Paul M Gignac

To date, several studies describe post‐hatching ontogenetic variation in birds; however, none of these studies document and compare ontogenetic variation of the entire skull in multiple avian species. Therefore, we studied ontogenetic skull variation of two bird species with very different ecologies, Pica pica , and Struthio camelus , using μCT based 3D reconstructions. For each specimen, we performed bone‐by‐bone segmentation in order to visualize and describe the morphological variation of each bone during ontogeny and estimated the average sutural closure of the skulls to identify different ontogenetic stages. Although bone fusion of P . pica occurs more rapidly than that of S . camelus the general sequence of bone fusion follows a similar trend from posterior to anterior, but a more detailed analysis reveals some interspecific variation in the fusion patterns. Although growth persists over a longer period in S . camelus than in P . pica and adults of the former species are significantly larger, the skull of the most mature S . camelus is still less fused than that of P . pica . Different growth and fusion patterns of the two species indicate that the interspecific ontogenetic variation could be related to heterochronic developments. Nevertheless, this hypothesis needs to be tested in a broader phylogenetic framework in order to detect the evolutionary direction of the potential heterochronic transformations.

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.