An Osteo‐Developmental Baseline for Cranial Casque Anatomy of Southern Cassowaries Better Informs Functional Interpretations

Abstract

Seemingly bizarre cranial ornaments can be found among most major tetrapod groups. The casques (i.e., dorsal skull projections of bone surrounded by keratin sheathing) of modern cassowaries (Casuarius) are perhaps the most enigmatic of modern Aves. Both the biological role(s) and anatomical configuration of this headgear has been debated for the past 150 years, limiting our understanding of the life history of these unique birds as well as the selective conditions under which cranial elaborations appear in archosaurs. Because these structures are poorly understood, it is essential to set an osteo‐developmental foundation from which biomechanical, behavioral, physiological, and functional studies can radiate from. To address the commonly proposed roles of cassowary casques (e.g., ramming, vocalization, thermoregulation, display), we conducted μCT analyses of cranial anatomy in a developmental series of southern cassowaries (C. casuarius). This allowed us to describe details of both external and internal anatomy as well as timings of sutural fusions and inflations of casque elements. We compared the developmental timing of casques to physical, behavioral, and reproductive maturity to determine the plausibility of each function. Our sample (n = 130) indicates the majority of casque growth occurs prior to sexual maturity; although, casque morphology changes substantially throughout adulthood. Deviation from midline is particularly common in the casques of C. casuarius and such asymmetries appear to be primarily dextral. Form does not support functions for either physical ramming or vocalization; however, thermoregulation and display are not refuted by ontogeny nor adult phenotype, necessitating more focused future study.Support or Funding Information2019 New York Institute of Technology College of Osteopathic Medicine (Akinobu Watanabe)2019 Western Interior Paleontological Society ‐ Karl Hirsch Memorial Grant (Todd L. Green)2018 American Association of Anatomists ‐ Visiting Scholarship (Todd L. Green)2017 National Science Foundation ‐ Major Research Instrumentation Grant (Paul M. Gignac)2016 Western Interior Paleontological Society ‐ Karl Hirsch Memorial Grant (Todd L. Green)2015 National Science Foundation ‐ Early‐concept Grant for Exploratory Research (Paul M. Gignac)2015 National Science Foundation ‐ Division of Environmental Biology (Paul M. Gignac)