| dc.description.abstract | Modern cassowaries (Casuarius spp.) are flightless birds best known for their elaborate cranial casques, keratin and bone headgear located above the orbit and neurocranium. Because cassowaries are rare, seclusive, and potentially dangerous, there have been few in-depth studies regarding headgear biology despite functional speculations and suggestions of the casque as a modern analog for the ornaments of extinct taxa, which they resemble. Without baseline anatomical, ontogenetic, disparity, and functional studies, the casque's position in comparative and evolutionary studies remains uncertain. I address this uncertainty by elucidating the biological role(s) of the casque through micro-computed tomography imaging (uCT), evaluation of allometric scaling, two-dimensional shape analyses, and testing for similarities and differences in the comparative anatomy of osseous headgear across extant and extinct casqued archosaurs. I find that southern cassowary (C. casuarius) casques are comprised of eight distinct cranial bones and are far more complex than previously realized. Additionally, the central, dorsal element appears to be neomorphic and unique to cassowaries. The keratinous and osseous portions of C. casuarius casques scale with strong positive allometry, reaching the majority of their maximum size by sexual maturity. Casques of C. casuarius do not appear to be sexually dimorphic in ontogenetic trajectory nor adult shape. However, I find that casque shape may differ between certain C. casuarius regional populations and that casques are shaped significantly differently between the three extant species of cassowaries (C. bennetti, C. casuarius, C. unappendiculatus). Taken together, ontogenetic scaling of C. casuarius casques implicates their potential roles in signaling maturity or status, and shape analysis supports casques functioning in species differentiation. Because cassowary casque composition differs substantially from those of most other archosaurs (including neognathous birds), I find that the casques of modern neognaths (e.g., Macrocephalon maleo, Numida meleagris) may represent more appropriate anatomical analogs for ornament patterning and homology studies in extinct, non-avian dinosaurs with comparably simple cranial ornaments. Nonetheless, cassowary casques appear to be particularly important for our understanding of elemental elongation, developmental timing with whole-body indicators of maturity (e.g., feather and apteria coloration), multi-functionality, and ornament disparity among archosaurs. | |
