Meeting Abstract

P3-36   -   Cassowaries are unicorns: Ontogenetic micro-CT analyses of Casuarius casques reveal a likely neomorphic cranial bone Green, TL*; Gignac, PM; Watanabe, A; New York Institute of Technology College of Osteopathic Medicine, Old Westbury, NY; Oklahoma State University Center for Health Sciences, Tulsa, OK; New York Institute of Technology College of Osteopathic Medicine, Old Westbury, NY tgreen09@nyit.edu

Cranial ornamentation is evident across amniotes, including the archosaur lineage comprising avian and non-avian dinosaurs. Among birds, cassowaries possess conspicuous casques, composed of a keratinous sheath and underlying bone. The skull bones that contribute to the bony casque fuse together in adulthood, obscuring suture boundaries. Because casque osteology is difficult to interpret from direct observations alone, it has led to numerous conflicting anatomical descriptions in prior literature. To reconcile this conflict, we formally documented casque ontogeny using a combination of micro-computed tomography imaging, digital visualization, and gross dissection across an extensive growth series of southern cassowaries (Casuarius casuarius; n = 54). Tracking dorsal skull element borders and expansions beginning at pre-hatching when sutures are pronounced allowed us to determine the individual bones that comprise the casque in neonate, immature, and mature animals. Three paired (nasals, lacrimals, and frontals) and two unpaired bones (median casque element, mesethmoid) make up the complex casque osteology in C. casuarius. The median casque element, described for the first time in our study, is a superficial midline bone not present in closely related species, such as emus (Dromaius novaehollandiae) and ostriches (Struthio camelus). To our knowledge, no other extant avian species possess a similar midline cranial element, suggesting the median casque element is derived in Casuarius. The next step is to determine the specific bone type of the median casque element via clearing and staining and histology. Classifying detailed casque anatomy in cassowaries allows for future comparisons of ornaments in other extant birds and extinct non-avian dinosaurs, increasing our understanding of cranial ornament configuration and evolution.