Odocoileus virginianus (Zimmermann, 1780)

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-

Odocoileus virginianus

Figures 11 , 14 , 15

An illustration of the basicranial aspect of the ear region of Odocoileus is found in Edinger and Kitts (1954) with a general discussion of variability in the foramen ovale.

The bulla is relatively loosely attached to the skull in this taxon, essentially held in place by its enclosure of the tympanohyal via the stylohyal fossa (fig. 14). If the tympanohyal is broken, the bulla easily separates from the skull. Figure 15 is an adult skull (SBU-MAR 20) with the bulla and the tympanohyal removed to show the petrosal in situ. The skull preserves sutures very clearly. AMNH-M 244789 is an adult specimen and demonstrates the skull with the bulla in situ.

Both figures 14 and 15 show several features of the orbit; the optic canal is generally round but reaches a point at its inferior aspect. The optic foramen lies anterior and slightly superior to the sphenorbital fissure. The large sphenorbital fissure is teardrop shaped with the apex positioned superolaterally at the junction of the parietal, alisphenoid, and basisphenoid. At the base of the sphenorbital fissure is the very small anterior opening of the pterygoid canal. Directly posterior to the sphenorbital fissure is the foramen ovale, which is contained entirely within the alisphenoid, however, the lateral margin of the foramen ovale very closely approximates the alisphenoid-squamosal suture. The suture between the alisphenoid and the basisphenoid is obliterated.

The posterior aspect of the hard palate is flat (fig. 14). The posterior spine is absent on the palate and the medial edges of the palatine bones do not extend as far posteriorly as do the lateral edges. The vomer is exposed in the basipharygneal canal, extending well past the posterior edge of the palatine bones and almost as far posteriorly as the posterior edge of the entopterygoid process. The posterior aspect of the vomer terminates as a small, pointed projection of bone and this extends posterior to the presphenoid-basisphenoid suture (fig. 14). This projection is only partially in contact with the roof of the basipharyngeal canal. The presphenoid has a small exposure in the superior aspect of the basipharyngeal canal. The margins of the pterygoid versus the palatine and alisphenoid bones are very clear. The entopterygoid process forms a small hamulus with a blunt end, and the hamulus is essentially level with the palate (fig. 14). There is almost no development of a pterygoid fossa and little development of an ectopterygoid process. The pterygoid canal appears to have two posterior openings (one anterior to the other) that unite to open together at the anterior opening of the pterygoid canal in the orbit (fig. 14). The more anterior of these passes directly into the orbit, and the more posterior travels along the superior part of the pterygoid toward the orbit. There is no craniopharyngeal canal.

In the midline, there is a small basipharyngeal canal (fig. 15). The tympanic processes are circular, roughened areas that do not project significantly from the skull base (fig. 14). These processes have no contact with the auditory bulla. Flexion stops at the anterior aspect of the occipital condyles are very pronounced and deeply excavated (fig. 15). The flexion stops abut the tympanic processes posteriorly. The occipital condyles approximate each other at the anterior midline but do not touch. They have distinct ventral and posterior surfaces that meet at a right angle forming an occipital ridge. The paracondylar process is a triangular plate in lateral view and is widest superiorly at its base where it contacts the auditory bulla. The apex of the process extends as far ventrally as the alveolar line of the upper dentition. The process hooks slightly medially from superior to inferior (particularly clear when seen in posterior view). The paracondylar process terminates in a small knob (fig. 14). These processes lie adjacent to the mastoid region of the petrosal, which is exposed with clear sutures between it and the exoccipital and squamosal.

There is both a hypoglossal foramen and a condyloid canal (fig. 15). Both are positioned superior to the occipital condyles and are partially hidden by them. The hypoglossal foramen is positioned relatively anterior and medial; it is a direct opening to the superior aspect of the foramen magnum. The condyloid canal is adjacent to the hypoglossal canal and runs more extensively within the exoccipital before opening endocranially. The condyloid canals are variable and tend to be larger externally than internally and open in a nonbilaterally symmetrical fashion on the deep surface of the condyle.

Several foramina and channels are visible with the bulla in situ (fig. 14). Starting anteriorly, medial to the foramen ovale (described above), there is a distinct sulcus running near the junction of the basisphenoid and the alisphenoid. A sharp crest on the alisphenoid marks the sulcus laterally, and the sulcus runs from the middle ear toward the ptergygoid fossa, eventually leading to the posterior opening of the pterygoid canal. The sulcus is most pronounced between the middle ear and the pterygoid fossa, becoming much more faint as it approaches the posterior opening of the pterygoid canal.

