Hippopotamus amphibius, 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-

Hippopotamus amphibius

Figures 18 , 21 , 22

The ear region of Hippopotamus and its closely related fossil relatives has been described by several authors ( Leidy, 1853; Kampen, 1905; Pear- son, 1927; 1929; Orliac et al., 2014; Rakotovao et al., 2014) with Orliac et al. (2014) providing CTbased reconstructions of cranial vasculature.

USNM-M 182395, an adult specimen, has the bulla in situ on the right side and removed on the left side (fig. 21). I describe both sides below. Both occipital condyles have previously been removed prior to this study as have parts of the left pterygoid bone and left squamosal. Additional information, such as details on the occipital condyles, was observed on USNM-M 162981 and AMNH M-24289.

The bulla in this taxon is held firmly in place laterally and posteriorly with the external acoustic meatus fused to the squamosal laterally and the bulla fused to the exoccipital posteriorly (fig. 21). Exposure of the petrosal required sawing the bulla off at the medial edge of the external acoustic meatus.

The sphenorbital fissure can be seen in figure 22; it is an oval opening with the long axis oriented superomedially to inferolaterally. It is substantially larger than the optic foramen (not shown) and lies inferolateral to it. The anterior opening of the pterygoid canal appears to be approximately 10 cm anterior to the sphenorbital fissure, at the superior aspect of the exposure of the pterygoid bone within the orbit. It is not apparent where the posterior opening of this canal lies (if it is present at all) even though the pterygoids are well exposed on the sides and roof of the basipharyngeal canal. The thin pterygoid bones extend posteriorly from the basipharyngeal canal almost to the carotid foramen/incisure. The entopterygoid process that gives rise to the hamulus is not well developed, nor is there a clearly differentiated ectoptyergoid process projecting from the alisphenoid (fig. 21). Thus, there is almost no pterygoid fossa except for a small invagination immediately lateral to the hamulus. The hamulus does not extend far superiorly. The vomer emerges posteriorly from the basipharyngeal canal, and its right and left alae each terminate as a point and diverge in the midline exposing the overlying presphenoid (fig. 21). The pterygoids each taper to a point posteriorly adjacent to the location of the very small posterior opening of the pterygoid canal (fig. 21). There is no particularly distinct sulcus running between this opening and the middle ear. The following sutures are not typically visible in the basicranial view of adult specimens: basisphenoid-basioccipital, alisphenoid-squamosal, and basioccipitalexoccipital. The craniopharyngeal canal is absent (fig. 22). Two round, knob-shaped tympanic processes lie immediately adjacent to the bulla and most likely spanned the basisphenoid-basioccipital junction. These processes have a rough texture with numerous small bumps. The tympanic processes lie medial to the lateral edge of the basisphenoid-basioccipital, and they do not contact the bulla. The knobs are in contact with each other posteriorly and give rise to a median crest that extends posteriorly from the point at which they meet. There is a single hypoglossal foramen on each side that is almost as large as the jugular foramen and gives very direct passage into the foramen magnum (figs. 21, 22). The occipital condyles (not figured) are smoothly convex and exposed on the ventral and posterior surfaces of the skull. There is no distinct occipital ridge. The anterior borders of the occipital condyles are well separated on the ventral surface of the skull and flexion stops are absent. There is no regularly positioned, distinct condylar foramen, although parts of the occiput are pocked with small, irreg- ularly sized and spaced foramina. The paracondylar process is small and triangular shaped. It projects downward with a slight posterior inclination and comes to a knob-shaped point inferiorly. The medial border of the paracondylar process is connected to the bulla by a bony ridge, and the bulla and the exoccipital fuse at the base of the paracondylar process.

The external surface of the bulla has a roughened texture; it is marked with a few spikes of bone and pocked with tiny foramina (fig. 21). The bulla is ovoid with the long axis running from anteromedial to posterolateral. At the anteromedial edge there is a flattened, triangular styliform process. From the ventral tip of the styliform process extends a sharp bullar ridge that runs to the posterolateral corner of the bulla, where the external acoustic meatus disappears between the postglenoid and posttympanic processes of the squamosal bone. The bullar ridge is briefly truncated about one-third of the distance from the posteromedial corner of the bulla. Posterior to the bullar ridge is a deep, columnar-shaped, angled passageway that contains both the stylohyal fossa medially and the stylomastoid foramen laterally. This passageway is set so deeply that it is somewhat obscured in figure 21. The ventral end of the tympanohyal is visible only by looking directly into this opening. The tympanohyal has as a small, circular, and flattened tip. The stylohyal fossa is a very shallow excavation in the bulla; the bulla barely conforms to the tympanohyal at all, contacting it only on its anterior surface where the two structures may even be fused. It appears that a full bony separation between the stylohyal fossa and the stylomastoid foramen is absent. The paracondylar processes are short structures with a wide base superiorly that extend only a short distance inferior to the bulla in lateral view. Crests run between these processes and fuse to the bulla on either side of the stylohyal fossa such that there is a distinct excavation in the paracondylar process that forms the posterior edge of the stylohyal fossa.

