phylogenetic relationships and classification of didelphid marsupials ...

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44 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 posterior limb tends to be dorsoventrally flattened and laterally expanded, forming part of the floor of the external ear canal to a greater or lesser extent. 12 The anterior and posterior limbs of the didelphid ectotympanic are always widely separated by a broad posterodorsal gap, the incisura tympanica. Caenolestids, dasyurids, most peramelemorphians, and Dromiciops essentially resemble didelphids in most aspects of ectotympanic morphology. The anterior limb is suspended loosely but directly from the basicranium, and it is separated from the posterior limb by a broad posterodorsal incisura tympanica. However, whereas the ectotympanic annulus in didelphids, caenolestids, dasyurids, and peramelemorphians is prominently exposed on the lateral aspect of the bulla, the ectotympanic is uniquely concealed from lateral view within the auditory bulla of Dromiciops (see Segall, 1969a: fig. 3, 1969b: fig. 4). The ectotympanics of Recent diprotodontians differ strikingly from those of didelphids and other marsupials, principally by forming tubelike extensions of the ear canal, by ankylosis or fusion with neighboring bones, and by dorsal migration and narrowing or closure of the incisura tympanica (Aplin, 1990). The outermost auditory ossicles of didelphids conform to a pattern that is widespread among other plesiomorphic marsupials. In particular, the malleus invariably has a long, sharply inflected neck, a small orbicular apophysis, a well-developed lamina, and a manubrium that is approximately parallel to the anterior (tympanic or gracile) process; the body of the incus is distinctly differentiated from the long and short processes of that bone (Segall, 1969b: figs. 7, 9; Henson, 1974: fig. 20A). Although taxonomic differences in 12 We were previously incorrect in describing the expansion of the posterior limb of the ectotympanic as ‘‘medial’’ in taxa with direct suspension (Voss and Jansa, 2003: 30). As noted by Wible (2003: 165), the expansion of the posterior limb of the ectotympanic in Monodelphis is lateral to the sulcus tympanicus, and therefore contributes to the floor of the external ear canal (acoustic meatus) rather than to the bullar floor. This is also true of most other didelphids with direct ectotympanic suspension. Only in Chacodelphys, Cryptonanus, Gracilinanus, Lestodelphys, Thylamys, and some species of Marmosa does the ectotympanic also make a small bullar contribution. certain details of didelphid mallear and incudal structure were noted by Sánchez- Villagra et al. (2002) and Schmelzle et al. (2005), we did not attempt the requisite dissections to assess the phylogenetic distribution of the features they described. The didelphid stapes, however, exhibits striking morphological variation that can be determined without dissection. Despite some intraspecific variation noted by Gaudin et al. (1996), most didelphids normally exhibit one or the other of two different stapedial morphotypes defined by Novacek and Wyss (1986). Most didelphids (e.g., Philander; Novacek and Wyss, 1986: fig. 5F) have a more or less triangular or stirrup-shaped, bicrurate stapes that is perforated by a large stapedial (obturator or intercrural) foramen. Other didelphids have a columelliform (or columnar) stapes that is imperforate (or microperforate: with a foramen whose maximum diameter is less than the width of a surrounding crus; Gaudin et al., 1996). Taxa with columelliform stapes include Caluromysiops, Lestodelphys, and several species of Monodelphis (e.g., M. peruviana, M. theresa). The ossicular morphology of nondidelphid marsupials exhibits significant taxonomic variation. Whereas the mallei of caenolestids, dasyurids, and peramelemorphians essentially resemble those of didelphids in the features described above, the malleus of Dromiciops has a short, uninflected neck, no lamina, and a manubrium that forms an open angle with the anterior process (Segall, 1969a, 1969b). The stapes of Dromiciops is bicrurate (with a large foramen, like didelphids), but the stapes of caenolestids, dasyurids, and peramelemorphians are columelliform and imperforate (Novacek and Wyss, 1986; Schmelzle et al., 2005). Notoryctes and phalangeriform diprotodontians have ossicles that are strikingly unlike those of didelphids and other plesiomorphic marsupials (Segall, 1970; Sánchez- Villagra and Nummela, 2001). OCCIPUT: Fusion of the supraoccipital with the interparietal is common to all didelphids and occurs very early in postnatal life as has already been discussed, so the dorsal part of the lambdoid crest is not associated with any visible sutures, nor does it develop sesamoids for the insertion of
