Almost no direct information is available for determining what thyreophorans ate. Nevertheless, their teeth and jaws clearly show adaptations to herbivory. Because of their low height with relation to their sauropod (Chapter 10) and ornithopod (Chapter 11) companions during much of the Mesozoic, thyreophorans were most likely low-level browsers. An ability of stegosaurs to raise up on their hind legs to increase their browsing height was proposed as a possible adaptation to high-level browsing, but this scenario is unlikely. Similarly, there is little doubt that ankylosaurs were anything but low-level browsers, and even the most imaginative paleontologists refrain from depicting ankylosaurs as sitting up or otherwise behaving in a sprightly manner.

The advent of flowering plants by the Early Cretaceous was no doubt a factor in thyreophoran food choices throughout the Cretaceous, but Jurassic ankylosaurs and stegosaurs must have eaten ferns and other low-lying undergrowths in forest and shrubland ecosystems. The anatomical arrangement of the teeth and portions of thyreophoran skulls supports this supposition. For example, ankylosaur teeth are small, but their beaks were well developed for cropping plants. Some possible differences in dietary preferences between ankylosaurids and nodosaurids have been suggested on the basis of their beak shapes. Nodosaurids had narrow beaks, which would have nipped at plant stems and leaves more precisely. In contrast, ankylosaurids had wider beaks that would have grabbed swaths of vegetation. Ankylosaurs also had larger than normal hyoids, which were bones in the throat that would have supported a tongue. This tongue would have moved chewed food to the cheeks for temporary storage, followed by movement from the cheeks back into the mouth. Ankylosaurs were therefore capable of much chewing, although an examination of only their teeth would surely lead to a different conclusion.

No toothmarks or coprolites attributable to thyreophorans have been reported from the geologic record. Likewise, gastroliths have been correlated only with remains of the nodosaurid Panoplosaurus. Using sauropods as a model for comparison, most thyreophorans should have had gastroliths to aid their digestion of plant material because their teeth and jaws were poorly-adapted for grinding or otherwise processing their food. Future excavations of thyreophoran skeletal remains should test this hypothesis. However, if such investigations prove fruitless, then paleontologists will have to provide alternative explanations for why gastroliths were not required.

One hypothesis proposed for how ankylosaurs digested food is that their considerable hindquarters provided space for massive amounts of fermentation in the hindgut. This hypothesis is supported by the wide space allowed by the costae (ribs) toward the posterior of most ankylosaurs. This space gave sufficient room for a gastrointestinal tract holding symbiotic anaerobic bacteria that would have aided in the breakdown of cellulose in plants. One of the metabolic by-products of such breakdown is methane. So if ankylosaurs and other herbivorous dinosaurs employed such digestive systems, the Mesozoic atmosphere likely would have had a noticeably different scent to that of today.

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