The first occurrence of a probable pachycephalosaurian hyoid element

Derived from the second pharyngeal arch, the hyoid complex is a free-floating midline structure that is situated posteroventrally to the lower jaw. While there are many known free-floating osseous structures—such as dermal ossifications, dinosaurian ossified tendons/myosepta, and the mammalian baculum/baubellum—the hyoid complex is one of the few examples of these ‘floating’ bony elements that is found widely across the vertebrate bauplan. In terrestrial vertebrates, the hyoid complex is an important structure for vocalization, feeding, and respiration. A gracile, delicate, and multi-component structure in close proximity to heavily predated tissues, the hyoid complex is not a taphonomically favored structure; and yet, despite these taphonomic challenges, within Dinosauria, the primary osseous portion of the hyoid complex, the ceratobranchial, is sufficiently documented within many clades—save Pachycephalosauria. Previously, there were no known or suspected ceratobranchials reported from any pachycephalosaurian, making it one of the few osseous elements entirely unknown from this clade. Here, we report on a bony element which we hypothesize to be a right ceratobranchial from the holotype of the late Maastrichtian “ Dracorex hogwartsia .” This element was found associated with the holotypic material—though never reported—and its overall morphology, such as being anteroposteriorly elongate and transversely thin ( e.g ., ‘rod-like’), is congruent with ceratobranchial morphology from other dinosaurian taxa. If our identification is correct, this not only marks the first occurrence, and one of the remaining ‘blank spots’ of the pachycephalosaurian skeleton to be ‘filled in,’ but also has potential physiological and osteohistological implications for better understanding this enigmatic dinosaurian clade.