Potamochoerus porcus (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-

Potamochoerus porcus

Figures 4 , 7 , 8

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) also provided details regarding the auditory bulla of Potamochoerus .

Descriptions of the bulla in situ and removed are based on adult specimens AMNH-M 238330 and AMNH-M 53703, supplemented by AMNH- M 238329. Descriptions were also supplemented by examination of subadult specimen AMNH-M 100670. The lateral side of the bulla is firmly fused to the skull with the external acoustic meatus tightly sutured to the adjacent squamosal. In the adult, the petrosal in situ can be observed only with the bulla sawed off.

The optic foramen and sphenorbital fissure, which are out of view in the illustrations, are set very close together with the round optic foramen positioned slightly superior and anterior to the much larger and more oval sphenorbital fissure. About 3 cm anterior to the base of the sphenorbital fissure is a pin-sized hole that may be the anterior opening of the pterygoid canal but is too minute to probe for continuity with a posterior opening.

The surface of the palate is flat in a horizontal plane and ends in a small, superiorly directed posterior spine that receives contributions from the right and left palatine bones. There is no roughening or raised ridge where the two palatine bones meet at the midline suture. The ento- and ectopterygoid processes are platelike with sharp edges that are well separated ventrally, demarcating a distinct pterygoid fossa. The pterygoid fossa is deepest ventrally and flattens dorsally where it is defined only by crests on the medial and lateral sides. Where the ecto- and entopterygoid processes meet ventrally, the palate surface is very roughened. The hamulus is a distinct prong extending posteriorly, ventrally, and slightly laterally from the rest of the entopterygoid process (figs. 7, 8). The hamulus terminates in a roughened knob.

In the roof of the basipharyngeal canal, the vomer, presphenoid, and pterygoid bones are visible (although the vomer sometimes covers the presphenoid entirely), however, the complete contours of each bone are hard to distinguish in the adult due to fusion with the basisphenoid and alisphenoid. The posterior end of the vomer lies just anterior to the presphenoid-basisphenoid suture and is apparent in AMNH-M 53703 (fig. 8). At the junction of the basisphenoid and the pterygoid is a pin-sized hole that is the posterior opening of the pterygoid canal. There is no particularly pronounced sulcus leading back from it to the middle ear. As noted above, the pterygoid canal is too narrow to easily establish continuity with a probe between its anterior and posterior openings.

The midline of the skull lacks a craniopharyngeal canal. A distinct median crest runs along the basisphenoid and basioccipital terminating just anterior to the occipital condyles. The tympanic processes are two small knobs positioned between the midline and the basicapsular fissure that have no contact with the bulla (fig. 8). The occipital condyles approximate each other in the midline but do not touch. They lack flexion stops and an occipital ridge, although the condyles do have distinct ventral and posterior planes. A single hypoglossal foramen opens into the anterior aspect of the foramen magnum (figs 7–8). The condyloid canal is absent. The paracondylar process is extremely long, extending well ventral to the bulla; it is oriented vertically and slightly anteriorly (fig. 7). It is of relatively uniform thickness from the base to the tip.

The bulla is ovoid, mediolaterally compressed and, as noted previously ( Kampen, 1905), extends a good distance from the skull base with its longest dimension ventrodorsal (fig. 7). Its texture is smooth with pin-sized holes irregularly positioned on its surface. In ventral view a subtle bullar ridge that runs anteromedially to posterolaterally demarcates the ventralmost extreme of the bulla. At its inferiormost point the ridge forms a pointy bump that marks the ventral extreme of the bulla. A crest of bone is visible at the anteromedial margin of the bulla; this crest is slightly scrolled and marks the medial side of the opening for the auditory tube.

The external acoustic meatus is extremely long relative to the mediolateral dimensions of the rest of the bulla, and wraps up the side of the skull such that the porus acusticus externus is substantially superior to the middle ear region (fig. 7). Distinctions between the postglenoid and posttympanic processes of the squamosal are difficult to discern because these processes are entirely fused to the elongate external acoustic meatus. Even in subadult specimens such as AMNH-M 100670, fusion of the squamosal and the ectotympanic is already largely complete: just a trace of a suture remains between the ectotympanic and the squamosal anterior to the external acoustic meatus, and no suture is visible between the squamosal and the ectotympanic posterior to the external acoustic meatus (the squamosalexoccipital suture is clear in subadults). Using the subadult as a guide, one can estimate the positions of the sutures between the squamosal and the exoccipital and between the squamosal and the anterior aspect of the external acoustic meatus. The longest dimension of the glenoid fossa is mediolateral and the preglenoid process is just the ridgelike structure at the anterior part of the joint. The glenoid fossa is gently convex and lies in the same transverse plane as the skull base (fig. 7). The postglenoid foramen is absent. The postglenoid process is weakly developed in the adult and its identification is aided by comparison with subadult specimens. The postglenoid process is a small knob positioned at the lateral extreme of the joint.

