Clade Marginocephalia, consisting of the relatively rare pachycephalo-saurs but also the very common
Why Study Marginocephalians?
ceratopsians, includes one of the most famous dinosaurs known, the Late Cretaceous Triceratops of North America. However, perhaps less known is that Marginocephalia is a group of dinosaurs that rivals Theropoda (Chapter 9) and Ornithopoda (Chapter 11) in its diversity. Largely because of the ceratopsians, Marginocephalia is also among the best-represented of Late Cretaceous dinosaurs through their skeletal remains, first discovered in the late nineteenth century and still regularly uncovered today. Ceratopsians are exceedingly common in museums as a result of their abundance and completeness. Three marginocephalian genera (Protoceratops, Psittacosaurus, and Triceratops) rank in the top ten of all dinosaurs owned by museums. Unlike most dinosaur clades, many of its members were described on the basis of plentiful fossil material. More than 50% of ceratopsian genera and species are named from nearly complete or complete skeletons.
Marginocephalians were among the most endemic of all dinosaur clades in that their body and trace fossils have only been found in North America, Asia, and Europe. They also are the most limited in geologic time, nearly all comprised of Cretaceous examples, meaning that they were relative latecomers in Mesozoic ecosystems. This circumstance of limited biogeography and time was probably linked, thus affecting their evolutionary history until their extinction at the end of the Mesozoic.
Of all dinosaurs, marginocephalians are best known for their heads, which went to extremes in the development of thick or broad skulls. Indeed, a few ceratopsian species have the largest heads of any known land-dwelling animal. On these thick or otherwise enormous heads were some of the most elaborate and gaudy accessories seen in any dinosaur, such as numerous horns and spikes with various shapes, bosses, and broad frills. In terms of marginocephalian locomotion, rare ceratopsian tracks, recognized recently, fill a supposed gap in their paleontological record but contribute to ensuing controversies about their locomotion based on skeletal analyses. Pachycephalosaur tracks are so far either undiscovered or unrecognized, but at least their robust skull parts were taphonomically predisposed toward being preserved.
Some of these ornate animals may have traveled in herds, as suggested by mono-specific bonebeds and paleobiogeography. Such gregariousness would have created stunning vistas on Late Cretaceous landscapes, but also would have had a considerable impact on plant life. On a more individual basis, intraspecific competition in dinosaurs is perhaps most vividly conjured by the image of thick-skulled pachy-cephalosaurs running into one another at high speeds. Added to this scenario are similar ones of jousting ceratopsians, deterring either predation or rivals by simply turning their heads for full visual effect. Thus, the heads of marginocephalians are
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