Marginocephalia (/ˌmɑːrdʒənoʊsəˈfæliə/ Latin: margin-head) is a clade of ornithischian dinosaurs that is characterized by a bony shelf or margin at the back of the skull.

[2][3] They basally used gastroliths to aid in digestion of tough plant matter until they convergently evolved tooth batteries in Neoceratopsia (or "new Ceratopsia") and Pachycephalosauria.

[6] Pachycephalosaurs, or "thick-headed reptiles", have primitive features that include basally small sized bodies, obligate bipedalism, and simple teeth with one row in operation at a time that are replaced as they are worn down.

Recent research suggests the flat heads could be a juvenile state before developing the dome shape in the adult stage.

[14] There is also evidence of advanced adductor musculature that extends from a large coronoid process on the mandible up to the ceratopsian frill, which would increase chewing force.

[16] Marginocephalian remains reveal significant evidence of being social creatures, much of which is related to the many possible functions of the bony skull margins.

Some possible functions of the variably shaped and sized margins are to ward off predators, display, ritualized combat, defense of territory, or establishing social ordering.

[21] Thus, the heads could have been used as ornamentation or to butt into the softer flank of other pachycephalosaurs for intraspecific communication or as a form of agonistic behavior.

[23] Other possible functions include intraspecific communication for mating purposes[24] or as a visual display of territorial protection, as seen in many common day organisms such as red-breasted robins.

[30] Horns also could have been used for thermoregulation as indicated by isotope analysis,[31] as aid in knocking down vegetation,[32] or for horn-locking agonistic behavior.

[39] Less is known about bipedal pachycephalosaur locomotion, although they must have had a fairly broad girth in order to make room for their enlarged gut.

[40] Stenopelix Wannanosaurus Goyocephale Homalocephale Pachycephalosauridae Micropachycephalosaurus Chaoyangsaurus Psittacosauridae Liaoceratops Archaeoceratops Leptoceratopsidae Coronosauria Cladogram after Butler et al., 2011.

[41] Wannanosaurus Goyocephale Homalocephale Pachycephalosauridae Micropachycephalosaurus Chaoyangsaurus Stenopelix Yinlong Psittacosauridae Liaoceratops Archaeoceratops Leptoceratopsidae Coronosauria