phylogenetic relationships and classification of didelphid marsupials ...

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26 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 scales, each of which is soft and flexible (yielding easily to the tip of a probe on fluidpreserved material), the scales on the underside of the base of the tail are heavily cornified, forming hard raised tubercles in Glironia (see da Silva and Langguth, 1989: fig 1a), Caluromysiops, and some species of Caluromys. The tail is a slender, muscular organ in most didelphids, but Thylamys and Lestodelphys have incrassate tails (Morton, 1980) in which fat is seasonally deposited. Incrassate tails can be recognized superficially by their characteristically swollen outline and soft texture in fresh and fluid-preserved material, and by their flattened, grease-stained appearance in most skins. When the external aspect of the tail leaves some room for doubt, the presence or absence of subcutaneous adipose tissue is easily determined by dissection. For example, a midventral incision that we made near the base of the tail of MZUSP 32097 provided unambiguous confirmation of Carmignotto and Monfort’s (2006) statement that the tail of Thylamys macrurus is incrassate (contra Palma, 1997; Creighton and Gardner, 2008). Among other marsupials, representative caenolestids, dasyurids, and peramelemorphians that we examined have nonprehensile tails that are uniformly pigmented, annularly scaled, and lack basal extensions of body pelage; most are slender, muscular organs, but those of Rhyncholestes and some dasyurids (e.g., Sminthopsis crassicaudata) are incrassate. The tail of Dromiciops is incrassate, annularly scaled, and distally prehensile, with a conspicuous extension of body pelage onto the basal 1/4 to 1/3 of the caudal dorsum. Cranium and Mandible The best general description of didelphid cranial morphology is Wible’s (2003) account of the skull of Monodelphis brevicaudata, which includes a review of the literature on didelphid cranial ontogeny, myology, and other relevant topics. Whereas Wible’s description is mostly limited to external cranial features, Rowe et al. (2005) provided a detailed description of internal structures of the nasal skeleton of a closely related species, M. domestica. Together, these publications provide detailed and abundantly illustrated accounts of the didelphid head skeleton as exemplified by two species in a single genus that is increasingly popular among biomedical researchers. Other didelphids, however, differ from Monodelphis in numerous aspects of cranial morphology (Voss and Jansa, 2003), including some characters that remain undescribed in the literature. Additionally, the literature contains no comprehensive account of the cranial features by which didelphids as a group differ from other marsupials, although important comparative observations were made by Osgood (1921), Tate (1947, 1948b), Archer (1976a), Reig et al. (1987), Marshall et al. (1995), Wroe (1997), Muizon (1998), and Sánchez-Villagra and Wible (2002), among others. Following a brief consideration of overall cranial shape, the following accounts summarize relevant cranial comparisons in a roughly anterior to posterior sequence along the dorsolateral contours of the skull, starting with the rostrum and nasal cavity and proceeding to the orbital region and braincase. A second sequence describes ventral cranial features beginning with the palate and proceeding to the basicranium and occiput. The principal osteological features of the didelphid skull and mandible are illustrated in figures 6 and 7. SHAPE VARIATION: Didelphids exhibit conspicuous variation in overall cranial shape that is illustrated in the taxonomic accounts below (figs. 37–54). Although some shape variation is obviously allometric (larger opossums tending to have relatively larger rostra and temporal fossae but relatively smaller orbits and braincases than smaller opossums), size-independent shape contrasts are also obvious. Thus, the rostrum is relatively short and broad in both Hyladelphys and Lutreolina, whereas the rostrum is relatively long and narrow in Marmosops and Metachirus. 5 A continuous taxonomic series of intermediate proportions makes any qual- 5 We quantified rostral proportions in exemplar skulls from taxa that were visually judged to represent morphological extremes in this respect. We measured rostral length (RL) from the anterior margin of one orbit to the tip of the ipsilateral nasal bone, and we measured skull size as condylobasal length (CBL). The values we obtained for the index (RL 4 CBL) 3 100 ranged from about 30% (in Lutreolina) to about 45% (in Metachirus).
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 27 Fig. 6. Dorsal and ventral cranial views of Marmosa murina showing principal osteological features mentioned in the text. Abbreviations: als, alisphenoid; atp, alisphenoid tympanic process; bo, basioccipital; bs, basisphenoid; cc, carotid canal; ect, ectotympanic; fm, foramen magnum; fpj, frontal process of jugal; fro, frontal; gf, glenoid fossa; gpa, glenoid process of alisphenoid; ip, interparietal; jug, jugal; lac, lacrimal; max, maxillary; mp, mastoid process (of petrosal); nas, nasal; occ, occipital condyle (of exoccipital); of, orbital fossa; pal, palatine; par, parietal; pcf, paracanine fossa; pogp, postglenoid process (of squamosal); pop, postorbital process; pre, premaxillary; prgp, preglenoid process (of jugal); pro, promontorium (of petrosal); ps, presphenoid; pt, pterygoid; rtp, rostral tympanic process (of petrosal); sq, squamosal; tcf, transverse canal foramen; tf, temporal fossa; za, zygomatic arch. itative coding of such variation an arbitrary exercise, but shape differences are visually conspicuous and are sometimes useful for diagnosing clades discovered by analyzing other data. The nascent literature on didelphid cranial morphometrics is not large, but it includes studies of intraspecific ontogenetic variation (Abdala et al., 2001; Flores et al., 2003), multivariate comparisons of conspecific species (e.g., Ventura et al., 1998, 2002; Cerqueira and Lemos, 2000), analyses of evolutionary rates (Lemos et al., 2001), geometric modeling of taxonomic variation (Astúa de Moraes et al., 2000), and prelim- inary attempts to explain shape differences in functional terms (e.g., Medellín, 1991). ROSTRUM: The premaxillae of many didelphids (Caluromysiops, Chacodelphys, Chironectes, Cryptonanus, Didelphis, Glironia, Hyladelphys, Lestodelphys, Lutreolina, Metachirus, Monodelphis, Philander, Thylamys, Tlacuatzin) are short, terminating abruptly in front of the incisors, and often lack a definitive suture between the left and right elements (fig. 8B). By contrast, in other didelphids the premaxillae are produced anteriorly as a more or less acutely pointed shelflike process that extends the bony
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