The bulla is flask shaped with a spherical medial portion (figs. 11, 14). A tube-shaped external acoustic meatus projects posteriorly and terminates at the lateral edge of the skull; there is a spike-shaped styliform process. The styliform process extends only as far ventrally as the bulla or a little beyond. The styliform process is flattened anteriorly and concave on its anteromedial surface. The bulla has a number of irregularly positioned pin-sized holes. The only other distinctive feature of the bulla is a knob projecting from its center at the most ventral part of the bulla. A ridge extends posterolaterally from this knob and marks the lateral edge of the stylohyal fossa (fig. 14). The bulla surrounds the tympanohyal only partially, such that the tympanohyal is partly exposed posterolaterally. The tympanohyal has a flattened inferior tip and does not project ventral to the bulla. The stylomastoid notch lies directly posterolateral to the tympanohyal and there is no pronounced sulcus for the facial nerve on the external aspect of the stylomastoid foramen.

The anterior carotid foramen is not visible with the bulla in situ because the foramen is positioned very deeply between the petrosal and bulla. The posterior carotid foramen is distinguished only by a slight indentation in the bulla near the jugular foramen and is also essentially out of view with the bulla in situ (figs 11, 14). However, when the bulla is in situ, the carotid foramen, middle lacerate foramen, basicapsular fissure, and jugular foramen are all visible and contiguous (fig. 14). The petrosal and bulla as a unit are not appressed closely to midline skull structures. The petrosal is also partially exposed even with the bulla in place, and specifically, its pars cochlearis lacks extensive contact with the surrounding alisphenoid, basisphenoid, basioccipital, and exoccipital. It is the pars canalicularis of the petrosal, particularly the mastoid region, that is wedged firmly between the exoccipital and squamosal (sutures remain visible). The contact between the pars canalicularis of the petrosal and the squamosal also contributes to holding the petrosal in place.

With the bulla removed it is apparent that the piriform fenestra is bordered by the alisphenoid and squamosal anteriorly and by the petrosal posteriorly (fig. 15). This fenestra is elongate, extending from the basisphenoid medially to the squamosal laterally, and is divided in two by a posteriorly directed triangular process from the alisphenoid. The carotid foramen lies at the anteromedial corner of the petrosal between the lateral margin of the basisphenoid/basioccipital and the jagged anterior edges of the flanges of the pars cochlearis of the petrosal. There is a very narrow middle lacerate foramen posterior to the carotid foramen and contiguous with it. The basicapsular fissure is an elongate, broad oval. A projection from the lateral margin of the basioccipital/exoccipital (the suture is obliterated) meets the posteromedial flange of the petrosal (the two structures just barely touch) and separates the basicapsular fissure from the jugular foramen in some specimens (SBU-MAR 20), but the entire area is much more open in others (AMNH-M 244789). The jugular foramen is round with an additional small, slitlike continuation at its lateral edge that extends posterior to the fenestra cochleae of the petrosal (fig. 15).

When the bulla is removed it is apparent that the external acoustic meatus forms a complete tube from the ectotympanic alone (fig. 11). The bone of the tube is roughened in places, but the tube does not fuse to the squamosal against which it lies. The part of the bulla adjacent to the ectotympanic annulus is marked by numerous small foramina. The bulla itself is completely hollow and devoid of bony struts or cancellous structures. The canal for the auditory tube is relatively broad with roughened edges and terminates in the styliform process (which varies in size between the two specimens examined; fig. 11 shows a relatively small styloid process). Canals that mark the places where the anterior and posterior carotid foramina would be formed as the bulla met the petrosal are very subtle. The position of the posterior carotid foramen in SBU-MAR 20 is marked by an irregular small hook of bone .

The glenoid fossa is convex anteriorly (the preglenoid process) and concave posteriorly (fig. 15). The postglenoid process is positioned predominantly toward the medial aspect of the joint in SBU-MAR 20 and runs posterolateral to anteromedial. There is a medial glenoid pit at the medial aspect of the glenoid fossa. When the skull is articulated with the mandible no part of the mandibular condyle contacts this pit. The postglenoid foramen is large and partly obscured by the bulla and the postglenoid process.

CETRUMINANTIA – RUMINANTIA – TRAGULIDAE

Figures

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).

FIGURE 14. Left basicranium of Odocoileus virginianus (Cetruminantia, Ruminantia, Cervidae; AMNH-M 244789) with auditory bulla in situ. Scale bar = 1 cm. Photograph: S. Goldberg.

FIGURE 15. Left basicranium of Odocoileus virginianus (Cetruminantia, Ruminantia, Cervidae; SBU-MAR 20) with auditory bulla removed to reveal petrosal in situ. Scale bar = 1 cm. Artist: U. Kikutani.