phylogenetic relationships and classification of didelphid marsupials ...

2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 87
Didelphids have often been compared with
dasyurids, but the literature contains no
adequate statement of the many characters
that distinguish these superficially similar yet
highly divergent clades. Among the external
and craniodental features treated in this
report, dasyurids consistently differ from
didelphids by their small, nonopposable
hallux; lack of a grooming claw on pedal
digit II; a nonprehensile tail (provided with a
terminal tuft of hairs that is never present in
opossums); anteriorly truncated and medially
notched nasals; a small interparietal that,
when present in juvenile skulls, is suturally
distinct from the supraoccipital; incomplete
posterolateral palatal foramina; an undivided
vomer that extends posteriorly to underlie the
presphenoid within the mesopterygoid fossa;
well-developed squamosal epitympanic sinuses;
four upper and three lower incisors; a
distinct posterior lobe on the unworn cutting
edge of i3; vestigial milk premolars; and a
distinct posterior cingulid on the lower
molars.
Originally described as a didelphid by
Thomas (1894) and long maintained in that
family by subsequent authors (e.g., Simpson,
1945), Dromiciops differs from opossums by
its basicaudal cloaca; a foramen rotundum
recessed in a common vestibule with the
sphenorbital fissure; frontal-squamosal contact;
a large interparietal that is suturally
distinct from the supraoccipital; an undivided
vomer that underlies the presphenoid within
the mesopterygoid fossa; left and right
pterygoid bones that contact one another in
the midline; keeled basisphenoid; a concealed
ectotympanic; a specialized malleus (with a
short uninflected neck, no orbicular apophysis,
and no lamina); lack of a paroccipital
process; a discontinuous stylar shelf on M1;
an unstaggered i2 alveolus; and a vestigial
anterior cingulid on the lower molars (of
which m1 lacks a hypoconulid notch).
Caenolestids, peramelids, and other marsupials
are sufficiently distinct from opossums
that explicit comparisons would be
pointless here. Some stem metatherians (e.g.,
herpetotheriids) that are strikingly similar to
didelphids in most respects (including ear
morphology; Gabbert, 1998) differ from
didelphids by having a distinct posterior
cingulid on the lower molars in addition to
the diagnostic endocranial and postcranial
features described by Sánchez-Villagra et al.
(2007). 23
REMARKS: A wide range of fossil taxa have
at one time or another been regarded as
didelphids (e.g., by Simpson, 1935, 1945;
Clemens, 1979; Marshall, 1981; McKenna
and Bell, 1997), but phylogenetic analyses
suggest that most of the extinct forms once
thought to be closely related to Recent
opossums (e.g., {Alphadon, {Andinodelphys,
{Glasbius, {Herpetotherium, {Jaskhadelphys,
{Pediomys) are stem metatherians and not
members of the crown group Marsupialia
(Rougier et al., 1998; Wible et al., 2001; Luo
et al., 2003; Sánchez-Villagra et al., 2007).
Herein we explicitly restrict Didelphidae to
living didelphimorphians, their most recent
common ancestor, and all of its descendants.
Even more restrictive concepts of Didelphidae
have been proposed, but none is
widely accepted. Hershkovitz (1992b), for
example, used Didelphidae to include just the
large opossums with 2n 5 22 chromosomes
(Chironectes, Didelphis, Lutreolina, Philander),
whereas Kirsch and Palma (1995)
excluded Glironia, Caluromys, and Caluromysiops
from the family. Because Didelphidae
in any of these applications (ours,
Hershkovitz’s, or Kirsch and Palma’s) is
monophyletic, the choice among them must
be justified by other criteria. In our view, the
name Didelphidae and its colloquial equivalent
(‘‘didelphids’’) are so deeply entrenched
in the literature as referring to all Recent
opossums that more restrictive applications
would serve no adequate compensatory
purpose.
Although didelphid monophyly is impressively
supported by nucleotide sequence data
23 As noted above, the posterior cingulid is absent in
didelphids, caenolestids, microbiotherians, and peramelids, so
it optimizes as an unambiguous marsupial synapomorphy on all
metatherian topologies in which these groups are recovered as
basal clades (e.g., Meredith et al. 2008). Because most
Cretaceous and early Tertiary metatherians are known only
from teeth, this trait provides a potentially useful criterion for
distinguishing stem taxa from members of the crown clade. For
example, several of the Paleocene Brazilian fossils classified as
didelphids by Marshall (1987) are described by that author as
having posterior cingulids and might plausibly be regarded on
that basis as stem metatherians. By contrast, Marsupialia (in the
crown-group sense) does not seem to be diagnosable by any
upper molar trait.