DESCRIPTION OF THE FACIAL SKELETON

The bauplan of the facial skeleton is surprisingly modern, including several indicators suggesting prehensile proboscis development similar to that of extant tapirs. Among the most obvious of these indicators is a deeply retracted nasoincisive incisure (narial incision), which reaches, or is close to, the level of the anterior orbit (Figure 5). On the horizontally flattened AMNH skull, the incisure lies well over the orbit and on the sagitally compressed SDSM skull at the level of the anterior orbit. This retraction is correlated with the posterior telescoping of the dorsal facial skeleton, and with the lack of a postorbital constriction ( Radinsky 1965 ). The premaxilla does not contact the nasal, and the ascending process of the maxilla is marked by a distinctive trough that terminates posteriorly in a shallow fossa that curls onto the posterodorsal nasals and anterodorsal frontals. This trough and fossa accommodates a cartilaginous meatal diverticulum in Tapirus, and following Witmer et al. (1999) are called the meatal diverticulum fossa and trough (Figure 5). A similar trough on the ascending process of the maxilla in Protapirus differs by extending posteriorly over the supraorbital process of the frontal rather than terminating in a fossa. The lateral border of this meatal diverticulum trough anterodorsal to the orbit, comprises the lacrimofrontal ridge, which served as the attachment area for the levator labii superioris muscle.

The relatively robust premaxilla is slightly downcurved (see Radinsky 1965) and has three incisors. These conditions are shared with both Tapirus and Protapirus, although the premaxilla of Tapirus is more robust than the others. The nasal process of the premaxilla does not reach the nasal, extending posterodorsally to the level of the posterior diastema, immediately anterior to the level of the first premolar. At this point it lies just lateral to the margin of the narial opening, resting on the dorsal edge of the facial maxilla. The CT data show the posterior margin of the premaxilla extending as a wedge-shaped intrusion between the nasal and facial surfaces of the maxilla. This relationship extends the entire sutural length, from dorsal margin to the alveolar margin. Unlike Tapirus, the third incisors are not caniniform. The interpremaxillary suture is open on the AMNH skull, and the left and right sides are slightly displaced. Although crushed, the flattened medial premaxillary margins of the nasoincisive incisure suggest that the cartilaginous nasal septum was clasped by the premaxillae, or possibly a maxilloturbinate that lined the narial face of the premaxillae (Figure 6). Both elements embrace the nasal septum in Tapirus. This contact with the cartilaginous nasal septum is situated immediately posterior to the dorsal symphysis (see Witmer et al. 1999). A single confluent palatine fissure notches the palatal premaxilla.

The maxilla lacks a canine, canine alveolus, and the corresponding alveolar ridge on its facial surface. Otherwise, the maxilla generally resembles that of Protapirus and Tapirus. Colodon is similar to Tapirus in having a large conchal sinus recess and apparently no maxillary sinus. The dorsal free edge of the facial maxilla is thickened and bears a variably developed crista conchalis on its ventromedial margin. Only fragments of the maxilloturbinate, which sutures to this crista, remain. Viewed laterally, the profile of the narial margin is marked by two broad scallops, which meet at the midpoint of the incisure margin as a low peaked eminence (Figure 7). This arrangement is similar to Protapirus, although the peak is more pronounced in Protapirus. Protapirus also differs in having the peak associated with a shallow fossa on the facial maxilla. The flattened and expanded medial surface of the peaked area of Colodon and Protapirus is interpreted to be a sutural contact surface for the septal portion of the maxilloturbinate. The septal maxilloturbinate of Tapirus is a medially directed lamina that contacts the cartilaginous nasal septum (see Witmer et al. 1999). In coronal section the nasal face of the maxilla is broadly concave below the crista conchalis (Figure 8), and compares favorably with the conchal recess described for Tapirus by Witmer et al. (1999).

The ascending process of the maxilla extends posterior to and lateral to the descending process of the nasals. At this area the ascending maxilla is dorsally concave and contributes to the meatal diverticulum trough. The lateral margin of the maxilla is raised and contacts the frontal and lacrimals along the lacrimofrontal ridge. In Tapirus, this ridge serves for attachment of the levator labii superioris muscles, a prime mover of the proboscis (Witmer et al. 1999). The infraorbital foramen opens at a level above P3/P4. CT data reveal an alveolar canal arising from the infraorbital canal immediately posterior to the infraorbital foramen.

Posteroventrally, the maxillary tuber floors the orbit. The unerupted M3s of both specimens are deeply lodged within their crypts. The damaged palatal processes of the maxillae are notched anteriorly for a single palatine fissure. The perpendicular lamina of the palatines has an extended contact with the maxilla.

The horizontal laminae of the palatines are not well preserved, but their choanal margin is near the level of M1/2. Note that during the ontogeny of living tapirs the choanae migrate posteriorly with the eruption of the molar series. Rostrally the horizontal laminae intrude between the maxillae, to about P3 level. As in Tapirus, the posterior perpendicular lamina of the palatine extends posterior to the anterior margin of the choanae along the medial surface of the maxilla, extending to the posterior face of the maxillary tuber, where there is a small posteriomedial suface for the suture with the pterygoid. The pterygoid is not preserved. Although the palatine foramina are damaged and difficult to see, the palatine canal is seen in the CT data extending along the medial margins of the maxillary tuber (e.g., Figure 9), and opening into the orbit immediately posterior to the maxillae tuber. This is similar to the palatine canal of Tapirus.

What is preserved of the nasals is similar to those of Tapirus, having an anteriorly tapering rostral process; the posteromedial margin of the rostral process with a small notch; and a short descending process resting on the posterior ascending process of the maxillae. Viewed dorsally, the rostral process is roughly triangular, widening posteriorly to the posterior nasoincisive incisure. The anterior medial rostral process of the nasals is broken. A median process of the frontals intrudes between the nasals posteriorly (Figure 10). Lateral descending processes of the nasals extend rostrally along the dorsal margins of the maxillae to the level of the thickened dorsal stem of the maxilloturbinal.

The maxilloturbinals are represented by fragments. As noted above, the thickened dorsal stem is only partially preserved (Figure 10). However, considering their fragmental nature, the arrangement of the crista conchalis and of the presumed sutural contact area for the septal maxilloturbinate (Figure 7) at the peaked narial border of the maxilla indicate that the maxilloturbinals were generally similar to those of Tapirus. The partially preserved vomer (Figure 8, Figure 9) has a deep septal sulcis, which lodged the cartilaginous nasal septum and the perpendicular plate of the ethmoid. Only fragments of the ethmoidal turbinals are preserved, although a delicate perpendicular plate can be seen in coronal and horizontal CT sections.

Two tubercles mark the facial face of the lacrimal, similar to the condition observed in Tapirus. The medial of these tubercles is continuous with the frontolacrimal ridge on the margin of the orbit. The lacrimal foramen, or foramina, and lacrimal canal are poorly preserved, but appear similarly positioned to those of Tapirus.

The frontals of Colodon resemble Tapirus, being antero-posteriorly compressed, dorsally inflated, and having a median process that intrudes between the nasals anteriorly. The CT data reveal large frontal sinuses that invest almost the entire element (e.g., Figure 11). This differs from the condition in Protapirus, which retains elongated frontals having no sinus cavities that overlie the cranial cavity. The meatal diverticulum fossa and trough are developed on the anterior frontals, and on rostrolateral processes (Witmer et al. 1999). These rostrolateral processes suture with the ascending maxillae, lacrimals, and posteriorly with the posterior descending processes of the nasals. The supraorbital processes are similar to those of Tapirus, extending posteriorly from the frontolacrimal ridge.