18 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 Lateral carpal tubercles (fig. 2) are more widespread than medial carpal tubercles, occurring in all taxa that exhibit any externally obvious sexual dimorphism in the wrist. We found these structures to be consistently present in large adult male specimens <strong>of</strong> Cryptonanus, Gracilinanus, Marmosops, Tlacuatzin, <strong>and</strong> most species <strong>of</strong> Marmosa. In addition, we observed medial carpal tubercles (Lunde <strong>and</strong> Schutt, 1999: fig. 3) in Marmosa demerarae, M. mexicana, M. paraguayana, M. regina, M. robinsoni, <strong>and</strong> M. rubra. Other <strong>didelphid</strong>s appear to lack sexually dimorphic carpal tubercles, 2 but we were not able to determine whether or not such structures are present in Chacodelphys <strong>and</strong> Glironia because no fully adult male specimens <strong>of</strong> either genus are currently available for study. The water opossum Chironectes has carpal tubercles that, uniquely, are neither sexually dimorphic nor ontogenetically variable. In this taxon, juveniles <strong>and</strong> adults <strong>of</strong> both sexes possess a large, fleshy process on the outside <strong>of</strong> the wrist, resembling a sixth finger, that is supported internally by the pisiform (Augustiny, 1942: fig. 16; Mondolfi <strong>and</strong> Medina, 1957: fig. 14; Oliver, 1976: fig. 1b). No examined non<strong>didelphid</strong> marsupial has carpal tubercles <strong>of</strong> any kind. MANUS: The <strong>didelphid</strong> h<strong>and</strong> is provided with five well-developed clawed digits (fig. 3). When the digits are flexed, their tips converge toward the center <strong>of</strong> the palm, <strong>and</strong> it seems likely that at least the arboreal <strong>and</strong> scansorial forms—which tend to have relatively longer fingers than terrestrial taxa (Lemelin, 1999; Kirk et al., 2008)—are capable <strong>of</strong> manual prehension. In most <strong>didelphid</strong>s, the manual digits are more or less evenly spaced, but in some arboreal taxa (e.g., Caluromys, Caluromysiops, Marmosa) the gap between dII <strong>and</strong> dIII is somewhat larger than the gaps separating other pairs <strong>of</strong> adjacent fingers, <strong>and</strong> it is possible that these taxa are incipiently schizodactylous (sensu Haines, 2 Prochel <strong>and</strong> Sánchez-Villagra (2003) reported that adult males <strong>of</strong> Monodelphis domestica, a species that lacks external evidence <strong>of</strong> sexual dimorphism in the wrist, nevertheless have significantly larger <strong>and</strong> more robust pisiforms than females. This condition could obviously be interpreted as a state intermediate to complete absence <strong>of</strong> sexual dimorphism in the wrist on the one h<strong>and</strong> <strong>and</strong> the very marked dimorphism exhibited by species with male carpal tubercles on the other. 1958). The pollex (dI) <strong>of</strong> Chironectes is set <strong>of</strong>f from the other manual digits by a wide gap <strong>and</strong> appears to be pseudo-opposable (sensu Napier, 1961); in fluid-preserved specimens it is <strong>of</strong>ten folded across the palm (Augustiny, 1942: fig. 16). According to Tate (1947), the third <strong>and</strong> fourth digits <strong>of</strong> the <strong>didelphid</strong> manus are subequal <strong>and</strong> longer than the other fingers (fig. 3C), proportions that correspond to the paraxonic morphotype defined by Brown <strong>and</strong> Yalden (1973). Not all <strong>didelphid</strong>s have paraxonic forefeet, however. Instead, many have a mesaxonic manus in which dIII is distinctly longer than the other fingers; taxa that exhibit this condition include Chacodelphys, Chironectes, Didelphis, Lestodelphys, Lutreolina, Marmosops (fig. 3B), Metachirus, Monodelphis (fig. 3A), Phil<strong>and</strong>er, <strong>and</strong> Thylamys. Yet another alternative condition is seen in Caluromys <strong>and</strong> Caluromysiops, in which dIV is slightly but distinctly longer than dIII (fig. 3D). Many <strong>didelphid</strong>s have small, weakly recurved manual claws that do not extend much (if at all) beyond the fleshy apical pad <strong>of</strong> each digit (fig. 3B, C), but some arboreal taxa such as Caluromys (fig. 3D) <strong>and</strong> others that are strictly terrestrial such as Monodelphis (fig. 3A) have large manual claws that extend well beyond the apical pads. The large manual claws <strong>of</strong> Glironia (not illustrated) are unusually deep, laterally compressed, <strong>and</strong> strongly recurved. Although illustrated taxonomic differences in claw length are striking, intermediate morphologies observed among other forms (e.g., Hyladelphys) make it difficult to recognize discrete character states for taxonomic analysis. Like most other plantigrade mammals (Whipple, 1904; Brown <strong>and</strong> Yalden, 1973), <strong>didelphid</strong>s usually have two well-developed metacarpal pads (thenar, hypothenar) <strong>and</strong> four interdigital pads on the hairless ventral (plantar or volar) surface <strong>of</strong> the manus. These six pads encircle a central palmar region that is either smooth or sparsely tubercular (as in most <strong>didelphid</strong>s; see Hershkovitz, 1997: fig. 6A) or densely covered with small convex tubercles (as in Chacodelphys, Lestodelphys, <strong>and</strong> most species <strong>of</strong> Thylamys; see Carmignotto <strong>and</strong> Monfort, 2006: fig. 3C). In almost all species, the epidermis that covers
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 19 Fig. 3. Dorsal views <strong>of</strong> right forefeet <strong>of</strong> Monodelphis brevicaudata (A, AMNH 140466), Marmosops incanus (B, MVZ 197629), Marmosa robinsoni (C, AMNH 259983), <strong>and</strong> Caluromys phil<strong>and</strong>er (D, AMNH 7433) illustrating generic differences in claw size <strong>and</strong> digital proportions. Monodelphis <strong>and</strong> Marmosops both have mesaxonic forefeet in which the third digit (dIII) is longest, whereas Marmosa has a paraxonic forefoot in which dIII <strong>and</strong> dIV are subequal. In Caluromys, dIV is the longest manual digit. the plantar pads is sharply differentiated from that <strong>of</strong> the surrounding plantar surface because it is provided with dermatoglyphs (friction or papillary ridges; Hamrick, 2001) resembling those on human fingertips. Plantar pads tend to be larger <strong>and</strong> to have more pronounced dermatoglyphs in arboreal opossums (e.g., Caluromys) than in terrestrial forms (e.g., Monodelphis), but a few <strong>didelphid</strong>s exhibit qualitatively different morphologies.
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