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Of these, the latter factor is the most likely, and recognition in more recent years of this human-induced diversity has decreased the number of ceratopsian taxa. Thus, among taxonomists the "lumpers" triumphed in part over the "splitters" (Chapter 5). This is not to say that Triceratops did not undergo speciation over 3 million years

FIGURE 13.3 Typical ceratopsian lower jaws. (A) Dentary and predentary of Protoceratops, Late Cretaceous of Mongolia; Dinosaur Adventure Museum, Fruita, Colorado. (B) Unidentified ceratopsian dentary and predentary, Late Cretaceous, western North America; College of Eastern Utah Prehistoric Museum, Price, Utah.

FIGURE 13.3 Typical ceratopsian lower jaws. (A) Dentary and predentary of Protoceratops, Late Cretaceous of Mongolia; Dinosaur Adventure Museum, Fruita, Colorado. (B) Unidentified ceratopsian dentary and predentary, Late Cretaceous, western North America; College of Eastern Utah Prehistoric Museum, Price, Utah.

or that most taxonomy of ceratopsians is scientifically unjustified. Nonetheless, the great number of genus and species names that have been given to them may be overstating their actual diversity.

All known marginocephalians were herbivores, as evidenced by the teeth and jaws of both pachycephalosaurs and ceratopsians. Pachycephalosaurs had heterodont dentition, with canines in the anterior part of the mouth and cheek teeth that were small, laterally compressed, and crowned with denticles. These teeth and their medial placement within the jaw resemble those of thyreophorans, partially reflecting their common ancestry within Genasauria (Chapter 12). In contrast, ceratopsians had cheek teeth develop into dental batteries that could have competed with those of some iguanodontians (Chapter 11). The anterior part of the mouth consisted of sharp beaks covered by horn (keratin) with a premaxillary rostral bone above it occluded with a pointed predentary and dentary below (Fig. 13.3). The members of each clade were so different in their eating apparatuses that contemporaneous species must have been specialized for different food sources. Most forms were probably low-level browsers because of their low heights or propensity toward quadrupedalism.

Both marginocephalian clades also differed considerably in their locomotion. Judging from the short fore limbs seen in some specimens, pachycephalosaurs were apparently bipedal, although limb proportions for some species are unknown. Likewise, although the manus for a few specimens is known, no pachycephalosaur pes has yet been found. Consequently, pachycephalosaur foot anatomy cannot be compared to tracks that might have been made by these animals. Such information would certainly help to test assumptions about obligate bipedalism. Some basal ceratopsians were also obligate bipeds, but the majority of ceratopsians were heavily built and obligate quadrupeds. Only a few might have been facultative bipeds. Advanced ceratopsian foot anatomy consisted of five toes on the manus and four on the pes; later quadrupedal forms had shortened but stout phalanges that ended in hooves. However, ceratopsian manus tracks should only show impressions from four toes (digits I through to IV), because digit V is markedly reduced in comparison to the other toes.

Undoubted marginocephalian trace fossils are so far only represented by ceratopsian trackways from Late Cretaceous strata in the western USA (but nowhere else) and gastroliths in Psittacosaurus, a primitive ceratopsian. Ceratopsian tracks are identified on the basis of the following data:

■ They match the aforementioned foot anatomy (Fig. 13.4).

■ Size comparisons of track compression shapes also match those of known ceratopsian foot sizes, especially when flesh is "added" to the foot.

Ceratopsian Tracks
FIGURE 13.4 Ceratopsian track interpreted from the Laramie Formation (Late Cretaceous) near Golden, Colorado.

■ They occur in strata formed in continental environments that are age-equivalent with known ceratopsian remains.

The gastroliths that occur in Psittacosaurus are localized masses of about 50 similarly-sized, rounded stones found within the rib cage of more than one specimen. The fulfillment of such specific criteria for gastroliths leaves little doubt that these stones are indeed trace fossils related to ceratopsian digestion. As far as other trace fossils related to feeding are concerned, no toothmarks (or beakmarks) have been described for marginocephalians, nor have any coprolites been linked to them (Chapter 14).

Numerous dinosaur nests in Late Cretaceous strata of Mongolia, often containing bountiful egg clutches, were originally attributed to the ceratopsian Protoceratops. Updated analyses and new evidence have rendered this identification suspect or in some cases falsified it entirely (Chapter 9). Consequently, paleontologists are beginning with a clean slate with regard to the body fossils (eggs and embryos) and trace fossils (nest structures) associated with marginocephalian reproduction. In contrast, possible trace fossils of marginocephalian behavior related to mating or territorial disputes, such as scars or fractures inflicted on one another, as evident in healed injuries in bone, may provide more insights on how pachycephalosaurs and ceratopsians interacted with one another.

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