Choeropsis liberiensis (Morton, 1849)

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-

Choeropsis liberiensis

Figures 18–20

Aspects of the ear region of Choeropsis liberiensis have been described by several authors ( Leidy, 1853; Pearson, 1927; 1929; Orliac et al., 2014) with Orliac et al. (2014) providing CTbased reconstructions of cranial vasculature in this region.

The bulla of Choeropsis liberiensis is very tightly fused to the skull particularly in the region of the external acoustic meatus (fig. 19). No specimen was available with the bulla removed and the petrosal in situ (a specimen with the petrosal completely removed is described in O’Leary, 2010). Nonetheless, a number of basicranial features can be seen in the adult specimen AMNH-M 81899, which is described below, supplemented by information from AMNH-M 148452 and from the dissected bulla of specimen UCR 3116.

The posterior orbit and palate are visible in figure 19. The sphenorbital fissure is relatively round and is set posterior and slightly lateral to the much smaller optic foramen. There are several very small openings anterior to the sphenorbital fissure, one of which may be the anterior opening of the pterygoid canal, but it is too small to verify continuity with the posterior opening of the pterygoid canal using a typical probe (more on that below). The postorbital process of the frontal juts toward the orbit but does not meet the jugal.

The posterior aspect of the palate has paired posterior spines of irregular shape and size, each terminating in a small knob (fig. 19). The plane of the palate is very flat with a roughening in the midline at the suture between the two palatine bones. AMNH-M 81899 has some asymmetry of the major palatine foramina; there is one on each side at the anterior aspect of the suture between the palatine and the maxilla and a third, on the right side only, that is more posterior and exclusively within the palatine (this last one is visible in fig. 19, which is the right side [drawing reversed]).

Visible in the roof of the basipharyngeal canal is the posterior end of the vomer, as are faint indications of a suture between the vomer and the pterygoids (fig. 19). The presphenoid is either not exposed or its suture with the basisphenoid is obliterated. The sutures between the pterygoids and the alisphenoids in the lateral walls of the basipharyngeal canals are almost entirely obliterated, although at the posterior edge of the pterygoid, the separation between pterygoid and the alisphenoid is clear. In the roof of the basipharyngeal canal almost halfway between the midline and the lateral walls are distinct posterior openings for the pterygoid canals. Although unambiguous, these openings are so small that their continuity with an anterior opening cannot be easily verified with a probe. Running posteriorly from them to the middle ear region is a distinct and elongate sulcus (fig. 19). There is little development of a pterygoid fossa, with the ento- and ectopterygoid processes being clearly distinct only at their ventral ends. The entopterygoid process terminates in a roughened ridge that runs anteroposteriorly. The hamulus (partially broken in fig. 19; observations based on AMNH- M 148452) extends ventrally well below the entopterygoid process. It is oriented in a parasagittal plane and forms a hook posteriorly and terminates in a knob.

In the midline a craniopharyngeal canal is absent and the suture between the basisphenoid and the basioccipital is obliterated (fig. 19). The basioccipital and basisphenoid bones have an overall sculpted appearance. The tympanic processes are distinct round knobs that lie just posterior to bilateral oval depressions with distinct outlines at their anterior edges. The occipital condyles are widely separated and lack both flexion stops and a distinct occipital ridge, although the condyles do have both a ventral and a posterior surface. There is one large hypoglossal foramen just anterolateral to the anterior aspect of the occipital condyle. It opens into the foramen magnum; the condyloid canal is absent. The paracondylar process is a small, straight structure that is of relatively uniform size from its base to its ventral tip. It extends ventral to the bulla and, in lateral view, angles posteriorly. At its base superiorly, the paracondylar process contacts the bulla via a small, sharp crest.

