Species-specific casque shapes in the genus Casuarius and implications for visual display

Abstract

Cranial ornamentation is a widespread phenomenon across both extinct and extant vertebrates (e.g., fishes, amphibians, reptiles, mammals, and birds). These seemingly bizarre structures are of particular interest to paleontological reconstructions of extinct archosaur (e.g., pterosaur, non-avian dinosaur) life histories and morphological evolution. In this study, we quantitatively demonstrate that casque shapes differ between each cassowary (Casuarius spp.) species. Deciphering the evolutionary history and function, or functions, of such ornaments - even in extant taxa - can be challenging due to the dearth of studies that bridge behavioral and morphological variation. Here, we quantify ornament characteristics in modern cassowaries to gain clarity on the function of their conspicuous cranial casques. Early taxonomic studies of these birds qualitatively recognized that casques appear to be distinctly shaped between species (i.e., C. bennetti, C. casuarius, C. unappendiculatus). We implement two-dimensional (2D) geometric morphometrics to compare casque shape (lateral and rostral views) between the three currently recognized cassowary species. Although the casque morphospace across the genus had some areas of species overlap, species-specific mean casque shapes are significantly different from one another. Casque shapes of C. bennetti and C. casuarius are particularly unique, whereas those of C. unappendiculatus are most frequently misclassified due to their shared shape characteristics with the other two species. As the most comprehensive casque variation study on cassowaries to date, these findings provide important context for better-informed interpretations of cassowary biology, casque function, and cranial ornament evolution in this unique group of birds. Such context with an extant group is also essential in building our understanding of cranial ornament functional morphology, paleobiology, and evolution in extinct archosaurs more broadly.