Redescription of the Panoplosaurus Mirus (ankylosauria: Nodosauridae) Holotype Skull Using Computed Tomography, and a New Comparative Scheme for Panoplosaurin Cranial Ornamentation

The late Campanian nodosaurid Panoplosaurus mirus Lambe 1919 was named on the basis of a complete skull and partial postcranium. However, as is typical of ankylosaurs, the constituent elements of the holotype skull (CMN 2759) are obscured both by their extensive co-ossification and by the overlying bony caputegulae, complicating taxonomic referral of other material. The holotype skull is re-examined here using computed tomography (CT) imaging, revealing new information about the internal anatomy. Previously unknown regions of the skull are described, including the basicranium and palate, and the internal anatomy in Panoplosaurus is examined from CT-rendered models of the brain and nasal cavities. The occurrence of a vomerine canal or groove and convoluted nasal passages are confirmed in Panoplosaurus, while decreased brain flexure relative to other nodosaurids suggests there is greater variability in the paleoneurology of Nodosauridae than previously appreciated. The unique pattern of cranial ornamentation in Panoplosaurus is also determined to be taxonomically significant (as it is in Ankylosauridae), and the generic separation of Panoplosaurus and the closely related Edmontonia is supported. Further, intraspecific variation in the cranial ornamentation of Panoplosaurus is revealed, based on the presence or absence of the internasal caputegulum, although the nature of this variation (e.g., ontogeny, sexual dimorphism, individual variation) remains elusive.