phylogenetic relationships and classification of didelphid marsupials ...

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