Except in a couple of very primitive genera the cheek teeth of ornithischians are inset with a lateral space that was roofed by the overhanging maxilla and floored by the massive dentary. This lateral space was delimited by cheeks and the mouth was small. The cheek muscles were not homologous to the Musculus buccinatoris of mammals, but the function was the same, to prevent the loss of food from the sides of the jaws during chewing. The evolution of self‐sharpening cheek teeth and of cheeks enabled ornithischians to eat much more resistant plant material and resulted in ornithischians replacing prosauropods as the ‘small to medium sized’ terrestrial herbivores (up to