Pachycephalosaurs

Pachycephalosaurs were two-legged plant-eaters with a thick, domed skull roof, often ringed with bony knobs and spikes. Pachycephalosaurus, the largest, gave the group its name, which means "thick-headed lizard".

They are known almost only from the Cretaceous of North America and Asia, and mostly from skulls, since the rest of the skeleton rarely survived. Whether they butted heads, shoved flanks or used their domes for display is still debated.