Potentially Neomorphic Cranial Bone Revealed in First Micro‐CT Study of Southern Cassowary Casque Development

Abstract

Modern cassowaries are a group of flightless birds that possess unusual cranial ornamentation. These bony and keratinous dorsal skull projections, referred to as casques, have been of interest to anatomists for the past two centuries. The osteological configuration of cassowary casques has been described by numerous authors during that period; however, interpretations have differed widely. This ambiguity, arising from purely direct observational studies has prompted the need to redescribe the osteology of cassowary casques across an extensive developmental series (n = 54) using micro-computed tomography (μCT) imaging, digital visualization, and gross dissection. Tracking cranial elements from embryo to adult form in southern cassowaries (Casuarius casuarius) and comparing those bones to non-casqued ratites (Dromaius novaehollandiae and Struthio camelus)revealed that eight distinct cranial bones comprise the casques of adult C. casuarius. The cranial pattern of three paired (nasals, lacrimals, frontals) and two unpaired elements (median casque element, mesethmoid) is unique among the bony ornaments of extant avians. Our analysis documents the median casque element as being distinct from the mesethmoid and absent in other, closely related flightless birds, suggesting this bone is neomorphic in modern cassowaries. This research sets an anatomical baseline for future studies regarding comparative ornament ontogeny and casque function in these poorly-understood birds, as well as the evolutionary and developmental origins of cranial ornamentation across vertebrates.