The teeth, as well as the muscle arrangement, particularly the impressive volume of the abdominal region, in the pachycephalosaurs point towards foraging on the low growing plantations. At the front of the jaws were simple, peg-like gripping teeth, the last of which were sometimes enlarged in a canine-like fashion. It is likely that these teeth were surrounded by a small, horny rhamphotheca as in all ornithischians. Further back, the cheek teeth of pachycephalosaurs were uniformly-shaped, with small, triangular crowns. The difference in the tooth wear amongst different pachycephalosaurs implies that different pachyceophalosaurians fed on different kinds of vegetation.

It is likely that the pachycephalosaurs had a large internal vat for fermenting their food, indicated by the bone arrangement; disproportionately broad ribcage and a great girth that extends till the base of the tail. These anatomical modifications of the more primitive condition seen in ceratopsians, ornithopods, and other ornithischians suggest a backward migration and enlargement of the digestive tract to occupy a position between the legs and under the tail. Much like the case for thyreophorans, simple styles of chewing may have combined with more extensive chemical digestion via the development of a huge gut to solve the problem of making a living as a plant-eating dinosaur.

References;
Fastovsky, D.E. & Weishampel, D.B (2005) The Evolution and Extinction of the Dinosaurs(2nd Ed.), Cambridge University Press: Cambridge, UK.