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 45 nuchal muscles (as in peramelemorphians, see below). The crest itself is only weakly developed in juveniles and in adult specimens of the smaller forms, but it is prominent in fully adult specimens of all of the larger opossums. The mastoid (pars mastoideus of the petrosal) is always prominently exposed on the lateral occiput, where it is bordered by the supraoccipital, exoccipital, squamosal, and sometimes the parietal. All didelphids have a well-developed mastoid process at the ventrolateral corner of the occiput that is sometimes provided with a sesamoid. The dorsal margin of the foramen magnum is formed by the exoccipitals and the supraoccipital in most didelphids (fig. 13A). In taxa that conform to this morphology, the foramen has a distinct middorsal emargination, the incisura occipitalis, that separates the left and right exoccipitals. By contrast, medial processes of the left and right exoccipitals are joined at the midline above the foramen magnum—excluding the supraoccipital from the dorsal margin of the foramen and occluding the incisura (fig. 13C)—in adult specimens of Didelphis, Lutreolina, Metachirus, andPhilander. Juvenile (and some subadult) specimens of these genera, however, resemble other didelphids in their occipital morphology, such that both conditions (presence and absence of a supraoccipital margin) can be observed in ontogenetic series of conspecific skulls (Abdala et al., 2001: fig. 3). The medial processes of the exoccipitals are closely approximated in Chironectes, but in all of the specimens we examined the supraoccipital still forms part of the dorsal margin of the foramen magnum (contra Sánchez-Villagra and Wible, 2002), and the incisura occipitalis persists as a narrow middorsal notch. The paroccipital process of the exoccipital is inconspicuous in most didelphids, consisting of a low, rounded or subtriangular bony mass for muscle attachment that is broadly adnate to the petrosal; the indistinct apex of the process in taxa that conform to this widespread condition is directed posteroventrally (fig. 13A, B). By contrast, in Chironectes, Didelphis, Lutreolina, Metachirus, and Philander the paroccipital process is much larger, more erect, and points almost straight ventrally in most examined specimens (fig. 13C, D). Among other plesiomorphic marsupials, dasyurids are essentially similar to didelphids in most occipital features, but mastoid processes are indistinct in caenolestids, and Dromiciops lacks paroccipital processes. Peramelemorphians differ from all other marsupials in having unique osseous structures that are sutured between the parietals and the supraoccipital on each side of the skull. Commonly illustrated but seldom labeled or described by authors, these are the lambdoid sesamoids of Filan (1990), who hypothesized that they function as force multipliers for M. rectus capitis dorsalis and other head extensors. Another occipital feature that might be unique to peramelemorphians is a sesamoid that is probably associated with the origin of the posterior belly of the digastric; this small bone occupies the apex of the paroccipital process in some peramelemorphians (e.g., Perameles), but it attaches to the otherwise smooth ventrolateral corner of the exoccipital in taxa that lack a paroccipital process (e.g., Microperoryctes). MANDIBLE: The didelphid mandible consists of an anteroposteriorly elongate horizontal ramus that contains the dental alveoli, an ascending ramus with well-developed coronoid and condylar processes, and a posteroventral angular process. The mandibular symphysis is never fused, and the retromolar space (between m4 and the base of the coronoid process) is either imperforate or pierced by tiny nutrient foramina of inconstant number and position. The masseteric fossa (a prominent concavity for the insertion of the eponymous muscle on the posterolateral surface of the jaw) is always imperforate, and its posteroventral border is defined by a distinct shelf. The articular condyle is transversely elongate and more or less semicylindrical. Although substantial variation in mandibular shape is apparent from the drawings that accompany our systematic accounts (figs. 37–54), other features are more readily characterized for taxonomic diagnoses and phylogenetic inference. Most didelphids have two mental foramina that perforate the lateral surface of the horizontal ramus, of which the more anterior is usually larger and located below p1 or p2, and the more posterior is usually smaller and located below p3 or m1. Chironectes, howev-
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