Pecari tajacu (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-

Pecari tajacu and Tayassu pecari

Figures 4–6

Pearson (1927) described and illustrated key features of the ear regions of living and extinct Tayassuidae , particularly anatomical variation in proximity to the articulation of the mandible. Kampen (1905) provided details on the bulla, and Edinger and Kitts (1954) provided an illustration of tayassuid basicranial openings in the ear region. Descriptions and phylogenetic analysis of the petrosal in Tayassu and extinct relatives, including the relationship of the petrosal to surrounding cranial bones, have also been published by Orliac (2013).

The bulla is very tightly fused to surrounding bones in these species (e.g., adult specimen SBU- MAR 8), particularly the external acoustic meatus. In UCM-VZ 1247, a specimen of Pecari tajacu , the bulla was dissected off to expose the petrosal in situ. The larger species, Tayassu pecari , is represented by SBU-MAR 8. Both specimens are adults based on dental eruption, but UCM-VZ 1247 has a larger number of intact cranial sutures suggesting that it may have been ontogenetically younger than SBU-MAR 8 at death.

The optic foramen is distinctly circular. It is superomedial to the sphenorbital fissure and smaller than it. The sphenorbital fissure is oval in Tayassu pecari and more circular/irregular in Pecari tajacu ; in each case the long axis of the fissure trends superomedial to inferolateral. No distinct anterior opening of the pterygoid canal is visible at the base of the sphenorbital fissure.

Both figures 5 and 6 show the posterior aspect of the basipharyngeal canal. The vomer terminates anterior to the posterior opening of the basipharyngeal canal, anterior to the suture between the presphenoid and the basisphenoid. The presphenoid forms the central and posterior part of the roof of the canal and terminates with a straight suture against the basisphenoid. The two pterygoids are visible on either side of the presphenoid and taper toward each other anteriorly, where they extend onto the bony palate. There is no conspicuous posterior opening for the pterygoid canal, and no conspicuous sulcus running anteriorly from the middle ear toward the pterygoid region. The hamulus is a small crest that flares laterally and is more pronounced in Tayassu pecari (fig. 5). The pterygoid fossa is shallow and flat with the ectopterygoid plate forming a particularly sharp posterior ridge.

The midline of the skull lacks a craniopharyngeal foramen. The suture between the basioccipital and the basisphenoid is preserved in Pecari tajacu but obliterated in Tayassu pecari (figs. 5, 6). The tympanic processes are irregular bumps on the lateral edge of the basioccipital. They do not contact the bulla and are separated from each other by a midline crest. The occipital condyles are widely separated at their anterior margins and lack flexion stops. The condyles are divided into posterior and inferior surfaces that meet at a right angle. There is only a slight hint of an occipital ridge. Both specimens have hypoglossal foramina posteromedial to the jugular foramen that are distinctive in having one conspicuous external opening and a second smaller, accessory opening posterior to the hypoglossal foramen (fig. 6). These double foramina are inset into a larger, round depression and both open into the foramen magnum.

The bulla is bulbous and comes to a small styliform process at its anteromedial edge (fig. 5). As previously noted elsewhere ( Kampen, 1905), the bulla does not protrude to a significant distance ventrally inferior to the glenoid fossa. The external surface of the bulla is smooth. The bone hyal is present, it is an extremely delicate and is thin walled and nearly transparent exposing relatively small structure deeply recessed in the underlying trabecular structure throughout (figs. small, circular stylohyal fossa. In the subadult 4, 5). The bulla does not have a particularly pro- specimen of T. pecari , AMNH-M 133513, an nounced bullar ridge marking the transition independent tympanohyal is not apparent. The from its medial to lateral side. No tympanohyal stylohyal fossa is completely enclosed within the is visible with the bulla in situ. If the tympano- posterior bullar wall and it is marked laterally by a small crest from the bulla (fig. 5). The fossa extends anteriorly as a shallow groove on the bulla, dividing it into a larger lateral and smaller medial convexity. The stylomastoid foramen is lateral and slightly posterior to the stylohyal fossa. The exoccipital forms the posterior border of both the stylomastoid foramen and the medially adjacent jugular foramen.

The external acoustic meatus is a very long tube that is completely fused to the squamosal and exoccipital such that it is not apparent as a distinct structure in ventral view (fig. 5). There is a distinct suture posterior to the postglenoid process; in the adult it is not possible to determine whether this suture represents the complete fusion of the postglenoid and posttympanic processes of the squamosal around an the elongate external acoustic meatus, or whether some part of the external acoustic meatus remains exposed after fusion. Pearson (1927: fig. 1) described a “post-tympanic process” in Pecari tajacu and labeled it as the platelike portion of the squamosal located lateral and superior to the paracondylar process (= paroccipital process in Pearson’s [1927: fig. 1] terminology). In mature specimens, however, this area does not conform to a shape that could be characterized as a process or projection that protrudes ventrally and is distinct from the ectotympanic, as is typical of a posttympanic processes (see examples illustrated across Mammalia in O’Leary et al., 2013: char. 555). The region in Peccari and Tayassu is instead a flat plate of bone that is highly fused. The mastoid region of the petrosal is also not visible on the external surface of the skull. The porus acusticus externus is out of view ventrally because it is situated posterior and distinctly superior to the bulla (fig. 5). Looking into the porus, a conspicuous circular scroll of bone can be seen. This scroll probably represents part of the ectotympanic, but the boundaries are difficult to discern due to its extensive fusion to the squamosal.

