Camelus dromedarius, 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-

Camelus dromedarius

Figures 2–4

The camelid skull has been described in the context of the gross anatomy of the species in a landmark study of the dromedary ( Smuts and Bezuidenhout, 1987). Lama guanicoe has also been illustrated and described with several basic landmarks labeled ( Starck, 1995), and variability in cranial openings has also been briefly discussed by Edinger and Kitts (1954). Descriptions here are based on two adult specimens: SBU- MAR 31, which was illustrated after the removal of the auditory bulla (the petrosal in isolation from the same specimen, opposite side, was described by O’Leary, 2010), and AMNH-M 14111, which has the auditory bulla in situ. Descriptions of those two adult specimens were supplemented by information from subadult AMNH-M 6368.

The bones of the intact basicranium of C. dromedarius are highly fused; the auditory bulla and external acoustic meatus are tightly adhered (often without visible sutures) to the exoccipital, basioccipital, basisphenoid, alisphenoid, and squamosal (fig. 2). As noted above, the auditory bulla had to be sawed off SBU-MAR 31 to reveal the underlying petrosal in situ, the promontorium of which does not make direct contact with the underlying bulla.

Orbit and palate structures are not shown in the illustrations, nor are they preserved in the dissected specimen, however, some observations based on specimen AMNH-M 14111 are noted here. The relatively round sphenorbital fissure is much larger than, and is set posterior and ventral to, the optic foramen. The optic foramen is circular and has a prong of bone on its lateral bor- der that extends several centimeters anteriorly. At the base of the sphenorbital fissure is a slit that is the anterior opening of the pterygoid canal. The pterygoid canal is short and straight such that a probe in this foramen emerges several centimeters through the posterior opening of the pterygoid canal at the posterior edge of the entopterygoid process in the roof of the basipharyngeal canal. In the specimen illustrated here, a long sulcus connects the posterior opening of the pterygoid canal to the anterior aspect of the middle ear region (figs. 2, 3).

Based on AMNH-M 14111, sutures on the palate are largely fused, making it possible to see only a weak indication of the suture where the pterygoid met the alisphenoid. In the roof of the basipharyngeal canal a large part of the vomer has extensive exposure posterior to the palate, obscuring the presphenoid-basisphenoid con- tact. The ento- and ectopterygoid processes are flanges that extend posteriorly and slightly laterally. The entopterygoid process with its hamulus has a much longer anteroposterior extent than does the ectopterygoid process and it is concave posteriorly. The pterygoid fossa is small and confined to the ventral one-third of the total height of the posterior wall of the basipharyngeal canal. The foramen ovale is a separate hole anterior to the middle ear region and is not confluent with any other adjacent skull-base openings (figs. 2, 3). It is situated entirely within the alisphenoid, but its lateral border lies very close to the suture with the squamosal. On its lateral side, the foramen ovale has a sharp crest that protrudes several millimeters ventrally and comes to a point anteriorly (fig. 3).

In the skull midline a craniopharyngeal canal is absent in the adults but present in the juvenile specimen. Only a hint of the suture between the basisphenoid and the basioccipital is visible in AMNH-M 14111, and it lies just anterior to the tympanic processes, which are situated entirely on the basioccipital (fig. 2). The tympanic processes are subtle structures represented only by roughened areas of bone set between the midline of the skull and the lateral edge of the basioccipital. There is no median crest in the midline of the basioccipital. The medial edges of the occipital condyles almost meet in the midline and there are flexion stops, each marked by a sharp ridge anteriorly, on the anterior aspects of the occipital condyles. The occipital condyles have a distinct occipital ridge dividing the ventral from the posterior surface of the condyle. The hypoglossal foramina lie in deep pits anterolateral to the flexion stops (figs. 2, 3). Bilateral accessory hypoglossal foramina lie anterior to the main hypoglossal foramina (fig. 2): in AMNH-M 14111 the accessory foramen opens into the foramen magnum; in SBU-MAR 13 it opens into the basicapsular fissure. The condyloid canal is absent upon inspection of the external aspect of the skulls, although I note that Smuts and Bezhuidenhout (1987) mention the presence of one in the roof of the hypoglossal canal. The paracondylar process of the exoccipital is a curved ledge of bone that is concave posteromedially; its lateral margin is relatively straight. The paracondylar process has a roughened texture and extends to approximately the same distance ventrally as the bulla.

As noted above the auditory bulla is highly fused to surrounding bones. It consists of a tubelike external acoustic meatus that is approximately twice as long as the bulla is wide mediolaterally (figs. 2, 4). The external acoustic meatus has a rough surface ventrally. The porus acusticus externus is a small, circular, and laterally facing opening; it lies approximately at the same level as the middle ear (not significantly superior to it). The bulla has a roughened and complex surface, and in some specimens is pocked with many pinsized foramina (fig. 2). The overall shape of the bulla is one of a very compressed ovoid with the long axis running anteromedial to posterolateral. A bullar ridge, an extensive crest running along the ventral surface of the auditory bulla from anteromedial to posterolateral, marks this axis (fig. 2). The bullar ridge lies anterior to the stylohyal fossa and runs along the ventral surface of the external acoustic meatus. Running posteromedial to the stylohyal fossa, the bulla also has a separate prong of bone that fuses with the paracondylar process of the exoccipital (fig. 2). Two accessory foramina lie where the bulla meets the paracondylar process (fig. 2). Anteriorly the bulla has a very small styliform process that does not extend as far ventrally as other aspects of the bulla, but does mark the position of the bony opening of the auditory tube.