As noted above, the auditory bulla is held in place extremely tightly, primarily by full fusion of the elongate external acoustic meatus to the postglenoid and posttympanic processes of the squamosal (fig. 19). The bulla is ovoid and oriented with its long axis anteromedial to posterolateral. Medially the bulla contacts the overlying petrosal extensively but not the alisphenoid, basisphenoid, or basioccipital (fig. 18). The bulla itself has a number of pinholesized openings (the largest of which is at the anterolateral aspect of the bulla and is out of view in the illustrations). It also has some areas of jagged-textured bone posteriorly. The external acoustic meatus is as wide as the bulla mediolaterally; the meatus wraps onto the lateral side of the skull such that the porus acusticus externus is substantially more superior than is the middle ear. The external acoustic meatus has mediolaterally elongate crests of bone on its ventral surface. The posttympanic process of the squamosal is not only fused to the external acoustic meatus, but also to the exoccipital. The styliform process of the bulla is a small pointed process (figs. 18, 19). Following it superiorly onto the medial side of the bulla, a channel emerges on the dorsomedial aspect of the bulla that marks the opening for the auditory tube (fig. 18). This channel is marked by a strong crest. Running posteriorly from the styliform process on the external (ventral) surface of the bulla is a gentle bullar ridge that follows the long axis of the bulla. The ridge terminates in one of the crests, noted above, that runs along the ventral surface of the external acoustic meatus. The crest lies anterior to the stylomastoid foramen. Sutures indicate that the anterior two-thirds of the edge of the stylomastoid foramen are formed by the bulla and its posterior one-third by the exoccipital. There is no sulcus external to the stylomastoid foramen. The stylohyal fossa is posteromedial to the stylomastoid foramen, and is a gentle depression in the posterior bulla. The stylohyal fossa is bordered posteriorly by the exoccipital, and the crest of bone from the paracondylar process (noted above) marks the medial border of the fossa. On the right side of the specimen (the left is slightly broken in this area) there is a small prong of bone deep within the stylohyal fossa (not visible in illustrations). This structure appears to be a very small, recessed tympanohyal and it rests against the exoccipital. There are no particular grooves or channels marking the posterior carotid foramen whether the bulla is in situ or removed. The anterior carotid foramen appears to be marked only by a slight indentation in the bulla and a small crest adjacent to it.

Anteromedially, medially, and posteromedially there is a wide and contiguous space between the petrosal-bulla complex and the surrounding bones (fig. 19). The piriform fenestra is a wide area that blends with the incisura ovalis such that there is little distinction between them (fig. 20). Similarly the carotid incisure/foramen is distinguished only by a small, medially oriented notch in the basisphenoid. This notch is fully open to the middle lacerate foramen and the basicapsular fissure. There is a slight approximation of the basioccipital/exoccipital and the petrosal/bulla complex between the basicapsular fissure and the jugular foramen, but these openings are still confluent. The jugular foramen is a large anteromedially to posterolaterally oriented opening that is fully visible with the bulla in place (figs. 19, 20).

When the bulla is viewed in isolation (fig. 18) it is clear that the ecotympanic annulus is one continuous structure with indistinct divisions between the two crura. The internal aspect of the bulla is cancellous and the bulla has extensive contact with the petrosal. The auditory tube is marked by a distinct trough, but the anterior and posterior carotid foramina leave only indistinct marks on the bulla.

The glenoid fossa has an elongate, oval outline and is gently concave along its orientation from anterolateral to posteromedial (fig. 19). There is no preglenoid process. The postglenoid process is a knob that lies at the posteromedial extreme of the joint only. There is no medial glenoid pit. A distinct postglenoid foramen is absent, as has been noted in much greater detail using micro CT scan–based reconstructions by Orliac et al. (2014). Those authors observed that while an opening remains patent, often through multiple minute passageways, there is no “fully individualized” postglenoid foramen in Choeropsis (Orliac et al., 2014: 2) . Such detail cannot be seen in the illustrations here, but in the specimens themselves there are small slits between the external acoustic meatus and the postglenoid process that most likely led to the passageways to the endocranium identified by Orliac et al. (2014). These slits are too small to penetrate with standard probes.

Figures

FIGURE 18. Left auditory bullae of Tragulus javanicus (Ruminantiamorpha, Ruminantia, AMNH-M 102091), Choeropsis liberiensis (Cetancodonta, Hippopotamidae, UCR 3116) and Hippopotamus amphibius (Cetancodonta, Hippopotamidae, AMNH 24289). Scale bar = 1 cm. Artists: U. Kikutani (Tragulus), C. Lodge (Choeropsis and Hippopotamus).

FIGURE 19. Left basicranium of Choeropsis liberiensis (Cetancodonta, Hippopotamidae, AMNH-M 81899) with auditory bulla in situ. Right side of specimen reversed. Scale bar = 1 cm. Artist: U. Kikutani.

FIGURE 20. Left basicranium of Choeropsis liberiensis (Cetancodonta, Hippopotamidae, AMNH-M 81899), angled close-up to show space between petrosal and surrounding bones. Scale bar = 1 cm.