phylogenetic relationships and classification of didelphid marsupials ...

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42 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 Dromiciops is almost exclusively formed by the alisphenoid (as in didelphids), but a broad petrosal shelf forms part of the sinus roof in caenolestids and some peramelemorphians. Diprotodontians exhibit a very wide range of bullar morphologies (reviewed by Aplin, 1990), many of which include substantial squamosal participation. The epitympanic recess of didelphids is a shallow concavity in the ventral surface of the petrosal pars canalicularis that encloses the mallear-incudal articulation (Wible, 2003: fig. 7D). 11 Although this morphology is widespread among other marsupials, a few (e.g., Macrotis, Perameles) have an enormously inflated epitympanic recess that vaults high above the ossicles, leaving the mallear-incudal joint effectively unenclosed by bone. The promontorium of the petrosal pars cochlearis is not marked by any vascular grooves in didelphids. Instead, the most conspicuous feature of this otherwise smooth, bulbous structure is the rostral tympanic process (sensu MacPhee, 1981; Wible, 1990). The rostral tympanic process is small, more or less conical, and perhaps serves only to anchor the posterior (ventral) limb of the ectotympanic annulus in some opossums (e.g., Glironia), but it is anteroposteriorly expanded, dorsally concave, and substantially contributes to the ventral enclosure of the middle ear in others (e.g., Marmosa; Reig et al., 1987: figs. 38–42). In Caluromys and Caluromysiops, the rostral tympanic process extends along the entire length of the promontorium from the fenestra cochleae to the anterior pole (Sánchez- Villagra and Wible, 2002). The rostral tympanic process of caenolestids and most peramelemorphians is small and more or less conical, but dasyurids, some peramelemorphians (e.g., Macrotis), and Dromiciops have an anteroposteriorly extensive and dorsally 11 Because the illustrations in Wible (1990) are so useful in other respects, it is important to note that the epitympanic recess is mislabeled in two views of the tympanic surface of the petrosal of Didelphis virginiana. As noted by Muizon (1998: 90), the concavity so labeled in Wible’s figure 4A and 4C is really the posterior part of the hypotympanic sinus roof; the true epitympanic recess is a smaller depression posterolateral to the hypotympanic sinus roof and separated from it by a low ridge, the petrosal crest. concave process that forms a substantial part of the bullar floor in these taxa. In most didelphids, the fenestra cochleae is exposed in lateral or ventrolateral view as illustrated for Didelphis by Wible (1990: fig. 1). In others (e.g., Caluromys, Caluromysiops, Lestodelphys, Marmosa rubra, Monodelphis emiliae, andThylamys), the fenestra cochleae is concealed within a chamber or sinus formed by a laminar outgrowth of the pars canalicularis that approximates or contacts the rostral tympanic process of the pars cochlearis. This lamina, which appears to be homologous with the caudal tympanic process of MacPhee (1981), is invariably fused posteroventrally with the paroccipital process of the exoccipital, which also throws forward a small ‘‘tympanic’’ outgrowth. The resulting auditory chamber—corresponding to the ‘‘periotic hypotympanic sinus’’ of Archer (1976a) and the ‘‘post-promontorial tympanic sinus’’ of Wible (1990)—is always small and uninflated in opossums. The fenestra cochleae is also exposed in caenolestids and some peramelemorphians (e.g., Echymipera), in which tympanic processes of the petrosal are either small or absent. In other peramelemorphians (e.g., Perameles) and in dasyurids, however, the fenestra cochleae is concealed within a postpromontorial sinus formed by the seamless fusion of the rostral and caudal tympanic processes; in these forms (as in Caluromys and other didelphids with similar petrosal features), the sinus is small and uninflated. The fenestra cochleae of Dromiciops is enclosed in an inflated (bubblelike) postpromontorial sinus that, although strikingly unlike the smaller sinuses of other plesiomorphic marsupials, is also assumed to have resulted from fusion of caudal and rostral tympanic processes (Sánchez-Villagra and Wible, 2002: fig. 11). No other bone (besides the alisphenoid, ectotympanic, and petrosal) participates in auditory sinus formation among Recent didelphids, which have only one or two hypotympanic cavities as described above. Caenolestids and Dromiciops are similar to didelphids in this respect, but Recent dasyuromorphians and some peramelemorphians (e.g., Macrotis and Perameles) have a welldeveloped epitympanic sinus formed by the
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 43 Fig. 17. Oblique ventrolateral view of left ear region in Marmosops impavidus (A, MUSM 13284) and Philander mcilhennyi (B, MUSM 13299) illustrating taxonomic differences in ectotympanic suspension. Whereas the ectotympanic (ect) is suspended from the skull by attachments both to the petrosal (pet) and to the malleus (mal) inMarmosops, the ectotympanic of Philander is suspended only from the malleus (there is no attachment to the petrosal). Other abbreviations: atp, alisphenoid tympanic process; pro, promontorium (of petrosal); rtp, rostral tympanic process (of petrosal); sq, squamosal. squamosal. This structure is a cup-shaped cavity that, in all skulls retaining vestiges of auditory soft tissues, appears to be covered by the membrana Shrapnelli (pars flaccida of the tympanic membrane). Archer (1976a: 304) remarked that, ‘‘[i]n some didelphids (e.g., Metachirus) there is a depression in the squamosal which is clearly the homologue of this sinus,’’ but nothing resembling the squamosal epitympanic sinus of dasyurids occurs in any living opossum. ECTOTYMPANIC AND OSSICLES: Although didelphids were described by van der Klaauw (1931: 26) as having a completely free ectotympanic, two distinct patterns of ectotympanic attachment can be recognized in the family. In most didelphids the anterior limb (or crus) of the ectotympanic annulus is directly connected to the skull near the point where the squamosal, alisphenoid, and petrosal are juxtaposed behind the postglenoid process. Where the connection can be seen clearly (dried remnants of soft tissues frequently obscure this feature), the actual attachment seems to be to the petrosal (fig. 17A). In six genera (Caluromys, Caluromysiops, Chironectes, Didelphis, Lutreolina, and Philander), however, the anterior limb of the ectotympanic is not directly attached to the skull, and the suspension is indirect, via the anterior (tympanic) process of the malleus (fig. 17B). In all didelphids with an indirect dorsal connection between the ectotympanic and the skull, the tympanic annulus is more or less ringlike because the posterior (ventral) limb is not expanded to form part of the floor of the middle ear cavity. By contrast, in some taxa with direct ectotympanic suspension, the
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