Ontogeny of Southern Cassowary, Maleo, and Helmeted Guinea Fowl Casques: Implications for the Study of Bony Patterning in Non‐avian Dinosaur Cranial Ornaments

Abstract

Cranial ornamentations (e.g. horns, domes, ridges, rugosities, crests, casques)—some intricately comprised of numerous bony elements—are common among living and extinct archosaurs. Yet, the developmental processes and selective regimes that bring about these metabolically expensive and seemingly bizarre structures remains a mystery. Such features have independently evolved at least 11 times among birds and are varied in their external morphology. Keratinous sheathing and obliterated sutures in late‐stage development have led to contradictory interpretations of cranial osteology for these species and has left it unclear whether the internal morphology of underlying bones of different casqued birds is arranged in a similar fashion. Here we compare the ontogeny for three of the independent casque acquisitions among Aves to clarify their constituent parts. To evaluate the null hypothesis that avian cranial ornaments possess similar anatomical patterns, we analyzed developmental series of skulls of southern cassowaries (Casuarius casuarius), maleos (Macrocephalon maleo), and helmeted guinea fowl (Numida meleagris) using μCT imaging. Sampling neonates and juveniles with incipient casques allowed us to track telescoping elements and measure growth. Our results point toward at least two modes of casque ontogeny among modern birds: (1) disunited, in which a midline chondrocranial element grows relatively slowly and posteriad to buttresses lateral dermatocranial bones (i.e. cassowaries), and (2) geminal, in which a rapidly growing casque is built from anteriad right‐left dermatocranial constituents only (i.e. Neognathae). These findings suggest that cassowaries are developmental outliers among extant casqued birds and extinct dinosaurs. Counterintuitively, this work identifies modern cassowaries—previously considered hallmark analogs for non‐avian dinosaurs—as poor osteo‐developmental models for studying the deep history of archosaur casque evolution.Support or Funding Information2018 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)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.