Ceratopsians, or horned dinosaurs, display a spectrum of variation in the system of depressions or sinuses on the dorsum of the skull. Derived non-ceratopsoid neoceratopsians such as Protoceratops and Zuniceratops variably possess a frontoparietal depression rostral to the dorsotemporal fenestrae. In ceratopsids, such as Triceratops, the depression is roofed secondarily by bone to form a supracranial sinus. The sinuses vary in their lateral extent, with the most restricted condition found in Centrosaurus and Styracosaurus. The sinuses are suggested to be pneumatic structures, although it cannot be determined if they are paranasal, paratympanic, or pharyngeal in origin. The inferred pneumatic diverticula that supplied the sinus extended from the dorsotemporal fenestrae into the sinus via a pair of channels on the dorsum of the skull. Ontogenetically, the sinus began as a shallow depression on the roof of the skull that was enclosed by excavation into the cranial bone and overgrowth of bone. Sinus morphology in bovid mammals, frequently invoked analogues for ceratopsids, is not correlated with behaviors such as head butting, but differences in sinus placement and anatomy between the groups temper the use of bovid sinuses as an analogy for ceratopsid sinuses. The development of a closed sinus in ceratopsids from an open depression was probably associated with an increase in skull size and the accompanying relatively greater loads applied to the horns (in order to maintain the structural integrity of the skull), as well as an anatomical reorganization of the ceratopsian skull.