A survey of casques, frontal humps, and other extravagant bony cranial protuberances in birds

Abstract

The occurrence of casques, frontal humps, and other bony cranial protuberances in birds is reviewed. Several previously overlooked examples are reported and casques of some rare taxa are for the first time figured. Bony cranial protuberances are most widespread among galloanserine birds, which is particularly true for helmet-like casques on top of the skull. In the species-rich clade Neoaves, by contrast, bony protuberances rarely occur on top of the skull and are mainly restricted to the beak. Similar structures have a different ontogenetic origin, with the casque of cassowaries being mainly formed by the mesethmoid and that of the Helmeted Guineafowl (Numida meleagris) deriving from the frontal bones. The pneumatic frontal humps of anseriform birds are formed through inflation of diverticula of the antorbital and/or fronto-ethmoidal sinus, and air sac diverticula are likely to be involved in the formation of other casques. In some Galloanseres, bony cranial protuberances are associated with integumentary structures that have a signaling function, but such correlations do not exist for other taxa, for which a physiological, acoustic or sensory function of the protuberances has to be considered. Casques also occur in pterosaurs and non-avian dinosaurs, but most comparisons between these extinct animals and birds were confined to the casques of cassowaries. The full spectrum of bony cranial outgrowths in birds has not yet been considered and the present survey may serve as a basis for future comparisons. Prominent casques on top of the skull are more likely to evolve in terrestrial or aquatic birds, in which the head is particularly exposed and plays a role in intraspecific signaling, but their absence in most neoavian taxa is noteworthy and requires future studies to address possible ontogenetic, ecological, or functional constraints on their formation.