With the bulla in situ it is difficult to view the piriform fenestra and the carotid foramen (discussed below). The anterior carotid foramen is set deeply on the anterior aspect of the bulla and lies adjacent to a sharp crest. Medially the bulla is in contact with, and partially fused to, the basioccipital, excavating it by way of a protrusion from the bulla (figs. 2–4). This configuration obscures a view of the basicapsular fissure and middle lacerate foramen while the bulla is in situ (fig. 2). This bullar contact with the basioccipital is a bulbous medial protrusion from the bulla (fig. 4) that leaves a smooth excavation in the basioccipital (fig. 3). Posterior to where the bulla abuts the basioccipital is the small and round posterior carotid foramen (called the fissura petrotympanooccipitalis in Smuts and Bezuidenhout, 1987: fig. 6). The posterior carotid foramen is partially separated from the larger jugular foramen by a crest of bone that juts posteromedially from the bulla. This crest does not fuse to midline bones. The large and round jugular foramen is clearly visible with the auditory bulla in situ (fig. 2). Smuts and Bezuidenhout (1987) described the jugular foramen as divided into medial and lateral parts. In AMNH-M 14111 there is essentially one opening, but it is partly divided by a small prong of bone that juts anteriorly from the exoccipital. The jugular foramen is very deep walled. It is not completely enclosed by the bulla as its posteromedial edge is formed by the exoccipital (fig. 2). Laterally, the tympanohyal is set deeply within the stylohyal fossa where it is tightly fused to the bulla anteriorly and the exoccipital posteriorly. The morphology of the subadult specimen suggests that the tympanic bulla encircles the tympanohyal completely. The tympanohyal has a flattened and broad ventral tip. Lateral to the tympanohyal is the stylomastoid foramen, a small round opening that is bordered by the exoccipital, the tympanic bulla, and the mastoid region of the petrosal, all of which are fused (fig. 2). There are no conspicuous grooves emanating from this foramen.

The bulla in isolation reveals a long and smooth internal surface to the external acoustic meatus, which makes an almost 90° bend along its course between the ectotympanic ring and the porus acusticus externus (fig. 4). With the bulla removed it is apparent that the anterior and medial parts of the pars cochlearis of the petrosal are very close to, but actually free of, contact with the surrounding alisphenoid, basisphenoid, and basioccipital bones (fig. 3). The petrosal is firmly held in place by sutures on its lateral and posterolateral sides. The petrosal overrides these other bony elements superiorly with several struts of bone extending across the basicapsular fissure without contacting midline structures. The mastoid region of the petrosal is somewhat obscured by fusion of the petrosal to the squamosal and exoccipital but is visible in ventral view. Figure 3 shows the elongate and straight squamosal contribution to the external acoustic meatus.

With the bulla removed (fig. 3) it is apparent that the piriform fenestra is an open, oval-shaped space that lies lateral to the pointed anterior extreme of the petrosal. The piriform fenestra is fully continuous with the carotid incisure/foramen, the latter being a larger, essentially oval opening close in size to that of the foramen ovale. The opening forms almost a complete circle by the approximation of the petrosal to the fused basisphenoid-alisphenoid, however, technically the pars cochlearis of the petrosal is not in contact with the midline bones (basisphenoid, basioccipital) and the carotid foramen is not closed. The basicapsular fissure is an irregular and elongate opening that connects the carotid incisure to the jugular foramen via a middle lacerate foramen winding between prongs of bone jutting from the midline and from the petrosal. The full extent of the jugular foramen does not differ greatly from that which was visible with the bulla in place. Forming the anteromedial border of the jugular foramen is a large and pointed rostral tympanic process that protrudes inferiorly from the petrosal. This process is most clearly visible in an isolated petrosal ( O’Leary, 2010: fig: 33).

The long axis of the glenoid fossa is oriented mediolaterally; the medial glenoid pit is absent (fig. 2). The fossa is relatively flat overall and oriented such that its posterior aspect is positioned more superiorly. The posterior aspect of the fossa is more concave directly anterior to the postglenoid process. The fossa has a small, inferiorly directed flange of bone at its most lateral extreme (out of view in the illustrations). The postglenoid process extends from the lateral edge to approximately the midpoint of the joint. It is a large lip of bone that is appressed to the anterior surface of the external acoustic meatus. The postglenoid foramen has openings on both the medial and lateral sides of the postglenoid process (figs. 2, 3). That these two postglenoid foramina are contiguous becomes apparent when the postglenoid process is removed, and it is the medial opening of the postglenoid foramen that leads into the endocranium. A blind recess lies lateral and adjacent to the medial opening of the postglenoid foramen (fig. 3). The posttympanic process of the squamosal is highly fused to the external acoustic meatus and does not extend much inferior to the external acoustic meatus, and certainly does not contact the postglenoid process. Due to its short extent, the posttympanic process is out of view in the illustrations.

Figures

FIGURE 2. Left basicranium of Camelus dromedarius (Camelidae; AMNH 14111) with auditory bulla intact. Illustration was done prior to removal of bulla for illustration in figure 3. Scale bar = 1 cm. Artist: C. Lodge.

FIGURE 3. Left basicranium of Camelus dromedarius (Camelidae; SBU-MAR 31) with auditory bulla removed to reveal petrosal in situ. Scale bar = 1 cm. Artist: C. Lodge.

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.