phylogenetic relationships and classification of didelphid marsupials ...

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24 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 and share a continuous mucosa. In life, both openings are normally recessed in a common sinus (cloaca), but this feature is obscured in male specimens with everted genitalia. Chironectes uniquely lacks a cloaca because the urogenital and rectal openings are widely separated by furred inguinal skin (Nogueira et al., 2004: fig. 1). Conflicting descriptions of didelphid cloacal morphology by Hershkovitz (1992a, 1992b) were discussed and refuted by Voss and Jansa (2003). All examined nondidelphid marsupials have an inguinal cloaca resembling the common didelphid condition, with the conspicuous exception of Dromiciops, in which the cloaca is basicaudal (Hershkovitz, 1999: fig. 12). Descriptions of didelphid male genitalia (e.g., by Ribeiro and Nogueira, 1990; Martinelli and Nogueira, 1997; Nogueira et al., 1999a, 1999b; Nogueira and Castro, 2003) were recently reviewed and summarized by Nogueira et al. (2004), who tabulated morphological comparisons among 18 species in 12 genera. All examined opossums have a bifid penis (contra Hershkovitz, 1992b), but didelphid male genitalia exhibit conspicuous taxonomic variation in length, shape, disposition of the urethral grooves, presence/ absence of diverticula, and other details. Unfortunately, the genitalic characters of many species remain unstudied, and we were unable to extend Nogueira et al.’s (2004) observations for this report. TAIL: Most didelphids have a tail that is substantially longer than the combined length of the head and body, but some taxa are much shorter tailed (table 5). The known range of relative tail length in the family is bracketed on the one hand by Gracilinanus emiliae, an arboreal species with a tail that may be almost twice as long as the combined length of its head and body (Voss et al., 2005: table 2), and on the other by some terrestrial forms (e.g., Monodelphis spp.) with a tail that may be less than half as long as the head and body. However, arboreal taxa are not always longer tailed than terrestrial forms. Treedwelling Glironia venusta, for example, has a tail that is subequal in length to its head and body, whereas ground-dwelling Metachirus nudicaudatus is much longer tailed. Body pelage (soft fur composed of ordinary coat hairs that are not associated with TABLE 5 Relative Tail Length of Exemplar Species from 18 Didelphid Genera LT/HBL a Caluromys (Caluromys) philander 1.49 Caluromys (Mallodelphys) lanatus 1.52 Caluromysiops irrupta 1.19 Chacodelphys formosa 0.81 b Chironectes minimus 1.20 Cryptonanus unduaviensis 1.16 Didelphis marsupialis 1.04 Glironia venusta 1.07 Gracilinanus agilis 1.40 Hyladelphys kalinowskii 1.44 Lestodelphys halli 0.67 Lutreolina crassicaudata 0.96 Marmosa (Marmosa) murina 1.30 Marmosa (Micoureus) regina 1.46 Marmosops noctivagus 1.33 Marmosops pinheiroi 1.45 Metachirus nudicaudatus 1.32 Monodelphis emiliae 0.47 Philander opossum 1.01 Thylamys karimii 0.77 Thylamys macrurus 1.29 Tlacuatzin canescens 1.00 a Length of tail (LT) divided by length of head and body (HBL), computed from mean values in table 4 except as noted. b From external measurements of the holotype (Voss et al., 2004a). epidermal scales) extends to a variable extent onto didelphid tails. Body fur extends onto the tail much farther dorsally than ventrally in Glironia (see da Silva and Langguth, 1989: fig. 1), Caluromysiops, and some species of Caluromys and Monodelphis (see Voss et al., 2001: fig. 29A). By contrast, body fur extends onto the tail dorsally and ventrally to about the same extent (from about one-sixth to about one-third the length of that organ) in Caluromys philander, Chironectes, Didelphis, Lutreolina, Philander, and some species of Marmosa (e.g., M. paraguayana). In most other didelphids, however, body fur does not extend more than a short distance, if at all, onto the tail base. The exposed skin of didelphid tails is variously pigmented. Some species have uniformly pigmented (concolored) tails that are usually some shade of grayish or brownish, but pale markings are not uncommon.