A very small paracondylar process projects from the exoccipital (figs. 5, 6). This process extends only a few centimeters inferior to the bulla itself in Tayassu pecari . In lateral view the paracondylar process is concave anteriorly and convex posteriorly. It has a slight flair at its ventral tip in Tayassu pecari but is a simple point that faces posteriorly in Pecari tajacu . At the base of the paracondylar process it is close to but not in contact with the auditory bulla.

With the exception of the anteromedial edge, the bulla is tightly appressed to the surrounding basioccipital, exoccipital, and squamosal. The carotid incisure/foramen and the incisura ovalis are, however, visible even with the bulla in situ (fig. 5). The anterior carotid foramen is a very deeply set structure, the entry to which is marked by a groove on the anterior bulla. When the bulla is in situ it is difficult to more than approximate the position of the posterior carotid foramen because it is so deeply set and it leaves no conspicuous marks on the bulla. With the bulla in situ, the jugular foramen is distinguished by a notch in the exoccipital (fig. 5). The basicapsular fissure is, however, not visible with the bulla in situ, because although the bulla is not fused to the midline, it is tightly appressed to the basisphenoid/basioccipital (fig. 5).

With the bulla removed it is apparent that the petrosal is held in place only on its lateral and posterolateral edges (fig. 6). It is not fused in place but wedged between the exoccipital and the squamosal where the small mastoid region is lodged. Anteriorly, the petrosal is not interdigitated into the surrounding bones. Thus, once the bulla is removed, the petrosal is also relatively easily removed, or, alternatively, the petrosal can be easily removed from the endocranial side of a bisected specimen with the bulla still in situ.

Figure 6 reveals the large, contiguous opening that separates the petrosal from the alisphenoid, basisphenoid, basioccipital, and, with the exception of a small strut of bone, also the exoccipital. The incisura ovalis is separated from the carotid incisure by a sharp prong of bone that does not reach the petrosal posteriorly. Laterally the incisura ovalis is fully open to the piriform fenestra. Adjacent to the piriform fenestra on the lateral side is a deep blind recess for the bulla. This recess contains an accessory foramen, not visible with the bulla in situ, which opens into the endocranium. Posteromedial to the carotid incisure are the middle lacerate foramen and the basicapsular fissure. Each of these is a narrower opening than the carotid foramen/incisure. The basicapsular fissure is only partially separated from the jugular foramen by a small strut of bone from the exoccipital. This strut approximates but does not fully contact the petrosal. With the bulla removed it is apparent that the jugular foramen is more than double the size of what is visible when the bulla is in situ.

The glenoid fossa lies entirely on the zygomatic arch, primarily on the squamosal, and is aligned roughly with the ventral margin of the bulla (figs. 5, 6). A distinct, ridge-shaped preglenoid process is formed primarily by the jugal as it meets the squamosal. The glenoid fossa is concave with an oval outline with the long axis oriented from anterolateral to posteromedial. The postglenoid process is a narrow strut of bone that projects ventrally from the medial extreme of the joint to about the same distance inferiorly as does the preglenoid process. There is a single, small postglenoid foramen that is deeply set and lies directly posterior to the postglenoid process. The postglenoid foramen is visible whether the bulla is in situ or not. In Tayassu pecari this foramen is round and in Pecari tajacu it is more elongate and slitlike.

When removed and examined from its dorsomedial surface, the bulla has a relatively long, smooth external acoustic meatus that extends from the ectotympanic annulus (fig. 4). Presumably what is exposed internally is largely ectotympanic but, again, caution should be observed because the bones are highly fused to the squamosal and exoccipital. Several meatal foramina occur at the medial end of the external acoustic meatus. The ectotympanic 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 more irregular cancellous structure otherwise present throughout the internal aspect of the bulla. As noted above, the auditory tube is marked by a distinct crest. The bulla had extensive contact with the petrosal and the squamosal (fig. 4).

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 5. Left basicranium of Tayassu pecari (Suina; SBU-MAR 8) with bulla in situ. Scale bar = 1 cm. Artist: U. Kikutani.

FIGURE 6. Left basicranium of Pecari tajacu (Suina; UCM-VZ 1247) with bulla removed to reveal petrosal in situ. Scale bar = 1 cm. Artist: C. Lodge.