phylogenetic relationships and classification of didelphid marsupials ...

88 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
(e.g., Jansa and Voss, 2000; Amrine-Madsen
et al., 2003; Meredith et al., 2008), unambiguous
morphological synapomorphies of the
family remain to be confidently identified.
Because our analyses did not include any
nonmarsupial outgroup, the position of the
marsupial root node was not determined, and
the phylogenetic interpretation of characterstate
transformations on the branch separating
didelphids from nondidelphid marsupials
is correspondingly equivocal. If, as most
recent analyses suggest, didelphids are the
basalmost branch of Marsupialia (Nilsson et
al., 2004; Amrine-Madsen et al., 2003;
Horovitz and Sánchez-Villagra, 2003; Asher
et al., 2004; Sánchez-Villagra et al., 2007;
Meredith et al., 2008; Beck, 2008), then such
transformations might be didelphid synapomorphies,
or they could be synapomorphies
of the unnamed clade that includes caenolestids
and Australidelphia.
Comparisons with stem metatherians that
are believed to be close outgroups to
Marsupialia, including {Mayulestes (see Muizon,
1998), {Pucadelphys (see Marshall et al.,
1995), and {Herpetotherium (see Gabbert,
1998; Sánchez-Villagra et al., 2007) suggest
that most of the craniodental traits by which
didelphids differ from other marsupials are
plesiomorphic, but one exception merits
comment. Bone homologies on the posterodorsal
braincase have received little attention
in the literature, but they appear to be
phylogenetically informative at many taxonomic
levels within Metatheria. In particular,
the presence of a large undivided interparietal
bone that is wedged between the parietals
anteriorly and fused to the supraoccipital
posteriorly may be a didelphid synapomorphy.
According to Muizon (1998: 38, fig. 6),
paired interparietal ossifications are fused to
the parietals in {Mayulestes, but the basis for
this interpretation (which has no analog
among living metatherians) is not explained,
and it seems equally possible that the
interparietal(s) is(are) absent in this taxon.
In {Pucadelphys, paired interparietals (‘‘postparietals’’)
are suturally distinct from the
parietals and from the supraoccipital (Marshall
et al., 1995: 50, fig. 12). The occiput of
{Herpetotherium has not previously been
described, but in a well-preserved specimen
that we examined (127684 in the AMNH
vertebrate paleontology catalog) there is no
bone wedged between the parietals and the
supraoccipital, which share a suture just
behind the lambdoid crest. Therefore, the
available evidence suggests that the didelphid
condition is unique.
Other alleged morphological synapomorphies
of didelphids include features of the
spermatozoa (Temple-Smith, 1987), postcranial
skeleton (Horovitz and Sánchez-Villagra,
2003), and petrosal (Ladevèze, 2007). However,
the phylogenetic interpretation of such
traits is compromised by sparse ingroup
sampling. Spermatozooa, for example, have
not been studied from many genera (e.g.,
Caluromysiops, Glironia, Hyladelphys, Lutreolina,
Marmosops, Thylamys), and published
phylogenetic analyses of postcranial
and petrosal characters have not included
basal didelphids (analyzed didelphid terminal
taxa in both of the osteological studies cited
above belong to the subfamily Didelphinae).
Hopefully, future studies will help fill in
many of these taxonomic gaps and contribute
to a better assessment of anatomical character
support for didelphid monophyly.
Only Recent didelphids are formally classified
and described below, but several South
American Neogene genera represented by
well-preserved cranial material ({Hyperdidelphys,
{Thylatheridium, {Thylophorops) are
clearly members of the didelphimorph crown
clade. Pending a phylogenetic analysis of
their relationships with living forms, we
follow current paleontological judgments of
taxonomic affinity in suggesting where these
fossils belong. By contrast, most other fossil
didelphids are only represented by dental
fragments from which few characters can be
scored, and their relationships to extant taxa
are correspondingly ambiguous.
Subfamily Glironiinae, new
CONTENTS: Glironia.
DIAGNOSIS: Members of this clade differ
from other didelphids by the extension of soft
fur along the dorsal surface of the tail from
base to tip (the dorsal surface of the tail is
macroscopically naked distally in other opossums),
by strongly recurved and laterally
compressed manual claws (manual claws are
much less strongly recurved and laterally