phylogenetic relationships and classification of didelphid marsupials ...

84 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
subfamily Didelphinae, and he did not use
tribal nomenclature to indicate relationships
among them. In effect, his treatment of
Recent didelphids amounted to no more
than an alphabetic list of generic names,
conveying even less information about inferred
relationships than the classifications of
his predecessors.
Cabrera’s (1958) checklist of South American
mammals was one of the last attempts to
classify Recent opossum diversity by traditional
(prephylogenetic) criteria, and it remained
more or less unchallenged until the
advent of molecular systematics in the mid-
1970s. Despite the fact that at least some
contemporaneous researchers (e.g., Reig,
1955) recognized suprageneric groups among
living opossumlike marsupials, Cabrera did
not. Instead, only his taxonomic sequence
and his use of subgenera for Monodelphis and
Marmosa hint at inferred patterns of relationships.
Most of the generic names that
Cabrera recognized as valid are still used in
the same sense today, with the conspicuous
exception of Marmosa: whereas the nominotypical
subgenus contained (in addition to
Marmosa sensu stricto) species now referred
to Marmosops, the subgenus Thylamys contained
(in addition to Thylamys sensu stricto)
species now referred to Chacodelphys, Cryptonanus,
and Gracilinanus.
Reig et al. (1985) provided the first classification
of opossumlike marsupials based on
an explicitly phylogenetic analysis (subsequently
published by Reig et al., 1987), and
their classification was also the first to
incorporate results from molecular and cytogenetic
research (Kirsch, 1977b; Reig et al.,
1977). Not surprisingly, this classification
(table 17) departed from its predecessors in
several respects. Among them, Dromiciops was
once and for all removed to a separate family,
‘‘caluromyines’’ and didelphines were distinguished
as separate subfamilies, and several
tribes of the latter were recognized: all of the
large opossums with a diploid number (2n) of
22 chromosomes were referred to the Didelphini,
all of the smaller taxa with 2n 5 14–18
chromosomes were referred to the Marmosini
(thus implying a close relationship between
Monodelphis and the ‘‘murine’’ opossums),
and a monotypic tribe was created for
Metachirus.
The classification proposed by Hershkovitz
(1992b) was not supported by any
phylogenetic analysis, nor were phylogenetic
criteria cited to support any of its novel
features, including the recognition of four
groups of Recent opossums as separate
families (Marmosidae, Caluromyidae, Glironiidae,
Didelphidae). Taxonomic rank aside,
these taxa represent significant departures
from previous classifications by grouping
Metachirus with Monodelphis and the ‘‘murine’’
opossums, by the absence of any formal
relationship between this group (Marmosidae)
and the large opossums with 22 chromosomes
(Didelphidae), and by the separation
of Glironia on the one hand from
Caluromys and Caluromysiops on the other.
Also unlike other classifications, four ‘‘murine’’
genera (Gracilinanus, Marmosops, Marmosa,
Micoureus) were associated in a single
taxon (Marmosinae) that excluded Thylamys
and Lestodelphys.
Kirsch and Palma (1995) formalized the
results of their DNA-DNA hybridization
experiments in a classification that, for the
first time, associated Metachirus (as a monotypic
tribe) with the large 2n 5 22 opossums
(Didelphini), associated Monodelphis with
Marmosa and Micoureus, and recognized
the other ‘‘murine’’ opossums (Gracilinanus,
Lestodelphys, Marmosops, Thylamys) as
forming a separate clade. All of these taxa
were referred to the family Didelphidae,
whereas Glironia, Caluromys, andCaluromysiops
were referred to the Caluromyidae.
McKenna and Bell’s (1997) classification
of Recent opossums followed Reig et al.
(1985), but they used the name Monodelphini
for the group that those authors called
Marmosini. This was the system subsequently
adopted by Gardner (2005, 2008) who
lumped all of the old ‘‘murine’’ genera
together with several new taxa in the tribe
Monodelphini. No subordinate taxa (within
Monodelphini) were recognized by these
authors to indicate relationships among the
many genera assigned to this group.
A Revised Phylogenetic System
As reviewed above, all previous classifications
of Recent opossums contain nonmonophyletic
groups. Examples include ‘‘Marmo-