The internal cranial morphology of an armoured dinosaur Euoplocephalus corroborated by X-ray computed tomographic reconstruction

Abstract

Internal cranial anatomy is a challenging area to study in fossilized skulls because of small sample sizes and varied post-mortem preservational alterations. This difficulty has led to the lack of correspondence between results obtained from direct osteological observation and from more indirect reconstruction methods. This paper presents corroborating evidence from direct osteological observation and from reconstruction based on computed X-ray tomography (CT) on the internal cranial anatomy of the ankylosaurid dinosaur Euoplocephalus tutus. A remarkable specimen of Euoplocephalus preserves rarely observed internal cranial structures such as vascular impressions in the nasal cavity, olfactory turbinates and possible impressions of conchae. Comparison with fossils and CT models of other taxa and other Euoplocephalus specimens adds osteological evidence for the previously reconstructed nasal cavity in this dinosaur and revises the previously described braincase morphology. A new interpretation of the ethmoidal homology identifies a mesethmoid, sphenethmoid and ectethmoid. These ethmoidal ossifications are continuous with the mineralized walls of the nasal cavity. The location of the olfactory fenestra provides further evidence that the olfactory regions of the nasal cavity are pushed to the sides of the main airway. This implies that the function of the vascular impressions in the nasal cavity and the looping of the cavity are not related to olfaction. A byproduct of the elongate, looping airway is a dramatic increase in surface area of the nasal respiratory mucosa, which in extant species has been linked to heat and water balance. A role in vocalization as a resonating chamber is another possible function of the looping and elongation of the nasal cavity. Olfaction remains as a possible function for the enlarged olfactory region, suggesting that multiple functions account for different parts of the ankylosaurid nasal cavity that underwent substantial modification. Cranial endocasts show negligible variation within Euoplocephalus, which lends some confidence to interspecific comparisons of endocranial morphology.

Systematics, phylogeny and palaeobiogeography of the ankylosaurid dinosaurs

“…The laterosphenoid contributes to the postocular shelf, which appears to have been strongly developed as in Euoplocephalus (Miyashita et al 2011); this is unlike the condition in Zaraapelta. The orbitosphenoid and parasphenoid are difficult to discern, but the orbitosphenoid likely contributed to the lateral wall of the braincase and the medial wall of the orbit, and the parasphenoid contributed to the anteroventral wall of the braincase and to the interorbital septum, as in other ankylosaurs like Euoplocephalus (Miyashita et al 2011).…”

“…The jugal forms the ventral border of the orbit. The frontals cannot be distinguished in dorsal view and it is unclear if they possessed a scroll-like descending process as in Euoplocephalus (Miyashita et al 2011) and Zaraapelta.…”

“…The orbitosphenoid and parasphenoid are difficult to discern, but the orbitosphenoid likely contributed to the lateral wall of the braincase and the medial wall of the orbit, and the parasphenoid contributed to the anteroventral wall of the braincase and to the interorbital septum, as in other ankylosaurs like Euoplocephalus (Miyashita et al 2011). The ectethmoid contributes to the anterior wall of the orbit, and separates the orbit from the olfactory region.…”

Missing data estimation in tyrannosaurid dinosaurs: Can diameter take the place of circumference?

“…The most common cause of missing data in a palaeontological sample is taphonomic deformation (Chapman, 1990;Dilkes, 2001;Miyashita et al, 2011;Arbour & Currie, 2012;Tschopp et al, 2013;Hedrick & Dodson, 2013). The geological pressures that distort and destroy fossils as they form are beyond the control of palaeontologists; however, paucity in sample size can also be compounded by anthropological factors.…”

Unusual cranial and postcranial anatomy in the archetypal ankylosaur Ankylosaurus magniventris

“…Ankylosaurines have unusual nasal anatomy, with complex nasal vestibules, convoluted looping nasal passages, and a high degree of vascularisation in the posterior portions of the airway (Hill et al 2003;Witmer and Ridgely 2008;Miyashita et al 2011). The border of the external naris in ankylosaurines is typically demarcated by a distinct edge on the premaxilla and the external surface of the supranarial caputegulum.…”