phylogenetic relationships and classification of didelphid marsupials ...

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20 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 The plantar pads of two Brazilian species of Thylamys (illustrated by Carmignotto and Monfort, 2006: figs. 3a, 3b) are fused together and fill the entire palmar region rather than encircling a central palmar surface as in other didelphids. Additionally, only the apices of the plantar pads are provided with dermatoglyphs in T. velutinus, whereas the pads of T. karimii lack dermatoglyphs completely. Instead, the plantar pads are mostly (T. velutinus) or entirely (T. karimii) covered with small convex tubercles like those that occur on the center of the palm in other species of Thylamys. The ventral surface of the manus in the water opossum uniquely lacks any trace of plantar pads. Instead, the palm of Chironectes is essentially flat and densely covered with microscopically dentate tubercles whose fine structure was described and illustrated by Brinkmann (1911) and Hamrick (2001). Numerous smooth, hemisperical papillae (not mentioned by Hamrick, 2001), however, are scattered at regular intervals among the dentate tubercles and presumably have a different function (Brinkmann, 1911). Among other plesiomorphic marsupials, dasyurids and Dromiciops most closely resemble didelphids in manual morphology, both taxa having five well-developed clawed digits. However, whereas dIII and dIV are subequal in Dromiciops, dIII is distinctly longer than dIV in dasyurids. By contrast, dI and dV are conspicuously reduced and nail bearing in caenolestids (Osgood, 1921: pl. 2, fig. 4), and the same digits are vestigial—lacking claws or nails—in peramelemorphians (Lyne, 1951: fig. 11); of the remaining manual digits, dIII is distinctly the longest in both of these groups. Manual plantar pads are indistinct or absent in all examined peramelemorphians but they are distinct in dasyurids, caenolestids, and Dromiciops. Whereas the plantar pads of Dromiciops and at least some dasyurids (e.g., Murexia) have dermatoglyphs, the plantar pads of other dasyurids (e.g., Sminthopsis crassicaudata) are tubercular, and those of caenolestids are smooth. The central palmar surface is essentially smooth (or irregularly creased) in caenolestids and Dromiciops, but it is densely tubercular in examined dasyurids and peramelids. Fig. 4. Plantar view of right hind foot of Gracilinanus marica (redrawn from Boas, 1918: pl. 1, fig. 2). Pedal characters that distinguish didelphids from most other marsupials include eleuthrodactyly, a large opposable hallux (digit I), a grooming claw (Putzkralle) on digit II, and prominent dermatoglyph-bearing plantar pads. PES: Most didelphids have an eleuthrodactylous hind foot with five well-developed digits that are all separate and freely movable (fig. 4). The only exception is Chironectes, whose pedal digits are bound together by webs of skin to form a paddlelike swimming organ (illustrated by Augustiny, 1942: Mondolfi and Medina, 1957; Oliver, 1976; Hershkovitz, 1997). Although Bensley (1903), Tate (1933), and Kirsch (1977b) suggested that some didelphids exhibit incipient fusion of dII and dIII, we have not observed any specimen with a hind foot that even remotely resembles the syndactylous condition seen in peramelemorphians and diprotodontians. 3 3 Weisbecker and Nilsson (2008) likewise concluded that didelphids do not exhibit incipient syndactyly. The illustration in Hall (1987: fig. 1n) that appears to show an incipiently syndactylous foot in ‘‘Didelphis microtarsus’’ (5 Gracilinanus microtarsus) is a grotesque cartoon that does not accurately depict the pedal morphology of that species or any other opossum (for an anatomically accurate illustration of the hind foot of Gracilinanus, see fig. 4).
