Tragulus javanicus (Osbeck, 1765)

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-

Tragulus javanicus and T. napu

Figures 16–18

The basicranium of Tragulus javanicus has been figured ( Starck, 1995: 1020), but none of the structures are labeled; Kampen (1905) provided details on the auditory bulla of Tragulidae . Two specimens are figured here, one of T. napu with the bulla in situ (fig. 16) and one of T. javanicus with the bulla removed (fig. 17). The bulla is typically very tightly held in place or even fused to surrounding bones by maturity (e.g., AMNH-M 188310), and the view of the petrosal in situ without the bulla has been possible because the bulla was detached in the subadult T. javanicus specimen (AMNH-M 102091), which has unerupted upper and lower third molars. Observations on the adult anatomy with the bulla in situ were supplemented by examination of AMNH-M 32652.

Several posterior orbit structures are visible in figures 16 and 17. The optic foramen is oval and positioned anterosuperiorly relative to the sphenorbital fissure. The sphenorbital fissure is teardrop shaped, coming to a point at its superior aspect and it appears to be contained entirely within the alisphenoid.

A variably present accessory foramen lies anteromedially adjacent to the foramen ovale (only in the specimen in fig. 17) near the junction of the alisphenoid and the basisphenoid. The accessory foramen is present bilaterally in some specimens and absent in others. On the left side this foramen appears to open at the posterior base of the sphenorbital fissure and to have connections into the body of the sphenoid itself, however, on the right side the accessory foramen enters the body of the sphenoid but does not appear to emerge. This accessory foramen is absent in the mature AMNH-M 188310. A distinct pterygoid canal is either absent or very small, although specimens AMNH-M 188310 and 32652 have small holes inferior to the sphenorbital fissure (not illustrated), but it is not apparent that these foramina connect to openings more posteriorly.

Both figures 16 and 17 show the posterior aspect of the basipharyngeal canal. The vomer is not visible because it terminates anterior to the posterior end of the palatine bone and anterior to the presphenoid-basisphenoid suture. The pterygoid bones are not preserved in the subadult but are preserved in the mature specimen. The ento- and ectopterygoid processes are approximately the same size, both projecting only a small distance posterolaterally at their ventral margins. There are distinct sutures indicating where the palatine bone ends posteriorly and how the pterygoid and the alisphenoid contribute to the posterior tips of the pterygoid processes. There is little development of a pterygoid fossa; there is only a small space at the inferior aspect of the two pterygoid processes. The hamulus has a convex ventral border and projects posterolaterally, terminating in a subtle hook. The hamulus does not project ventrally to a significant distance relative to the palate. The posterior edge of the hard palate is straight where the two palatine bones meet in the midline, and the posterior parts of the palatine bones lack a nasal spine.

The midline has a craniopharyngeal canal in AMNH-M 10291 and 188310 (fig. 17; out of view in fig. 16) just anterior to the tympanic processes. The tympanic processes are elongate, oval bumps situated primarily on the basioccipital but also extending onto the basisphenoid. The two processes are more closely approximated to each other posteriorly than anteriorly. The tympanic processes are situated relatively close to the mid- line (yet each remains distinct from the other) marks the transition between the two surfaces. and they do not contact the auditory bulla. The paracondylar process is a simple projection

The occipital condyles of the exoccipital are that extends straight down from the skull base. distinctly separate anteriorly. They have gentle It does not extend as far ventrally as the ventral flexion stops at their anterior extreme. The edge of the auditory bulla. An accessory foraoccipital condyle has both a ventral and a pos- men (fig. 17) at the base of the paracondylar terior aspect, and a very subtle occipital ridge process is also present in the immature speci- men but absent in the mature specimen. The hypoglossal foramen is posteromedial to the jugular foramen. It is a single hole somewhat deeply set and separated from the jugular foramen by a distinct ridge. Visible in the foramen magnum there are some extremely small foramina (three on the left and one on the right; out of view in the illustrations) on the internal surface of the occipital condyle (AMNH-M 188310). These appear to terminate within the bone. A condylar foramen that actually pierces the condyle is absent in both specimens.

