Ceratopsia

The prominent horns of Triceratops were what first caught the attention of O. C. Marsh when he saw its skull in 1887, leading him to first identify it as a fossil bison. Later, more cautious examination revealed that the horned skull belonged to a dinosaur, and it was promptly renamed in 1889 to Triceratops horridus (= "horrid three-horned face"). Marsh is credited with naming this famous dinosaur and the Family Ceratopsidae (using the Linnaean system), but the first ceratopsian named from the geologic record was Monoclonius, described by his rival Edward Cope in 1876. Despite the presence of these heavyweights of dinosaur paleontology at the forefront of describing ceratopsians new to science, by far the most important single contributor to the study of ceratopsians was John Bell Hatcher (Chapter 3). Although he only lived for 42 years, Hatcher discovered 50 ceratopsian skeletons. He also wrote the majority of The Ceratopsia, the classic treatise on these dinosaurs, which was published in 1907, three years after he died. The study of these horned dinosaurs would not be nearly as advanced as it is today without Hatcher's seminal contributions, and his legacy is apparent whenever people see these dinosaurs depicted in fact and fiction.

A dinosaur belonging to Ceratopsia has several important characters:

■ A rostral bone anterior to the maxilla that paired with a predentary to form a sharp beak.

■ A frill formed by the parietals that hung past the rest of the skull.

■ Cheeks that extend laterally and posteriorly, giving the skull a triangular shape when viewed from above its dorsal surface.

■ A palate positioned high in the skull.

Notice that all of these features deal with novelties in the skull, which re-emphasizes the importance of how marginocephalians in general, and ceratopsians in particular, are largely identified by their cranial anatomy. In other words, a headless ceratopsian skeleton found in the field would present a greater challenge to classify than a disembodied head.

Ceratopsia is split into two sister clades, Psittacosauridae, based only on the genus Psittacosaurus, and Neoceratopsia, which includes all other ceratopsian genera. The oldest known ceratopsian is Chaoyangsaurus from the Middle-Upper Jurassic of China, but the most primitive ceratopsian is Psittacosaurus, a small (less than 2 meters long) ceratopsian that occurs abundantly as near-complete or complete specimens in Lower Cretaceous rocks of Asia. Ten species of Psittacosaurus have been identified so far, which either means that it diversified during the Early Cretaceous, or (more likely) was the victim of much taxonomic splitting. The latter might be attributable to the large amount of skeletal material available for study, which lends itself well to identifying small anatomical differences. Regardless, Psittacosaurus can be distinguished from other ceratopsians through:

FIGURE 13.7 Comparative anatomy between skulls of two small ceratopsians. (A) Early Cretaceous psittacosaurid Psittacosaurus of Asia, the oldest known ceratopsian and namesake of its clade. (B) Late Cretaceous neoceratopsian Protoceratops, also of Asia.

FIGURE 13.7 Comparative anatomy between skulls of two small ceratopsians. (A) Early Cretaceous psittacosaurid Psittacosaurus of Asia, the oldest known ceratopsian and namesake of its clade. (B) Late Cretaceous neoceratopsian Protoceratops, also of Asia.

1 its nares, which were elevated away from the rostral;

2 a loss of digit V on its manus, which left it with only three fingers; and

3 a loss of its antorbital fenestra (Fig. 13.7A).

Psittacosaurids are thought to have retained more primitive features than the neo-ceratopsians. They certainly had simpler-looking skulls, consisting of a barely detectable parietal shelf overhanging the occipital and a lack of horns. Its proposed membership in Ceratopsia was doubted until specimens were found with rostrals which, along with its triangular skull and toothless beak, confirmed its common ancestry with neoceratopsians. It is also one of the few ceratopsians interpreted as a facultative biped, and it currently is the only ceratopsian found with undoubted gastroliths. The unique aspects of psittacosaurids are interpreted as being a reflection of their more primitive ornithischian ancestry.

Neoceratopsians are the geologically youngest of the major groups of dinosaurs, with their geologic range restricted mostly to the Late Cretaceous. Although they only occur in North America and Asia, they are well represented by numerous body fossils of high variety and completeness. Their young geologic age and the excessively bony skulls of some species favored their preservation over older dinosaurs with less bone (Chapter 7). For some species that occur in Upper Cretaceous strata of Mongolia, the exceptional preservation provided by these deposits also contributed much of what is known about neoceratopsians today.

Neoceratopsians are best distinguished from the psittacosaurids by the considerable size increase in their skulls in proportion to their postcranial skeletons. This tendency resulted in a few species competing for the largest skull ever owned by a land-dwelling animal. Some frills reached lengths of 1.5 meters and total skull lengths approached 3 meters! One adaptation for supporting such huge heads was a fusion of the anterior cervical vertebrae, and another was the development of stout fore limbs. By default, most neoceratopsians must have been obligate quadrupeds with only a few smaller forms having a possibility of facultative bipedalism.

The cladistic classification of neoceratopsians has been a point of contention in recent years, mostly as a result of a large amount of character data that conflict with traditional classification schemes. For example, a venerable gradistic name was applied to a grouping of some of the smaller (1-3 meters long) and more basal neoceratopsians, Family Protoceratopsidae, with its star member Protoceratops (Fig. 13.7B). This was then used to fit its members into a clade by the same name, but subsequent cladistic analyses have failed to support its mono-phyletic grouping. Consequently, numerous basal neoceratopsians precede the other major clade within Neoceratopsia, Ceratopsidae, which also is based on a former family name.

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