Pachycephalosauria
Goyocephale
Micropachycephalosaurus
Homalocephaloidea
Homalocephale
Pachycephalosauridae
Pachycephalosauria is a group of ornithischian dinosaurs which evolved during the Cretaceous period in what is now North America and Asia. They were all bipedal herbivorous animals with very thick skulls, sometimes ridged with small to large tubercles and spikes. Initial speculation about the purpose of the reinforced skulls centered on mating disputes and intra-species "head-butting", but recent modeling and simulations show that head on collisions would most likely result in severe injury to the animal under these conditions (broken necks). The skulls were most likely a defense mechanism used against ramming predators. Species include Pachycephalosaurus and Stegoceras
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