The auditory bulla is ovoid, coming to a small pointed styliform process at its anteromedial edge (figs. 16, 18). The surface of the bone is smooth and the bulla lacks a conspicuous exit for the auditory tube. The most ventral aspect of the bulla forms a bullar ridge along its length that sweeps from the anteromedial to the posterolateral extremes of the bone terminating posterior to the tympanohyal. The tympanohyal invaginates the posterolateral edge of the bulla where it is tightly held in a stylohyal fossa that occupies the posterolateral one-third of the bulla (fig. 16). There is some fusion of the tympanohyal and the bulla superiorly. The bulla does not completely surround the tympanohyal, even in mature individuals. The tympanohyal is also appressed to the paracondylar process of the exoccipital. The tip of the tympanohyal is flat and broad and distinctly exposed emerging from the stylohyal fossa. The external acoustic meatus is a distinct tube that does not quite extend to the lateral edge of the skull but terminates directly posterior to the postglenoid foramen. The ectotympanic forms a complete tube of bone that sits within a shallow depression on the squamosal. The bulla contains a small accessory foramen on its anterolateral aspect (fig. 16). The stylomastoid foramen is a small, circular hole formed by the bulla abutting the mastoid region of the petrosal. It is located posterolateral to the tympanohyal and anterolateral to the paracondylar process. The paracondylar process broadens at the superior end and contacts the auditory bulla.

Medially and posteromedially, the bulla is tightly appressed to the basioccipital and exoccipital such that the wide openings between the petrosal and these other bones, described below, are almost completely obscured. In many specimens (e.g., AMNH-M 188310) the bulla is so tightly appressed to surrounding bones that there is only one conspicuous opening between the bulla and the basioccipital in ventral view of the skull: a confluent opening for the posterior carotid foramen and the jugular foramen (fig. 16). The shape of this foramen is somewhat variable. It is a small opening externally in AMNH- M 188310 that attenuates as a slit posterolaterally, but by contrast, in AMNH-M 32652 (not illustrated) there is a large, confluent pair of openings where the posterior carotid foramen and the jugular foramen would typically lie. Entry to this foramen at the posteromedial aspect of the bulla is distinguished by a gentle furrow on the bulla itself, a furrow that starts ventrally, medial to the bullar ridge, and disappears into the foramen (fig. 18). Besides the midline bones mentioned above, the bulla also makes contact with the alisphenoid and the petrosal, and its external acoustic meatus has extensive contact with the squamosal. The anterior carotid foramen is tucked deep within the anterosuperior aspect of the bulla and not visible with the petrosal in situ (see fig. 16). It is formed between the bulla and the petrosal. The groove for the auditory tube is not very distinct from the anterior opening of the carotid foramen (figs. 16, 18).

With the bulla removed the relationship of the petrosal to the surrounding bones can be seen (fig. 17). As noted above, the foramen ovale is contained entirely within the alisphenoid; it is not part of the petrosal complex. A large space separates the petrosal from the alisphenoid, basisphenoid, and basioccipital such that several openings into the skull are contiguous. The piriform fenestra is a gracile slit just lateral to the pointed epitympanic wing of the petrosal. It lies between the petrosal and the alisphenoid and squamosal. Posteromedial to the foramen ovale and on the medial side of the epitympanic wing is the carotid incisure at the anteriormost aspect of the large space between the petrosal and midline structures. More posteriorly there are no distinct boundaries for the middle lacerate foramen or the basicapsular fissure, but the space is widely opened. The basicapsular fissure is distinct from the jugular foramen with the exoccipital intervening between the two openings but not quite touching the petrosal and not completely closing off the jugular foramen from more anterior openings. The jugular foramen is elongate mediolaterally. The mastoid region of the petrosal is exposed between the exoccipital (just anterior to the paracondylar process) and the squamosal.

In isolation the smooth-surfaced bulla reveals the scroll of bone comprising the external acoustic meatus (fig. 18). The length of the meatus is relatively short compared to the full mediolateral length of the bulla. The inside of the bulla is filled with cancellous bone forming numerous chambers. The region adjacent to the ectotympanic ring has extensive contact with the petrosal. As noted above, the bullar contributions to the anterior carotid foramen and the auditory tube are not marked by strong grooves; its contribution to the posterior carotid foramina is more pronounced.

The glenoid fossa is situated entirely on the squamosal; it is mediolaterally elongate, convex laterally, and concave medially forming a medial glenoid pit (figs. 16, 17). Posterior to the medial glenoid pit a scroll of squamosal curves ventrally and wraps against the bulla. The lateral aspect of the glenoid fossa is situated on the zygomatic arch and not on the braincase. There is a single, small, circular postglenoid foramen, visible whether the bulla is in situ or not. On the left side of the immature specimen there is also a small accessory foramen adjacent to this.

Figures

FIGURE 16. Left basicranium of Tragulus napu (Cetruminantia, Ruminantia, Tragulidae; AMNH-M 188310) with bulla in situ. Scale bar = 1 cm. Artist: U. Kikutani.

FIGURE 17. Left basicranium of Tragulus javanicus (Cetruminantia, Ruminantia, Tragulidae; AMNH-M 102091) with bulla removed to reveal petrosal in situ. Scale bar = 1 cm. Artist: U. Kikutani.

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).