phylogenetic relationships and classification of didelphid marsupials ...

26 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
scales, each of which is soft and flexible
(yielding easily to the tip of a probe on fluidpreserved
material), the scales on the underside
of the base of the tail are heavily cornified,
forming hard raised tubercles in Glironia (see
da Silva and Langguth, 1989: fig 1a), Caluromysiops,
and some species of Caluromys.
The tail is a slender, muscular organ in
most didelphids, but Thylamys and Lestodelphys
have incrassate tails (Morton, 1980) in
which fat is seasonally deposited. Incrassate
tails can be recognized superficially by their
characteristically swollen outline and soft
texture in fresh and fluid-preserved material,
and by their flattened, grease-stained appearance
in most skins. When the external aspect
of the tail leaves some room for doubt, the
presence or absence of subcutaneous adipose
tissue is easily determined by dissection. For
example, a midventral incision that we made
near the base of the tail of MZUSP 32097
provided unambiguous confirmation of Carmignotto
and Monfort’s (2006) statement
that the tail of Thylamys macrurus is incrassate
(contra Palma, 1997; Creighton and
Gardner, 2008).
Among other marsupials, representative
caenolestids, dasyurids, and peramelemorphians
that we examined have nonprehensile
tails that are uniformly pigmented, annularly
scaled, and lack basal extensions of body
pelage; most are slender, muscular organs,
but those of Rhyncholestes and some dasyurids
(e.g., Sminthopsis crassicaudata) are incrassate.
The tail of Dromiciops is incrassate,
annularly scaled, and distally prehensile, with
a conspicuous extension of body pelage onto
the basal 1/4 to 1/3 of the caudal dorsum.
Cranium and Mandible
The best general description of didelphid
cranial morphology is Wible’s (2003) account
of the skull of Monodelphis brevicaudata,
which includes a review of the literature on
didelphid cranial ontogeny, myology, and
other relevant topics. Whereas Wible’s description
is mostly limited to external cranial
features, Rowe et al. (2005) provided a
detailed description of internal structures of
the nasal skeleton of a closely related species,
M. domestica. Together, these publications
provide detailed and abundantly illustrated
accounts of the didelphid head skeleton as
exemplified by two species in a single genus
that is increasingly popular among biomedical
researchers.
Other didelphids, however, differ from
Monodelphis in numerous aspects of cranial
morphology (Voss and Jansa, 2003), including
some characters that remain undescribed
in the literature. Additionally, the literature
contains no comprehensive account of the
cranial features by which didelphids as a
group differ from other marsupials, although
important comparative observations were
made by Osgood (1921), Tate (1947,
1948b), Archer (1976a), Reig et al. (1987),
Marshall et al. (1995), Wroe (1997), Muizon
(1998), and Sánchez-Villagra and Wible
(2002), among others. Following a brief
consideration of overall cranial shape, the
following accounts summarize relevant cranial
comparisons in a roughly anterior to posterior
sequence along the dorsolateral contours of
the skull, starting with the rostrum and nasal
cavity and proceeding to the orbital region
and braincase. A second sequence describes
ventral cranial features beginning with the
palate and proceeding to the basicranium and
occiput. The principal osteological features of
the didelphid skull and mandible are illustrated
in figures 6 and 7.
SHAPE VARIATION: Didelphids exhibit conspicuous
variation in overall cranial shape
that is illustrated in the taxonomic accounts
below (figs. 37–54). Although some shape
variation is obviously allometric (larger
opossums tending to have relatively larger
rostra and temporal fossae but relatively
smaller orbits and braincases than smaller
opossums), size-independent shape contrasts
are also obvious. Thus, the rostrum is
relatively short and broad in both Hyladelphys
and Lutreolina, whereas the rostrum is
relatively long and narrow in Marmosops and
Metachirus. 5 A continuous taxonomic series
of intermediate proportions makes any qual-
5 We quantified rostral proportions in exemplar skulls from
taxa that were visually judged to represent morphological
extremes in this respect. We measured rostral length (RL) from
the anterior margin of one orbit to the tip of the ipsilateral nasal
bone, and we measured skull size as condylobasal length (CBL).
The values we obtained for the index (RL 4 CBL) 3 100
ranged from about 30% (in Lutreolina) to about 45% (in
Metachirus).