Nick Snels wrote But I think one should make a distinction between the various pachy specimens. It could very well be that animals like Pachycephalosaurus or Stegoceras had quite robust necks, since they used their heads as a ram. The necks of the flat-headed pachy's, like Homalocephale and Goyocephale, could be much less robust, since they used to push each others head or flank. And Stygimoloch fully relied on its massive horns at the back of his head to avoid collisions with another male. That's maybe the explanation for his small dome and his "fragile" neck.