A very long and pronounced crest extends from the bulla, and runs anterior to the stylomastoid foramen to the porus acusticus externus (fig. 7). The precise composition of the entire crest is unclear because of the obliteration of the squamosal-ecotympanic suture, but the posterior face of the crest almost certainly consists of squamosal, consistent with descriptions by Pearson (1927). This crest is the posttympanic process. The squamosal also contributes a small amount of bone to the anterior aspect of the base of the paracondylar process (as noted by Pearson, 1927).

At the anteromedial edge of the bulla there is no significant formation of a styliform process (fig. 7). Passageways into the bulla in this area, such as the anterior carotid foramen and the opening for the auditory tube, are very deeply set and completely out of view when the bulla is in situ. The carotid incisure is a large opening that is visible even when the bulla is in situ. It is fully continuous with the basicapsular fissure. Posterior to the basicapsular fissure the basioccipital widens and almost contacts the bulla. This configuration partially separates the jugular foramen from the basicapsular fissure when the bulla is in situ. When the bulla is in place the jugular foramen is oval and small and is positioned directly anterior to the hypoglossal foramen. The bulla forms the anterior border of the jugular foramen; the medial and posterior borders are formed by the fused exoccipital/ basioccipital. Lateral to the jugular foramen the paracondylar process abuts the bulla closely but does not fuse to it. This arrangement fully separates the small, round stylomastoid foramen from the jugular foramen. Anterior to the stylomastoid foramen is a sharp crest of bone on the posterior bulla, which, as noted above, is contiguous with the posttympanic process of the squamosal. There is no distinct channel for cranial nerve VII external to the stylomastoid foramen. The stylohyal fossa is deep and vertically oriented and does not contain a visible tympanohyal.

With the bulla removed, the extent of the opening between the petrosal and the surrounding alisphenoid and basisphenoid is apparent (fig. 8). The piriform fenestra, incisura ovalis, carotid foramen, middle lacerate foramen, and basicapsular fissure all form a single, very large opening. Between the basicapsular fissure and the jugular foramen, however, the basioccipital and the petrosal approximate each other forming a partial separation between these two openings (however, technically the basioccipital and the petrosal do not touch). With the bulla removed it becomes apparent that the jugular foramen is large and irregularly shaped.

As noted above, the bulla is completely sutured to the skull, and the squamosal and the exoccipital have to be sawed to expose an in situ petrosal. AMNH-M 53730 does not have the bulla preserved, so descriptions are made from AMNH-M 238329, which is a disarticulated skull.

The bulla has extensive contacts with the petrosal in an irregular pattern around the ectotympanic ring (fig. 4). The bulla does not fuse with the petrosal. In complementary fashion, looking at the petrosal ( O’Leary, 2010; fig. 4), it is on the epitympanic wing and the flanges surrounding the promontorium where the bullar contact occurs. The bulla also has a large convex process (visible with bulla in situ) that abuts the basioccipital. 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. The porus acusticus externus is irregular and jagged. Meatal foramina are not conspicuous on the inside of the external acoustic meatus but may be very minute. The tympanic annulus is a distinct semicircle for approximately 180° with no clear demarcation between anterior and posterior crura. Jutting from the annulus are several regularly spaced struts of bone that are contiguous with the cancellous internal structure of the bulla. The bulla is cancellous throughout and is made up of bone that is so thick that when viewed externally the cancellous bone beneath is obscured. Extending toward the nasopharynx is a pronounced channel that forms the bullar contribution to the auditory tube. A sharp ridge delineates it. The anterior carotid foramen is not very distinctive and may have been contiguous with this or closely adjacent to it. The bullar contributions to the posterior carotid foramen and the jugular foramen are marked by a pair of subtle grooves (figs. 4, 7).

Figures

FIGURE 4. Left auditory bullae of Camelus dromedarius (Camelidae; SBU-MAR 31), Pecari tajacu (Suina; UCM- VZ 1247), and Potamochoerus porcus (Suina; AMNH-M 238329, reversed from right side) shown in dorsomedial view. Scale bar = 1 cm. Artists: L. Betti-Nash (Camelus), C. Lodge (Pecari); photograph: S. Goldberg.

FIGURE 7. Basicranium of Potamochoerus porcus (Suina; AMNH-M 238330) with bulla in situ. Illustration is left side (image reversed from right side). Scale bar = 1 cm.

FIGURE 8. Left basicranium of Potamochoerus porcus (Suina; AMNH-M 53703) with bulla removed to reveal petrosal in situ. Photograph: S. Goldberg. Scale bar = 1 cm.