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, 219 (6), 661-75

The Internal Cranial Morphology of an Armoured Dinosaur Euoplocephalus Corroborated by X-ray Computed Tomographic Reconstruction

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The Internal Cranial Morphology of an Armoured Dinosaur Euoplocephalus Corroborated by X-ray Computed Tomographic Reconstruction

Tetsuto Miyashita et al. J Anat.

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.

Figures

Fig. 1
Fig. 1
Schematic reconstructions of the nasal cavity morphology of two ankylosaur skulls. (A) The ankylosaurid Euoplocephalus tutus (AMNH 5405). The left half of the skull is derived from the reconstruction published by Witmer & Ridgely (2008), whereas the right half represents new information based on the osteological correlates of the soft tissues within and around the nasal cavity in UALVP 47977. The specimen (UALVP 47977) preserves parts of the main airway (in green), the olfactory region (in blue), and the endocranial cavity (in orange). (B) The nodosaurid Panoplosaurus mirus (ROM 1215) after Witmer & Ridgely (2008) for comparison. The nasal passage of Euoplocephalus is looped in more complex ways than that of Panoplosaurus, and the olfactory region of Euoplocephalus is pushed to the side of the main airway. bv, blood vessel trace; co, groove that possibly housed the concha; dac, dorsal alveolar canal; dpf, descending process fused to the ventral surface of the frontal; olfactory turbinate; ec, endocranial cavity; ee, ectethmoid; eth, ethmoidal complex; lnv, lateral nasal vessels; lw, lateral wall of the main airway; ma, main airway; mnv, medial nasal vessels; mnc, medial nasal canal; ns, nasal septum; nv, nasal vestibule; or, olfactory region; os, orbitosphenoid.
Fig. 2
Fig. 2
Illustration (A) and photograph (B) of the ventral view of an ankylosaurid skull roof (UALVP 47977, Euoplocephalus) from the Dinosaur Park Formation (Campanian, Upper Cretaceous), southern Alberta. Hatched lines indicate parts reconstructed with plaster. Impressions of the soft tissues, including the main airway, nasal arteries, and possible turbinates, are well defined. The ethmoidal elements are well ossified and separate regions of the nasal cavity from each other. No sutures are visible. aw, anterior wall of the cavity for the olfactory region; cor, cavity for the olfactory region; ls, laterosphenoid; od, orbital depression; sl, sulcus associated with the groove in the olfactory region. For other abbreviations, see Fig. 1.
Fig. 3
Fig. 3
Vascular impressions in the dorsomedial part of the nasal cavity proper of UALVP 47977. (A) Drawing of skull showing region of enlargement in diagram (B) and photograph (C). These ethmoidal vessels are likely to be part of the median nasal canal system. Hatched area in B represents broken nasal septum (mesethmoid). Anterior is to the right. For abbreviations, see Fig. 1.
Fig. 4
Fig. 4
Sagittal sections of a Euoplocephalus skull (AMNH 5405) from CT data of Witmer & Ridgely (2008) show a tunnel within the frontal bone, laterally positioned and passing medially and slightly posteriorly on both sides. The most lateral sagittal section for each side is where the canal disappears into the bone. Arrowhead indicates the tunnel within the frontal, and letters A–H indicate the levels of the CT slices on the skull. Anterior is to the right in all CT slices and in the 3D model of the skull. CT data are available from the website (http://www.oucom.ohiou.edu/dbms-witmer/3D-Visualization.htm).
Fig. 5
Fig. 5
CT-based reconstruction corroborates direct osteological observation. CT renderings of the skull roofs of two Euoplocephalus specimens, AMNH 5405 (A,C) and UALVP 31 (B,D). A and B show in dark grey the portion of the skull represented in C and D in relation to the entire skull, in oblique right anterior view. C and D are sliced to mimic the areas preserved in UALVP 47977, and show internal features of the skull in ventral view that correspond to those of UALVP 47977 (Fig. 2), with anterior towards the bottom of the page. oo, ocular osteoderm; pp, paroccipital process; q, quadrate. For other abbreviations, see Figs 1 and 2.
Fig. 6
Fig. 6
UALVP 47977 (Euoplocephalus) in occipital view. For abbreviations, see Fig. 5.
Fig. 7
Fig. 7
Comparison of cranial endocasts and that of braincases reveals minor variation amongst specimens referred to as Euoplocephalus. The braincase of AMNH 5405 in left lateral view (A), the cranial endocast of the same specimen in left lateral (B), dorsal (C) and ventral (D) views, the braincase of UALVP 47977 in right lateral view (E), the cranial endocast of the same specimen in right lateral (F) and ventral (G) views, and the cranial endocast of UALVP 31 in right lateral (H) and ventral (I) views. The images E–I were all inverted horizontally to show the right sides in the same orientation with the left side of AMNH 5405 for the purpose of comparison. In both UALVP 47977 and UALVP 31, the right side is better preserved. UALVP 47977 is represented by a line drawing of a latex cast, and AMNH 5405 and UALVP 31 are 3D models based on CT data. Roman numerals refer to either the foramen for, or the trunk of, the cranial nerve. amcv, anterior middle cerebral vein; amp, insertion site for M. adductor mandibulae posterior; sensuHolliday & Witmer, 2007; fl, flocculus; fv, fenestra vestibularis; ic, internal carotid artery; ocv, orbitocerebral vein; of, olfactory fenestra; pmcv, posterior middle cerebral vein; sp, sinus of pituitary. For other abbreviations, see Figs 1 and 2.

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