Babyrousa babyrussa (Linnaeus, 1758)

O'Leary, Maureen A., 2016, Comparative Basicranial Anatomy Of Extant Terrestrial And Semiaquatic Artiodactyla Maureen A. O’Leary, Bulletin of the American Museum of Natural History 2016 (409), pp. 1-1 : 1-

Babyrousa babyrussa

Figures 9–11

Prior descriptions provided details of the cranial anatomy in a related member of Suidae , Sus ( Parker, 1874; Sisson, 1911; Getty, 1975; Schaller, 1992), and Kampen (1905) provided details of the auditory bulla of Babyrousa .

The basicranium is described from two specimens: AMNH-M 152858, with the bulla in situ, and AMNH-M 2238, in which the bulla has been sawed off to expose the petrosal. I also consulted AMNH-M 152861, a subadult, to examine sutures, which are typically fused in adults. In the adult, the petrosal is not visible with the bulla in situ.

The sphenorbital fissure is oval with the long axis directed superior to inferior. It is inferolateral to the circular opening for the optic canal (fig. 10). The vomer is visible in the basipharyngeal canal; it has a very fragile and thin posterior edge that terminates in an irregular edge (figs. 9, 10). The hard palate is smooth and its posterior edge is inflected superiorly; a sharp ridge denotes the suture of the right and left palatines (fig. 9). Each palatine bone contributes to part of the posterior spine in the midline and the suture between the two palatine bones is visible. The much larger, single major palatine foramen lies just anterior to the third molar, and as many as three, more irregularly shaped, and much smaller minor palatine foramina lie posterior to it, closely adjacent to the third molar. The hamulus of the pterygoid is a mediolaterally compressed plate that terminates in a pointed, superiorly directed hook (fig. 9).

The longest dimension of the glenoid fossa is mediolateral, and the fossa is in the same transverse plane as the skull base, in other words, substantially superior to the inferior margin of the bulla (fig. 9). The fossa is gently convex and continuous with a preglenoid process anteriorly. There is a small medial glenoid pit. The postglenoid process of the squamosal is at the lateral edge of the glenoid fossa where it forms a small bump. The postglenoid foramen is absent. Sutures between the squamosal and the ectotympanic are almost entirely obliterated in the adult specimen and even in subadults (fig. 9). The suture between the exoccipital and the squamosal is clear and the squamosal contributes a small amount of bone to the anterior base of the paracondylar process. Figure 9 shows an estimate of the position of the anterior suture between the squamosal and the ectotympanic, the latter forming a very elongate external acoustic meatus. Posterior to the external acoustic meatus, a sharp and distinct ridge ascends from the bulla, runs anterior to the stylomastoid foramen and terminates at the inferior margin of the porus acusti- cus externus. This is the posttympanic process of the squamosal (following the general descriptions of Pearson, 1927), although it is somewhat ambiguous where the ectotympanic ends and the squamosal begins. The porus acusticus externus is substantially superior to the middle ear region.

In the midline the craniopharyngeal canal is absent. A roughened ridge indicates where the basioccipital meets the basisphenoid. The tym- panic processes are not prominent structures and are distinctly separate from the auditory bulla (fig. 9). The occipital condyles are not in contact. They have distinct ventral and posterior surfaces but lack a pronounced ridge separating these surfaces. There is a single, distinct hypoglossal foramen that opens widely into the foramen magnum.

The paracondylar process of the exoccipital is blunt tipped and uniform in shape over its entire length. In lateral view, this process extends well below the auditory bulla. At its superior aspect, the process has an abrupt bend as it descends from the skull base; below the bend it straightens and is angled anteriorly. At its superior end it is in close approximation to the auditory bulla but not in contact with it. When the bulla is in situ its distinct bullar ridge can be palpated and, coming off the ridge at its most inferior position, is a small pointy spike (fig. 9). The overall shape of the bulla is ovoid with a roughened surface. The bulla is mediolaterally narrow and extends ventrally well below the level of the glenoid fossa, as previously noted ( Kampen, 1905). The stylohyal fossa that invaginates the posterolateral edge of the bulla is defined on both its medial and lateral sides by ridges on the bulla (fig. 9). A tympanohyal is not visible in the stylohyal fossa. The ridge on the lateral aspect of the stylohyal fossa forms the medial aspect of the stylomastoid foramen. There is a groove external to the stylomastoid foramen running along the posttympanic process of the squamosal. Via fusion to the posttympanic process of the squamosal, the bulla is completely sutured to the skull anterior to the exoccipital and has to be sawed off to expose an in situ petrosal or the bulla’s dorsomedial side. The bulla does not, however, fuse with the petrosal.

