The late Cretaceous sun blazes down on the open plains. A rustling in the brush draws your attention, and soon a dome-headed figure steps into view. Its head glistens in the light, a thick bony dome reinforced with knobs and ridges. With quick, jerky movements, it fixes its large eyes on you. The Pachycephalosaurus lowers its head slightly, stomping the ground in warning. The air grows tense as it prepares, either to flee—or to charge.