phylogenetic relationships and classification of didelphid marsupials ...

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-