phylogenetic relationships and classification of didelphid marsupials ...

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 of
Cryptonanus, Gracilinanus, Marmosops, Tlacuatzin,
and most species of Marmosa. In
addition, we observed medial carpal tubercles
(Lunde and Schutt, 1999: fig. 3) in Marmosa
demerarae, M. mexicana, M. paraguayana,
M. regina, M. robinsoni, and M. rubra. Other
didelphids appear to lack sexually dimorphic
carpal tubercles, 2 but we were not able to
determine whether or not such structures are
present in Chacodelphys and Glironia because
no fully adult male specimens of 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 and adults of both sexes
possess a large, fleshy process on the outside
of the wrist, resembling a sixth finger, that is
supported internally by the pisiform (Augustiny,
1942: fig. 16; Mondolfi and Medina,
1957: fig. 14; Oliver, 1976: fig. 1b).
No examined nondidelphid marsupial has
carpal tubercles of any kind.
MANUS: The didelphid hand is provided
with five well-developed clawed digits (fig. 3).
When the digits are flexed, their tips converge
toward the center of the palm, and it seems
likely that at least the arboreal and scansorial
forms—which tend to have relatively longer
fingers than terrestrial taxa (Lemelin, 1999;
Kirk et al., 2008)—are capable of manual
prehension. In most didelphids, the manual
digits are more or less evenly spaced, but in
some arboreal taxa (e.g., Caluromys, Caluromysiops,
Marmosa) the gap between dII and
dIII is somewhat larger than the gaps
separating other pairs of adjacent fingers,
and it is possible that these taxa are
incipiently schizodactylous (sensu Haines,
2 Prochel and Sánchez-Villagra (2003) reported that adult
males of Monodelphis domestica, a species that lacks external
evidence of sexual dimorphism in the wrist, nevertheless have
significantly larger and more robust pisiforms than females. This
condition could obviously be interpreted as a state intermediate
to complete absence of sexual dimorphism in the wrist on the
one hand and the very marked dimorphism exhibited by species
with male carpal tubercles on the other.
1958). The pollex (dI) of Chironectes is set
off from the other manual digits by a wide
gap and appears to be pseudo-opposable
(sensu Napier, 1961); in fluid-preserved
specimens it is often folded across the palm
(Augustiny, 1942: fig. 16).
According to Tate (1947), the third and
fourth digits of the didelphid manus are
subequal and longer than the other fingers
(fig. 3C), proportions that correspond to the
paraxonic morphotype defined by Brown
and Yalden (1973). Not all didelphids 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), Philander, and Thylamys.
Yet another alternative condition is seen
in Caluromys and Caluromysiops, in which
dIV is slightly but distinctly longer than dIII
(fig. 3D).
Many didelphids have small, weakly recurved
manual claws that do not extend
much (if at all) beyond the fleshy apical pad
of each digit (fig. 3B, C), but some arboreal
taxa such as Caluromys (fig. 3D) and 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 of Glironia (not illustrated) are
unusually deep, laterally compressed, and
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 and Yalden, 1973),
didelphids usually have two well-developed
metacarpal pads (thenar, hypothenar) and
four interdigital pads on the hairless ventral
(plantar or volar) surface of the manus. These
six pads encircle a central palmar region that
is either smooth or sparsely tubercular (as in
most didelphids; see Hershkovitz, 1997: fig.
6A) or densely covered with small convex
tubercles (as in Chacodelphys, Lestodelphys,
and most species of Thylamys; see Carmignotto
and Monfort, 2006: fig. 3C). In
almost all species, the epidermis that covers