Meeting Abstract
Complexly constructed cranial ornaments, consisting of multiple bony partitions (e.g., cranial casques) are common among archosaurs. Yet, the developmental processes and selective regimes that bring about these metabolically expensive and seemingly bizarre structures remains a mystery. Among Aves, this is partially due to contradictory interpretations of cranial osteology, leaving it unclear whether the underlying bones of different casqued birds are arranged in a similar fashion. Here we compare the ontogeny of avian casques, independently derived in paleognathous and neognathous birds, to clarify their constituent parts. Flightless cassowaries (paleognaths) and volant birds such as helmeted guinea fowl, magpie geese, curassows, hornbills, and maleos (neognaths) possess casques of varying shapes and relative sizes. Casque bones can grow rapidly and are obscured by keratin sheathing in early ontogeny and sutural fusion in adulthood, rendering them difficult to study. To evaluate the null hypothesis that avian cranial ornaments possess similar anatomical patterns, we compared the skulls of southern cassowaries (Casuarius), maleos (Macrocephalon) and their non-casques relatives throughout ontogeny using µCT data. Crucially, sampling neonates and juveniles with incipient casques allowed us to track telescoping elements and measure growth. Although the neonatal skulls in our sample are broadly similar, our results point towards at least two modes of casque ontogeny: (1) disunited, in which a midline chondrocranial element grows slowly and posteriad to buttresses lateral dermatocranial bones, and (2) geminal, in which a rapidly growing casque is built from anteriad right-left dermatocranial constituents only.
