phylogenetic relationships and classification of didelphid marsupials ...

2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 81
of our knowledge, no strongly conflicting
valid results have been obtained in any other
published phylogenetic analysis of molecular
or morphological character data. 22 Although
this tree is fully resolved with high support
values at most nodes, several outstanding
problems merit comment.
As we have previously noted elsewhere,
Chacodelphys exhibits conflicting patterns of
derived morphological similarities with Thylamys
+ Lestodelphys on the one hand and
with Monodelphis on the other (Voss et al.,
2004a). Weak parsimony support for nodes
along the phylogenetic path between Thylamys
+ Lestodelphys and Monodelphis in the
combined-data analysis that includes Chacodelphys
(fig. 36) presumably reflects such
character conflict, although it is noteworthy
that Bayesian support for the same nodes is
unaffected by taxon addition. Although we
are convinced that Chacodelphys is closely
related to Lestodelphys and Thylamys by a
host of phenotypic resemblances too indefinite
to code as characters but too numerous
to ignore, a compelling analytic solution to
this vexing problem is unlikely to be forthcoming
until at least some of the missing
molecular data for Chacodelphys can be
obtained from fresh material.
However, even substantial amounts of
sequence data are clearly not enough to
resolve other phylogenetic uncertainties. Indeed,
it is not a little frustrating that, with
.7000 bp of protein-coding nuclear sequence
in hand, the relationships of Glironia, Cryptonanus,
and Tlacuatzin should still be
problematic. Glironia is of special concern,
because the two plausible phylogenetic resolutions
for this genus (fig. 34A, B) determine
the root of the didelphid radiation. The
alternative phylogenetic resolutions of Cryptonanus
and Tlacuatzin within their respective
groups do not seem like comparably weighty
issues, but each could affect ecobehavioral
character optimizations of significant evolu-
22 DNA-DNA hybridization results showing Gracilinanus
nested within Marmosops (Kirsch and Palma, 1995; Kirsch et al.,
1997) were based on taxonomic misidentifications (Voss and
Jansa, 2003: 57). Analyses of 12S gene sequences by Palma and
Spotorno (1999) recovered Metachirus and Marmosops as sister
taxa, but this grouping was not found in subsequent analyses of
12S sequence data (Steiner et al., 2005) for reasons that remain
unexplained.
tionary interest. Phenotypically, Cryptonanus
is certainly more similar to Gracilinanus than
it is to Thylamys or Lestodelphys, whereas
Tlacuatzin is undeniably more similar to
Marmosa than it is to Monodelphis. The
positions of these genera in our combineddata
trees are therefore plausible despite the
absence of compelling parsimony or Bayesian
support.
Although it might seem best to wait until
these few remaining issues are convincingly
resolved before proposing a formal classification,
there is no guarantee that additional
data will soon be forthcoming or useful. In
the meantime, other relationships that are
strongly supported by our results merit
nomenclatural recognition, and the contents
of some taxa currently recognized as valid
need to be revised on the basis of our analytic
results. As documented below, no previous
classification is consistent with what is now
confidently known about didelphid phylogenetic
relationships.
CLASSIFICATION
Most essays on marsupial classification
(e.g., Simpson, 1945; Kirsch, 1977a; Marshall,
1981; Aplin and Archer, 1987; Archer
and Kirsch, 2006) are uninformative about
the historical development of opossum systematics.
To be sure, the chronological and
bibliographic details of didelphid taxonomy are
of limited interest, so the following paragraphs
mention only a few milestones on the long road
to the currently accepted classification. Because
most of the technical information about how
and why names were formerly applied to taxa
was recently summarized by Gardner (2008),
this brief narrative is primarily intended to
serveasanintroductiontotherevisedphylogeneticsystemthatwepropose.
Linnaeus (1758) described only five species
of marsupials, all of which were didelphids.
Although the four Linnaean species currently
recognized as valid (marsupialis, philander,
opossum, and murina) are now referred to
different genera, the great Swede placed all of
them in the genus Didelphis. New generic
names for opossums proliferated over subsequent
decades of the 18th and 19th centuries
(table 15), but no consistent binomial usage
had emerged prior to Thomas’s (1888) land-