Pinacosaurus grangeri, (GILMORE, 1933)

Hill, Robert V., D, Michael D., Emic, Bever, G. S. & Norell, Mark A., 2015, A complex hyobranchial apparatus in a Cretaceous dinosaur and the antiquity of avian paraglossalia, Zoological Journal of the Linnean Society 175 (4), pp. 892-909 : 893-894

PINACOSAURUS GRANGERI ( GILMORE, 1933)

New referred specimen: IGM 100/3186, nearly complete skull and articulated cervical vertebral series, ossified and articulated hyobranchial apparatus, cervical ribs, first cervical osteodermal half ring, left radius, and associated osteoderms. A partial postcranial skeleton was also collected and is currently under preparation.

Age and locality: Middle Campanian, Death Row sublocality, Ukhaa Tolgod, Ömnögovi Aimag, Mongolia. Facies S, structureless sandstone indicative of masswasting sandslides ( Dingus et al., 2008). The burial of dinosaurs and other vertebrates at this locality was rapid and complete, resulting in well-preserved and articulated specimens.

Diagnosis

The specimen is referred to Pinacosaurus grangeri ( Gilmore, 1933) on the basis of the following characters: primary airway bounded by dorsally embayed osteodermal mass, lacrimal incisure (pinching of snout in dorsal view), accessory openings into premaxillary sinus, squamosal horns small and pyramidal, and width across squamosal horns not greater than width across supraorbitals (Arbour & Currie, in press; Hill, Witmer & Norell, 2003; Burns et al., 2011).

History of discovery and preparation

IGM 100 /3186 was discovered in the summer of 2005 at the Death Row locality of Ukhaa Tolgod, Ömnögovi Aimag, Mongolia ( Dingus et al., 2008). The specimen is characteristic of dinosaur fossils from this locality, being relatively completely preserved and with individual bones in complete articulation or close association. A block containing the skull and cervical vertebrae was excavated and jacketed .

The block was scanned by computed tomography (CT) using a GE Lightspeed VCT scanner at the Stony Brook University Medical Center. As a result of poor contrast between the bone and matrix, internal visualization of the block was limited, and so the specimen was scanned several times as mechanical preparation proceeded. Scans were taken at 0.625-mm slices and intervals, 120 kV, and varying amperage (120– 250 mA) in order to produce the clearest images. Although bone was evident on the surface, the first scans did not reveal any internal structures. As the block was mechanically prepared it became clear that a nearly complete skull was preserved within. The dorsal surface was prepared partially, revealing the skull roof, osteoderms overlying the snout, and the first cervical osteoderm half ring.

Mechanical preparation of the ventral surface of the specimen uncovered the mandible and part of the hyobranchial apparatus ( Fig. 1 ). Additional CT scans allowed some superficial digital segmentation of the hyobranchium; however, similar density between bone and matrix made it impossible to fully characterize the anatomy of this structure. The predentary bone was removed and prepared separately. Two additional ossifications, found immediately caudal to the left hemimandible, were recognized as the interorbital ossifications, i.e. the orbitosphenoid and laterosphenoid bones.

Further preparation isolated the hyobranchial apparatus, which was ultimately prepared separately. Scans of the isolated hyobranchium provided the contrast necessary to fully segment and digitally reconstruct the structure using AVIZO 7.0 (http:// www.vsg3d.com). Three displaced teeth and the left articular bone remained in close association with the rostral elements of the hyobranchium.

Moderate additional preparation of the palatal region exposed the basioccipital and exoccipital elements, which had become dislodged, and drifted into the palate. The relative independence and lack of fusion of these bones further attest to the juvenile age of the individual. Original CT data and movie files are publicly available as project P2101 at http://www.morphobank.org ( O’Leary & Kaufman, 2012).

