phylogenetic relationships and classification of didelphid marsupials ...

16 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
the frons [forehead] past the inner edge of each
ear (not including it), and straight backward
through scapulae and hips, where they again
approach the median line of the body and
merge with the tail. This pair of lines encloses
the major part of the dorsal area of head and
body, the color of the area being very dark
brownish-gray or grayish fuscous. The fuscous
area is pointed at front, projecting forward
between the ears, and narrows again to a point
as it merges with the dark color of the upper
surface of the tail. The second [lateral] area,
light gray in color, frequently tinged with buffy
or yellowish, extends [on each side] between the
dark dorsal region and the edge of the belly
color at the normal transition line. Ventral color
either buffy, grayish, or snowy white.
The hair bases of the dorsal fur are heavily
pigmented, usually dark gray (or grayish), in
most didelphids, but species of Didelphis
uniquely exhibit white dorsal underfur.
The ventral pelage also exhibits noteworthy
taxonomic variation among didelphids.
In some species the ventral fur is ‘‘graybased,’’
a descriptor that applies when the
individual hairs are grayish basally and
abruptly paler (usually whitish, yellowish,
or brownish) distally; the overall color of the
ventral fur then depends on the degree to
which the dark basal pigmentation shows
through the paler superficial hue. In other
species, the ventral fur is partially or entirely
‘‘self-colored,’’ a descriptor that applies when
the individual hairs have the same pigmentation
(usually whitish) from root to tip.
Because marked differences in the patterning
of gray-based versus self-colored ventral pelage
can occur among closely related (congeneric)
species, such variation is often described
and illustrated in the revisionary taxonomic
literature (e.g., Patton et al., 2000: fig. 41).
However, the existence of numerous intermediate
conditions spanning the entire range of
taxonomic variation in ventral color patterns
(e.g., from entirely gray-based to completely
self-white ventral fur) precludes meaningful
phylogenetic scoring of this character.
Whereas most of the pelage colors (and
color patterns) described above are preserved
on museum skins that have been protected in
dark cabinets from the bleaching effects of
light, other colors that can be striking in life
fade quickly after death. The ventral pelage
of live Monodelphis emiliae, for example, has
been described as ‘‘bright glowing violaceous
pink’’ (Emmons, 1997: 34), a lurid hue that is
not retained in any examined museum
specimens. What little is known about such
fugitive pigments (some of which fluoresce
under ultraviolet light) was summarized by
Pine et al. (1985).
Most other marsupials have unremarkable
body pelage that essentially resembles the
common didelphid condition as described
above, but postcranial vibrissae are reduced
or absent in a few clades (Lyne, 1959) and
peramelemorphians have stiff, dorsally grooved
guard hairs (illustrated in cross section by
Lyne and McMahon, 1951: figs. 51, 60) that
impart a distinctively harsh, spinous texture
to their fur. Most other marsupials also
resemble didelphids in having unpatterned
dorsal fur, although there are noteworthy
examples of apparently convergent markings
(e.g., those of Dromiciops somewhat resemble
Chironectes’; Marshall, 1978b) and some
strikingly divergent ones (e.g., the transverse
sacral barring seen in Perameles gunnii; Lyne,
1951: pl. 1B). All examined outgroup taxa
have dark dorsal underfur.
WRIST: The wrists of males and females
are morphologically similar and externally
featureless in most didelphids, but striking
sexual dimorphism is present in certain small
arboreal and scansorial forms (Lunde and
Schutt, 1999). Grossly enlarged glabrous
tubercles supported internally by carpal
ossifications are exhibited by large adult
males of Cryptonanus, Gracilinanus, Marmosops,
Tlacuatzin, and some species of Marmosa.
Two distinct kinds of tubercles can be
distinguished, consisting of lateral (‘‘ulnar’’)
tubercles supported internally by the pisiform,
and medial (‘‘radial’’) tubercles supported
by the prepollex (op. cit.). Although
some intraspecific variation in the development
of carpal tubercles has been documented,
most of it can be attributed to ontogeny:
tubercles are consistently present in the
largest adult male specimens of species in
which such structures occur, whereas they
may be lacking in some smaller (presumably
younger) conspecific males. Lunde and
Schutt (1999) plausibly suggest that these
structures function as clasping devices during
copulation.