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 25 Many species, for example, have parti-colored tails that are paler distally than basally. In most taxa with pale tail tips, the basal color is grayish or brownish and the transition is either gradual (the basal color fading to whitish distally; e.g., in Marmosops ocellatus) or irregularly mottled (e.g., as in Caluromys derbianus; frontispiece). However, in most specimens of Chironectes (frontispiece), Didelphis, Lutreolina, and Philander, the tail is blackish basally with an abruptly white tip. Dorsoventrally bicolored tails (which are grayish or brownish above and pale on the underside) represent another widespread pattern of didelphid caudal marking that is indistinct in some species but sharply defined in others. Different patterns of caudal markings often distinguish closely related species and can be used in combination with other external traits to identify congeneric taxa in the field (Voss et al., 2004b: table 4). The epidermal scales that cover the unfurred nonprehensile surfaces of didelphid tails differ taxonomically in shape and arrangement (Tate, 1933: fig. 2). Square or rectangular caudal scales in unambiguously annular series occur in Chacodelphys, Cryptonanus, Thylamys, Tlacuatzin, and most species of Gracilinanus. Although individual cutaneous scales are indistinct in Lestodelphys and Monodelphis, the annular arrangement of caudal bristles in both taxa suggest that they also conform to the Thylamys pattern. By contrast, rhomboidal (diamondshaped) or hexagonal scales in spiral series occur in Caluromys, Caluromysiops, Chironectes, Didelphis, Lutreolina, Marmosops, Philander, and most species of Marmosa. A few taxa, however, have caudal scales that appear to be morphologically intermediate between the annular and spiral morphotypes. In these taxa (e.g., Hyladelphys, Metachirus, Marmosa mexicana) the caudal scales are arrayed in annular series over the vertebral articulations and in more or less spiral series elsewhere. Glironia cannot be assigned to any of these caudal-scale patterns because the dorsal and lateral surfaces of its tail are covered by body fur and the entire ventral surface is modified for prehension (da Silva and Langguth, 1989: fig. 1). Three stiff, bristlelike hairs emerge from the posterior margin of each caudal scale in most didelphids, but four or more hairs usually emerge from each caudal scale in Lutreolina and Philander. Although individual cutaneous scales are hard to distinguish in Monodelphis, the caudal hairs of species that we examined usually emerge from the skin in triplets, so this genus appears to conform to the widespread didelphid condition. The number of hairs normally associated with each caudal scale is impossible to assess in Glironia (whose dorsal caudal surface is furry and whose ventral caudal surface is modified for prehension; see above and below) and Lestodelphys (on which no discrete clustering within rows of caudal hairs could be distinguished with confidence). The hairs that emerge from the posterior margin of each caudal scale are not grossly differentiated, varying in length but subequal in thickness, in most didelphids. However, in most species of Marmosops the middle hair of each caudal-scale triplet is conspicuously thicker than the lateral hairs, and it is often more darkly pigmented (Gardner and Creighton, 1989). Although all didelphids are perhaps capable of caudal prehension to some extent, external morphological features associated with this behavior are variably developed in the family. The unfurred caudal surfaces of Chacodelphys, Chironectes, Lestodelphys, Lutreolina, Metachirus, and Monodelphis are covered with unmodified scales from base to tip (for illustrations of Chironectes and Lutreolina, see Hershkovitz, 1997: fig. 7). In taxa conforming to this morphology, the tail tip is sometimes provided with a smooth terminal button but never with a ventrally expanded apical pad bearing dermatoglyphs. The tails of all other didelphids are provided with a distal prehensile surface that may be smooth or covered by modified scales (conspicuously unlike those of the caudal dorsum) but is always transversely creased and glabrous; in taxa conforming to this morphology, the tail tip is invariably provided with a ventrally expanded pad bearing dermatoglyphs (for illustrations of Didelphis and Philander, see Hershkovitz, 1997: fig. 7). Some taxa additionally exhibit caudal modifications for basal prehension. Whereas the unfurred ventral surface of the tail base in most didelphids is covered by smooth flat
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Page 5 and 6: ABSTRACT This report summarizes a d
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