DESCRIPTION

The skull exhibits morphology characteristic of derived ankylosaurids, being triangular in dorsal view, roughly as long as it is wide. The nasal region is heavily ornamented by osteoderms and other cranial rugosities ( Vickaryous, Russell & Currie, 2001; Hill et al., 2003; Arbour & Currie, 2013). The skull roof caudal to the frontonasal suture is devoid of osteoderms or ornamentation, allowing the sutures between more caudal skull elements to be easily discerned. The parietals are fused into a single element. The lateralmost supraorbitals are elaborated into a sharp, pyramidal horn, and the secondary osteoderms overlying the squamosals are also pyramidal and rugose.

IGM 100/3186 resembles other known specimens of Pinacosaurus in having multiple accessory apertures in the narial region ( Fig. 2 ). Five openings are present on each side of the snout in IGM 100/3186, similar to the condition in IGM 100/1014 ( Hill et al., 2003). The dorsalmost opening represents the true naris, which opens into the airway. Ventral to this are three more or less round openings (structures C1–C3; Hill et al., 2003), which open into a premaxillary sinus cavity. Caudal to these apertures is a slightly rostrocaudally elongate opening corresponding to structure B ( Hill et al., 2003). As with other specimens of Pinacosaurus , it is unclear whether this aperture actually opened into a sinus or was an externally located fossa that housed soft tissue.

The premaxillary palate is fenestrated by two large openings, each one located near the caudolateral margin of the premaxilla ( Fig. 2 ). CT scans demonstrate that these openings communicate with the premaxillary sinus, and are therefore continuous with the external openings into this space (i.e. apertures C1–C3; Hill et al., 2003). The bilateral symmetry and finished edges of these openings suggest that they are actual fenestrae, rather than artefacts of breakage. Premaxillary palatal fenestrae have not been previously described in detail in Pinacosaurus or other ankylosaurs, although they appear to be present in certain illustrations depicting the palate ( Burns et al., 2011), but are absent in others (Maryan´ ska, 1977; Hill et al., 2003). Because the bone in this region is exceedingly thin, the margins of the fenestrae are susceptible to breakage. Their presence and natural size may be misinterpreted in all but the most completely preserved specimens.

The first cervical half ring is preserved in association with the skull. It consists of six rectangular osteoderms that meet one another at deeply interdigitating sutures. In articulation, the ring encircles more than two-thirds of the circumference of the neck. In ventral view the arch of the mandible is clearly visible, as it is preserved in articulation with minimal distortion. A long, triangular mandibular osteoderm is associated with each hemimandible. The predentary was dislodged into the oral cavity during or after deposition, but was preserved whole and then prepared separately. The hyobranchial apparatus (described below) dominates the space between the hemimandibles. Caudally, the ventral aspects of the cranial cervical vertebrae are visible. Each vertebra remains associated with its two cervical ribs; however, some shearing in this region has moved the ribs out of direct articulation.

Figures

Figure 1. Pinacosaurus grangeri (IGM 100/3186). Stereophotographs of articulated skull and cervical region in ventral view, showing hyobranchial elements at the centre. Abbreviations: bs, basisphenoid; cb2, second ceratobranchial; cvr, cervical rib; eb, epibranchial; lpg, left paraglossalium; ls, laterosphenoid; mo, mandibular osteoderm; os, orbitosphenoid; pmx, premaxilla; ppf, premaxillary palatal fenestra.

Figure 2. Pinacosaurus grangeri from the Upper Cretaceous of Mongolia, showing accessory narial apertures. Top, ZPAL MgD II/1, juvenile specimen of P. grangeri showing intact narial region; inset at lower left shows homologous portion on IGM 100/3186. Lower right, ventral (palatal) view of rostrum, showing the opening of the right premaxillary palatal fenestra. Abbreviations: B, structure B in the narial region; C, apertures C in the narial region, opening into the premaxillary sinus cavity; mx, maxilla; mxo, maxillary osteoderm, pmx, premaxilla; ppf, premaxillary palatal fenestra; v, vomer.