All marginocephalians were probably herbivores, ascertained from their teeth, jaws, and other skeletal adaptations. For example, because they had deeply inset cheek teeth, they could have had cheeks used for temporarily storing masticated plants while chewing. Pachycephalosaurs had heterodont dentition comparable to that of other dinosaurs interpreted as herbivorous. However, this dentition was weakly developed in comparison to the dental batteries seen in some other ornithischi-ans, such as hadrosaurids (Chapter 11). Ceratopsians represent a departure from pachycephalosaurs in that their rostral and predentary formed beaks ideal for slicing through vegetation. Their cheek teeth also developed into dental batteries but ones more inclined toward vertically oriented shearing, rather than horizontal (back and forth) grinding. This movement must have caused much wear and so required the development of batteries that continually replaced worn-out teeth. Indeed, tooth-replacement rates estimated for Triceratops were 50 to 100 days, with as many as six teeth in line to replace any one already in the jaw. The prodigious head shields of some ceratopsids were originally interpreted as having evolved for the support of massive jaw musculature, which was most likely present for such active chewing. Nevertheless, muscle attachments to frills may also have been a secondary consequence of sexual selection in favor of large frills, rather than an adaptation primarily for chewing (Chapter 6).
The food eaten by ceratopsians is suggested to have been palms and cycads, which were non-flowering plants, and small shrubs or trees of angiosperms. However, pale-obotanists have pointed out that fossils of these plants are not abundant in areas where ceratopsid bones are most commonly found. Instead, they suggest that low-level flowering plants were the preferred food for these dinosaurs. To add fuel to the controversy, ceratopsid tracks in Upper Cretaceous sandstones of Colorado (mentioned earlier) co-occur with impressions of palm and angiosperm leaves (Fig. 13.11).
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