The exact mechanism of the stegosaurian forage is known, although the teeth structure and the stature indicate that they were exclusively herbivorous. The dentine structure of the stegosaurians is similar to that of modern turtles and birds; sharp rhamphothecas covered the fronts of both the premaxilla and the predentary bone of the lower jaw. The sharply edges of the rhamphotheca of the upper and lower jaws were not hooked like those of a modern cannibalistic birds or snapping turtle and was effective for tearing even the toughest of the plant matter. Only in Huayanogosaurus, where the upper rhamphotheca was relatively small, was this cropping ability aided by the premaxillary teeth, which seem to be absent from other stegosaurians.
The cheek region (premolar and molar) of the stegosaruians were relatively small, triangular and lacked regularly placed, well-developed wear surfaces on their crowns; perhaps evidence to suggest limited grinding. Furthermore, the jaw muscle structure, specifically the low coronoid process, is suggestive of limited jaw movement.
Given the limited jaw movement, gastroliths may be a possible alternative to disintegrate the food for digestion. Although this is reported in sauropodomorphs and psittacosaurus, as well as close relatives of the dinosaurs, stegosaurian fossils have not associated with gastroliths so far. Additionally, the inset position of the teeth suggests that the stegosaurians seems to have possessed cheeks, an adaptation normally associated with sophisticated oral food-processing, thus further adding to the confusion.
Nevertheless, more obvious aspects of stegosaurian foraging behaviour can be identified. The relatively narrow snouts observed in all stegosaurians suggest a degree of selectivity in the food. The great variation in the size between the forelimb and hindlimb in Huayangosaurus yield a characteristic downturn of the vertebral column in the shoulder and the neck region, indicating that they were principally low-browsing animals, perhaps consuming in great quantities the leaves and succulent fruits and seeds of such ground-level plants as ferns, cycads and other herbaceous gymnosperms.
Alternatively, R.T. Bakker has argued that some forms, particularly Stegosaurus, were able to rear up on their hindlimbs in order to forage at higher levels, perhaps even into the crowns of tree. This is possible, as with the center of gravity near the hips, the hindlimbs would already be supporting nearly 80% of the weight of the body, and with strong, flexible tail possibly acting as another support to form a tripod.
Fastovsky, D.E. & Weishampel, D.B (2005) The Evolution and Extinction of the Dinosaurs(2nd Ed.), Cambridge University Press: Cambridge, UK.
Serenom P.C. & Dongm Z.M. The skull of the basal stegosaur Huayangosaurus taibaii and a cladistic analysis of Steogsauria, Journal of Vertebrate Paleontology 12, 318-343 (1992).