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 21 The didelphid hallux (dI) is well developed, set off at a wide angle from the other pedal digits, and fully opposable. As in other marsupials, the hallux is claw- and nailless in opossums. Although this digit tends to be maximally developed in arboreal forms, it is always long enough to contact the tips of the other digits when they are flexed, even in such strictly terrestrial taxa as Lestodelphys, Lutreolina, and Monodelphis. The remaining pedal digits bear welldeveloped claws that are laterally compressed and dorsoventrally recurved like those of most other plantigrade mammals (Brown and Yalden, 1973). The claws on dIV and dV are more or less symmetrically crescentic, with an unguis that is equally extensive on the axial and abaxial surfaces, and a subunguis that is only exposed ventrally. Digit II, however, bears a strikingly asymmetrical grooming claw (Putzkralle; Boas, 1918) with an abaxially exposed subunguis and a rounded, spoon-shaped apex. The subunguis is sometimes also exposed abaxially on the claw of dIII, but the asymmetry is consistently less than that of the second digital claw, and the third claw never has a rounded tip. In the strictly terrestrial opossums Lestodelphys, Lutreolina, and Monodelphis, the third pedal digit is longer than the adjacent second and fourth digits, and the hind foot is therefore mesaxonic (sensu Brown and Yalden, 1973). By contrast, dII, dIII, and dIV are subequal (none distinctly longer than the others) in Didelphis albiventris and D. virginiana. Among all other examined didelphid taxa (including Didelphis marsupialis), the second, third, and fourth pedal digits progressively increase in length, such that dIV is the longest toe opposing the hallux. It is noteworthy that the relative lengths of pedal digits do not strictly covary with those of the manual digits, as they might be expected to do if transformations of the pes and manus were functionally or developmentally determined by the same factors. For example, whereas dIV is the longest pedal digit in Marmosops, dIII is the longest manual digit in that genus. In most didelphids (e.g., Philander; Hershkovitz, 1997: fig. 6B) the entire ventral surface of the hind foot is macroscopically naked from the apex of the heel to the tips of the toes. Under high magnification, a fine plantar pelage of very short hairs partially or completely covers the heel in some taxa (e.g., Marmosops), but intermediate conditions in other forms and variation among conspecific individuals suggest that such microscopic details are not a reliable basis for either phylogenetic analysis or taxonomic diagnosis. By contrast, a morphologically distinctive condition occurs in Lestodelphys and Thylamys (see Carmignotto and Monfort, 2006: fig. 3), whose heels are entirely covered by coarse (macroscopically visible) fur. Most didelphids have six separate plantar pads like many other plantigrade mammals (Brown and Yalden, 1973), but several patterns of taxonomic variation are noteworthy: (1) The position normally occupied by the hypothenar (lateral tarsal) and fourth interdigital pads in most didelphids is occupied by a single elongate pad in Caluromys, Caluromysiops, and Glironia; although this large structure is plausibly interpreted as the result of fusion (hypothenar + interdigital 4), this scenario is complicated by the occasional presence (e.g., in AMNH 273038) of a small proximal metatarsal pad that might be a vestigial hypothenar. (2) The hypothenar is absent or variably present but clearly vestigial in Monodelphis, Lutreolina, Philander, and Didelphis. (3) The thenar (medial tarsal) and first interdigital pads are often in contact or indistinguishably fused among arboreal taxa (e.g., Caluromys and Marmosa), but a continuum of intermediate conditions precludes any unambiguous distinction between these morphologies and the separate conditions seen in terrestrial forms (e.g., Monodelphis and Thylamys). (4) Interdigital 2 is much larger than interdigital 3 in most arboreal/scansorial forms (e.g., Marmosa, Marmosops), but they are subequal in some terrestrial taxa (e.g., Monodelphis and Lutreolina). The pedal plantar pads of most didelphids are provided with dermatoglyphs like those of the manual plantar pads. However, the dermatoglyph-bearing epithelium is much reduced in Thylamys velutinus, and in T. karimii the pedal plantar pads are entirely tubercular (lacking dermatoglyphs completely; Carmignotto and Monfort, 2006: fig. 3). As in so many other aspects of its appendic-
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