phylogenetic relationships and classification of didelphid marsupials ...

22 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
ular morphology, Chironectes is unique
among didelphids in lacking any trace of
pedal plantar pads.
The eleuthrodactylous hind foot of Dromiciops
resembles the common didelphid
morphotype in having a large opposable
hallux, a grooming claw on dII, dIV . dIII,
and dermatoglyph-bearing plantar pads
(Hershkovitz, 1999: fig. 21). By contrast, the
hind foot of peramelids is syndactylous (with
fused dII and dIII; Hall, 1987; Weisbecker
and Nilsson, 2008); the hallux is small and
effectively nonopposable in caenolestids (Osgood,
1921: pl. 2, fig. 3), dasyurids (Thomas,
1888: pl. 23, fig. 8), and peramelids (Lyne,
1951: fig. 12); dIII and dIV are subequal in
examined caenolestids and dasyurids, both of
which groups also lack a grooming claw on
dII; peramelids do not have distinct plantar
pads; the plantar pads of caenolestids are
smooth; and the plantar pads of some
dasyurids are tubercular. The plantar epithelium
of the heel is macroscopically naked in
most of these outgroup taxa, but the underside
of the heel is coarsely furred in some
dasyurids (e.g., Sminthopsis crassicaudata).
POUCH AND MAMMAE: Based on our firsthand
examination of parous adult female
specimens, pouchlike enclosures for nursing
young are unequivocally present or absent
among didelphids. Although our sample sizes
were always small, we observed no intraspecific
variation in this aspect of female
reproductive morphology, nor did we observe
any intermediate condition between
absence and presence of a pouch. Despite
the fact that several distinctly different pouch
configurations can be recognized among
opossums, we provisionally recognize all
pouches as homologous in the absence of a
priori evidence to the contrary.
We found no trace of a pouch in suitable
material (fluid-preserved specimens and carefully
prepared skins) of parous adult female
Cryptonanus, Glironia, Gracilinanus, Hyladelphys,
Lestodelphys, Marmosa, Marmosops,
Metachirus, Monodelphis, Thylamys, and
Tlacuatzin. By contrast, well-developed
pouches were consistently found to be
present in suitably prepared parous adult
females of Caluromys, Chironectes, Didelphis,
and Philander. Although Lutreolina was
described as pouchless by Thomas (1888),
Cabrera (1919), and Marshall (1978a), two
fluid-preserved parous females that we examined
(UMMZ 166634, USNM 536827)
had pouches exactly resembling the morphology
illustrated and described by Krieg (1924:
fig. 11). Caluromysiops is said to have a
pouch (Izor and Pine, 1987; Reig et al., 1987),
but no explicit description or illustration of
the female reproductive anatomy of this
genus is available, nor were we able to
examine suitably preserved parous female
specimens. The presence or absence of a
pouch remains undocumented for many
opossums, notably Chacodelphys. Contradictory
literature accounts of a pouch as present
or absent in Metachirus were discussed by
Voss and Jansa (2003).
The marsupium of Caluromys philander
uniquely consists of deep lateral skin folds
that enclose the nursing young and open in
the midline (resembling the morphology that
Tyndale-Biscoe and Renfree [1987: fig. 2.8]
incorrectly attributed to didelphids in general).
In Caluromys lanatus, Didelphis, and
Philander, however, the lateral pockets are
joined posteriorly, forming a more extensive
enclosure that opens anteriorly (Enders,
1937: fig. 19). Yet another condition is
exhibited by Chironectes and Lutreolina, in
which the lateral pockets are connected
anteriorly, forming a marsupium that opens
posteriorly (Krieg, 1924: fig. 11A; Oliver,
1976: fig. 1B).
In all marsupials that possess a pouch the
mammae are contained within it, but the
mammae of pouchless taxa are variously
distributed (Tate, 1933: fig. 3). In most
pouchless didelphids (e.g., Glironia, Marmosa,
Metachirus) the mammae are confined
to a more or less circular inguinal/abdominal
array that occupies the same anatomical
position as the pouch in taxa that possess a
marsupium. However, other pouchless opossums
(e.g., Cryptonanus guahybae, Marmosops
incanus) have bilaterally paired mammae
that extend anteriorly well beyond the pouch
region. Although most of these anterior teats
are not actually located on the upper chest,
they are usually referred to as ‘‘pectoral’’ or
‘‘thoracic’’ mammae (e.g., by Tate, 1933;
Reig et al., 1987).
Most didelphids have, in addition to
bilaterally paired mammae, an unpaired