The carotid incisure and the incisura ovalis are visible in the space between the bulla and midline bones, even when the bulla is in situ (fig. 9). Anteriorly the opening of the auditory tube is inconspicuous in ventral view as it is tucked deeply into the skull base. The anterior carotid foramen does not leave a conspicuous mark on the bulla (fig. 11). The bullar contribution to the posterior carotid foramen is marked by a subtle groove just anterior to the bullar contribution to the jugular foramen. The bulla forms the anterior border of the jugular foramen, with the medial and posterior borders formed by the exoccipital (fig. 9).

With the bulla removed it is apparent that there is extensive space between the petrosal and the surrounding bones anterior and medial to it (fig. 10). Most laterally, the piriform fenestra is poorly defined and blends entirely with the inci- sura ovalis. The incisura ovalis is separated from the carotid incisure by a small, posteriorly directed prong of bone extending off what is probably the alisphenoid (sutures are fused). The carotid incisure is very large and wide, making a deep notch in the basisphenoid. This opening blends fully with the middle lacerate foramen and the basicapsular fissure, which is less clearly defined but is still a broad space between the petrosal and basisphenoid-basioccipital in the midline. The jugular foramen is the most separated of these skull-base openings and is defined by a prong of bone from the basioccipital that almost meets the petrosal (fig. 10). The squamosal part of the external acoustic meatus is smooth and wraps onto the lateral surface of the skull.

When the bulla is removed and examined from its dorsomedial surface, the relatively long, smooth external acoustic meatus is visible as it extends from the ectotympanic annulus (fig. 11). The bulla has extensive contacts with the petrosal in the vicinity of the ectotympanic annulus. As noted above, the bulla does not, however, fuse with the petrosal. In complementary fashion, it is on the flanges of the petrosal surrounding the bulging promontorium where in life the bulla contacts from below (see also O’Leary, 2010). The bulla also has a large convex process that abuts the basioccipital but does not fuse, or in some specimens (e.g., AMNH-M 152858) the bulla and basioccipital are very closely adjacent but not in contact. The ectotympanic part of the porus acusticus externus is irregular and jagged. Meatal foramina are not conspicuous on the inside of the external acoustic meatus. The ectoympanic annulus is a distinct semicircle for approximately 180° with no clear demarcation between anterior and posterior crura. Two irregular prongs of bone extend from the annulus and almost extend it to make a complete circle. Jutting from the annulus are several irregularly spaced struts of bone that are contiguous with the cancellous structure of the bulla. The bulla is cancellous throughout and its external bony surface is thin enough that the cancellous internal structure is visible through the bone in strong light. In the region of the ectotympanic, the edge of the bulla curves inward near where it makes contact with the petrosal. Extending from this curvature, a pronounced channel forms the canal for the auditory tube, and runs the length of the bulla. It is delineated by a sharp ridge. Markings on the dorsomedial surface of the bulla for the anterior and posterior carotid foramina are subtle channels.

Figures

FIGURE 9. Basicranium of Babyrousa babyrussa (Suina; AMNH-M 152858) with bulla in situ. Scale bar = 1 cm.

FIGURE 10. Basicranium of Babyrousa babyrussa (Suina; AMNH-M 2238) with bulla removed to reveal petrosal in situ. Scale bar = 1 cm. Artist: C. Lodge.

FIGURE 11. Left auditory bullae of Babyrousa babyrussa (Suina; AMNH-M 2238), Bos taurus (Cetruminantia, Ruminantia, Bovidae; SBU-MAR 14), and Odocoileus virginianus (Cetruminantia, Ruminantia, Cervidae; SBU-MAR 20), shown in dorsomedial view. Scale bar = 1 cm. Artists: C. Lodge (Babyrousa), L. Betti-Nash (Bos), and U. Kikutani (Odocoileus).