Marginocephalia

Marginocephalia is a clade of ornithischian dinosaurs defined by a distinctive shelf of bone along the margins of the skull. It comprises two principal lineages: Pachycephalosauria, the dome-headed dinosaurs, and Ceratopsia, the horned dinosaurs. The group first appears in the Late Jurassic and persists into the Late Cretaceous, with fossil remains primarily from North America and Asia.

Pachycephalosauria includes small to medium-sized herbivores characterized by thickened skull roofs that often form a dome. The skeletons indicate adaptations for both bipedal and facultatively quadrupedal locomotion. The domed skulls, sometimes with ear-like knobs or ornamentation, are thought to have been used in social display or combat, though exact behavior remains debated.

Ceratopsia encompasses beaked herbivores ranging from early, small forms such as Psittacosaurus to the later, large-frilled and horned ceratopsians like Triceratops. Derived ceratopsians possess ornate frills and facial horns and evolved robust dental batteries for processing tough vegetation. They are primarily known from the Early to Late Cretaceous and are especially diverse in North America and Asia.

Notable genera within Marginocephalia include Pachycephalosaurus and Stegoceras among the pachycephalosaurs; Psittacosaurus, Protoceratops, Centrosaurus, and Triceratops among the ceratopsians. Overall, marginal skull ornamentation and beaked mouths reflect shared adaptations for herbivory and social behavior within this clade.