phylogenetic relationships and classification of didelphid marsupials ...

12 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
mation. 1 In effect, such information comes
from commonly available materials that can
be examined without dissection or other
special preparations. Therefore, microscopic
features (e.g., those of the spermatozoa;
Temple-Smith, 1987) and visceral characters
(e.g., of the digestive tract; Santori et al.,
2004) are not reviewed below, nor are
osteological traits that require X-ray computed
tomography, serial sectioning, or
destructive methods for their study.
Size and External Features
Most opossums are externally unremarkable
mammals with pointed muzzles, large
rhinaria, well-developed vibrissae, prominent
eyes, membranous ears, nonspinous pelage,
subequal limbs, pentadactyl feet, and naked
tails. In many of these respects they resemble
other plesiomorphic marsupials (e.g., Dromiciops
and dasyurids) as well as certain
unspecialized placentals (e.g., solenodontids,
rice tenrecs, gymnures, and tree shrews).
Closer inspection, however, reveals numerous
distinctive and phylogenetically informative
details of didelphid external morphology.
SIZE: Didelphids are small to mediumsized
mammals. The smallest Recent species
is probably Chacodelphys formosa, the young
adult holotype of which had a head-andbody
length of 68 mm and probably weighed
about 10 g (Voss et al., 2004a). By contrast,
the largest living opossum, Didelphis virginiana,
can measure almost 500 mm in headand-body
length and weigh more than 3000 g
(Hamilton, 1958). Most didelphids, however,
range in head-and-body length from about
100 to 300 mm and weigh between about 20
and 500 g (table 4). Although some authors
have recognized ‘‘large’’, ‘‘medium’’, and
‘‘small’’ opossums, taxonomic assignments
to discrete size categories are often arbitrary
and sometimes misleading. For example,
Reig et al. (1987: character 25) scored
Metachirus as ‘‘large’’ and Caluromys as
‘‘medium’’ in their phylogenetic analysis,
but linear measurements and weights that
we compiled suggest that these taxa are
indistinguishable in size. More taxonomically
1 An important exception is postcranial skeletal morphology,
the topic of an independent study (Flores, 2009).
comprehensive compilations of morphometric
data will probably document a continuum
of didelphid size distributions.
By comparison, Old World marsupials
include some species that are smaller and
others that are much larger than any
didelphid. The smallest living Australian
species, for example, is said to be Planigale
ingrami, with an adult weight of only about
4 g, and the largest is Macropus rufus, males
of which are said to weigh as much as 85 kg
(Dawson et al., 1989). The extinct Pleistocene
species Diprotodon optatum, however, may
have weighed almost 2800 kg (Wroe et al.,
2004).
RHINARIUM AND MOUTH: The rhinarium,
a prominent pad of naked glandular skin
surrounding the nostrils, is divided by a
median crease or sulcus (sulcus medianus;
Ade, 1999) that extends from between the
nares to the upper lip in all examined
didelphids. The part of the rhinarium that
borders the upper lip (pars supralabialis) is
broad, and its ventral margin is notched by
one or two distinct grooves on each side of
the median sulcus (fig. 1). Two ventrolateral
grooves are present on each side in most
examined taxa, but only a single groove is
present in Chironectes, Didelphis, Lestodelphys,
Lutreolina, Metachirus, Monodelphis,
Philander, and Thylamys pallidior. Dasyurids
and Dromiciops have rhinaria that are
essentially similar in gross morphology to
those of didelphids, with a pars supralabialis
that makes broad contact with the upper lip;
however, only a single ventrolateral groove is
present in these taxa (Pocock, 1926: figs. 26–
28). By contrast, the supralabial part of the
rhinarium is reduced to a narrow philtrum in
caenolestids and peramelids, such that the
groove-bearing ventral rhinarial margin of
other marsupials is effectively absent.
The mouth is large in all didelphids, with a
posterior angle (angulis oris; Brown, 1971)
that extends posteriorly to a point below the
eye; the upper and lower oral margins are
smooth or irregularly wrinkled and anatomically
featureless. Most other marsupials have
anatomically featureless oral margins like
those of didelphids, but caenolestids are
uniquely provided with reciprocating fleshy
lappets of unknown function on the upper
and lower lips (Osgood, 1921: pl. 2; Bublitz,