The external acoustic meatus is very long (it exceeds the mediolateral width of the remainder of the bulla) and it wraps up the lateral side of rior carotid foramen is the clearer of the two and the skull such that the porus acusticus externus is marked by a jagged-edged furrow on the is not visible in ventral view of the skull (fig. 21). bulla. The opening of the auditory tube between As noted above, there is extensive fusion the bulla and petrosal is directly superior to the between the squamosal and the external acous- styliform process and is marked by a semicircutic meatus, however, the postglenoid and post- lar furrow in the bulla. This furrow has a sharp, tympanic processes of the squamosal do not crestlike lateral edge (fig. 18).

fully meet ventrally; they are separated by a nar- While the bulla is tightly held to adjacent row, mediolaterally oriented slit that exposes the bones via the external acoustic meatus laterally, external acoustic meatus. Medially, the anterior on the anterior, medial, and posteromedial sides, and posterior carotid foramina are both set so it does not contact the surrounding squamosal, deeply between the bulla and the petrosal that alisphenoid, basisphenoid, and basioccipital (fig. their positions can be approximated only when 21). The bulla is separated from each of these the bulla is in situ (fig. 21). The path to the ante- bones by several millimeters (note that in fig. 21, the styliform process of the bulla partially obscures a view of this continuous space between the bulla and the alisphenoid), and the petrosal is partially visible, even with the bulla in place. The bulla contacts but does not fuse to the pars cochlearis of the petrosal. When the bulla is removed its cancellous inner structure is visible. The ectotympanic ring extends slightly more than 180° with no differentiation where the two crura meet. The auditory tube is marked by a groove on the medial bulla but the carotid foramina leave only indistinct markings.

This lack of contact between the petrosalbulla complex and surrounding bones produces essentially one large opening that allows passage into and out of the middle ear/neck region and endocranium. Laterally the piriform fenestra lies between the petrosal/bulla posteriorly and the alisphenoid/squamosal anteriorly (figs. 21, 22). It is elongate and oriented anteromedial to posterolateral. The incisura ovalis makes a gentle indentation into what is probably the alisphenoid (although, as noted above, this should be interpreted with caution because the suture with the squamosal is obliterated). The carotid incisure, like the incisura ovalis, is visible even with the bulla in place. The carotid incisure is fully confluent with the middle lacerate foramen and the basicapsular fissure. Posterior to the basicapsular fissure a prong of bone from the exoccipital contacts but does not to fuse to the petrosal, thereby forming only a partial barrier between the basicapsular fissure and the jugular foramen. The jugular foramen is a large, squarish opening. Without the bulla in place the stylomastoid notch is indistinct but lies anterolateral to the jugular foramen. There is no exposure of the mastoid region of the petrosal on the ventral, posterior, or lateral aspects of the skull.

The glenoid fossa is rectangular to oval shaped and runs from anterolateral to posteromedial (fig. 21). The preglenoid process is absent. The surface of the postglenoid fossa is flat to convex in its center and the plane of the fossa faces posteriorly. At the lateral margin of the fossa is an elongate concavity. The postglenoid process lies primarily at the medial extreme of the joint and is a mediolaterally elongate ridge. A medial glenoid pit is absent. There is no conspicuous postglenoid foramen, but it is difficult to see the posterior aspect of the postglenoid process due to the proximity of the posttympanic process. There is a deep fissure where the postglenoid and posttympanic processes surround the external acoustic meatus anteriorly and posteriorly respectively. I cannot rule out the possibility that there may be a small postglenoid foramen contained within this space. Orliac et al.’s (2014) micro CT–based reconstructions of the postglenoid foramen in Hippopotamus showed that patent openings do exist in the area just noted, however, these are quite small and deep.

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 21. Left basicranium of Hippopotamus amphibius (Cetancodonta, Hippopotamidae, USNM-M 182395) with bulla in situ, petrosal is visible through the jugular foramen. Digitally reversed from right side. Scale bar = 1 cm.

FIGURE 22. Left basicranium of Hippopotamus amphibius (Cetancodonta, Hippopotamidae, USNM-M 182395) with auditory bulla removed to reveal petrosal in situ. Scale bar = 1 cm. Artist: U. Kikutani.