Osteological comparison of casque ontogeny in palaeognathous and neognathous birds: insights for selecting modern analogues in the study of cranial ornaments from extinct archosaurs

Birds, along with their dinosaurian precursors, possess a variety of bony cranial expansions. A deep understanding of the phenotypic complexity of these structures would be useful for addressing the development, evolution, and function of hard-tissue cranial ornamentation. Yet, the evolutionary significance and function of these structures have gone largely unaddressed because no unifying conceptual framework for interpreting bony cranial expansions currently exists. To provide such a framework, we examine osseous ornament variation in modern birds, using μ-CT imaging to examine the cranial casque components, structural composition, and developmental changes of two neognathous (Numida meleagris, Macrocephalon maleo) and one palaeognathous species (Casuarius casuarius) and survey the avian osteology literature of the 11 orders containing members with osseous cranial ornamentation. Our anatomical analyses suggest two broad configuration categories: (i) geminal, in which ornaments consist of paired elements only (i.e. within Neognathae) and (ii) disunited, in which ornaments consist of unpaired, midline elements along with paired bones (i.e. within Palaeognathae). Ornament bones contribute to casque elevation (proximal ornament support), elaboration (distal ornament shape), or both. Our results hold utility for unravelling the selection processes, particularly in difficult-to-decipher display roles, that shaped modern avian casques, as well as for the use of extant avians as comparative analogues of non-avian dinosaurs with ornamental head structures.