Marginocephalia consists of the bipedal Pachycephalosauria, the dome-headed ornith-ischians, and the quadrupedal Ceratopsia, the horned, parrot-beaked, frilled ornithischians. The group was largely restricted to the Cretaceous of Asia and North America, and is diagnosed by the presence of a variably-sized shelf that formed along the back of the skull.
Marginocephalians were gregarious animals, and species selection was likely a driving force in much of their behavior, a fact that is reflected in their morphology. The domes of pachycephalosaurs have been interpreted as structures designed for intraspecific competition: head and flank butting have been suggested. The striking morphological variety of horns and frill shapes, and cranial ornamentation in ceratopsians suggests a high level of intraspecific competition. In both groups, sexual dimorphism has been recognized. Ceratopsian gregariousness is also reflected by the presence of large monospecific bonebeds, suggesting that herds of ceratopsians roamed what is now the Great Plains of Canada and the USA.
All thyreophorans are genasaurs, which means that they chewed their food to a greater or lesser extent. While pachycephalosaur teeth don't reveal evidence of remarkable chewing adaptations and pachycephalosaurs likely gut-fermented in their capacious stomachs, cerat-opsians developed sophisticated chewing mechanisms including a robust coronoid process, dental batteries, a skull partitioned into cropping, diastem, and grinding sections, and powerful jaw adductor muscles that may have attached high on the frill.
Care of the young is known in ceratopsian dinosaurs. Nests of partially grown cera topsians have been found, suggesting parental care at the nest.
Ceratopsian evolution was characterized by increasing size, as well as by one or more migrations across the Bering Strait land bridge, from Asia to North America. While intraspe-cific competition was likely an important behavioral aspect of even the earliest ceratopsians, later forms evolved elaborate frill or horn displays.
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