurs known for their thick, rounded skull roofs. They belong to the clade Pachycephalosauria, a part of Marginocephalia, and are traditionally grouped with Homalocephalidae and Pachycephalosauridae, though exact relationships vary among analyses. They lived during the Late Cretaceous in what is now North America and Asia.

The most distinctive feature of pachycephalosaurs is the dome-shaped skull, formed by a thickened outer bone layer and extensive bone growth. Some skulls also show horns or bumps around the dome and along the devant wall. Outside the skull, they were generally small to medium-sized, with erect hind limbs and a build suitable for bipedal or facultatively quadrupedal movement. Their jaws and teeth were adapted to processing plant material, indicating a herbivorous or omnivorous diet.

Behavior and biology are inferred from skeletal remains and skull morphology. The oversized skull dome is widely thought to have evolved for social display and possibly head-butting in intraspecific interactions, such as dominance displays or combat. Growth patterns suggest the dome developed with age, and some researchers propose sexual dimorphism may have existed, though such conclusions remain debated.

Fossil records include genera such as Stegoceras, Pachycephalosaurus, Prenocephale, and Homalocephalus, dating from the Campanian to Maastrichtian stages of the Late Cretaceous. Pachycephalosaurs disappeared at the end of the Cretaceous period, along with most other dinosaur groups.