phylogenetic relationships and classification of didelphid marsupials ...

ABSTRACT
This report summarizes a decade of morphological and molecular research on the
phylogenetic relationships of didelphid marsupials (opossums), a substantially intact radiation
of New World metatherian mammals. We review the comparative morphology of Recent
opossums, emphasizing those anatomical systems from which taxonomically useful information
is available for the majority of living genera and species, namely the integument, cranium, and
dentition. Morphological similarities and differences among didelphids and other plesiomorphic
marsupials (caenolestids, microbiotheriids, dasyurids, and peramelids) are also described. These
observations, representing evolved differences in diverse functional-morphological systems,
together with karyotypic information gleaned from the literature, provide the basis for coding
129 phylogenetic characters that we scored for 44 ingroup and seven outgroup taxa.
Published information about the size, internal organization, chromosomal location, and
physiological properties of five nuclear genes (BRCA1, DMP1, IRBP, RAG1, vWF) sequenced
for this study suggest that these loci are unlinked, exist as single copies, are active in different
tissues, and encode protein products with widely divergent functions. All of the sequenced
fragments are long (.900 bp), free of ingroup alignment ambiguities, and translate to open
reading frame. Nucleotide data from a total of 7320 aligned sites were obtained from 43 ingroup
and seven outgroup taxa.
Separate parsimony, likelihood, and Bayesian analyses of these six data partitions
(morphology + karyotypes, five genes) resulted in highly congruent estimates of didelphid
phylogeny with few examples of conflict among strongly supported nodes. Analyses of
concatenated sequences and combined (nonmolecular + sequence) datasets effectively summarize
all of the common signal recovered from separate analyses: a completely resolved ingroup
phylogeny with high support statistics at most nodes. Remaining problems (not conclusively
resolved in this study) include the position of the ingroup root and the relationships of three
genera (Chacodelphys, Cryptonanus, Tlacuatzin) within their respective suprageneric clades.
The history of didelphid classification is reviewed, and all previous systems are found to
contain nonmonophyletic groups. A revised phylogenetic classification consistent with our
analytic results includes the following higher taxa: Glironiinae (for Glironia), Caluromyinae
(Caluromys and Caluromysiops), Hyladelphinae (Hyladelphys), Didelphinae (Marmosini,
Metachirini, Didelphini, and Thylamyini), Marmosini (Marmosa, Monodelphis, andTlacuatzin),
Metachirini (Metachirus), Didelphini (Chironectes, Didelphis, Lutreolina, andPhilander), and
Thylamyini (Chacodelphys, Cryptonanus, Gracilinanus, Lestodelphys, Marmosops, andThylamys).
The probable relationships of several Neogene fossil genera are also discussed. To
facilitate identifications, all Recent genera are redescribed, representative crania are illustrated,
and a key is provided.
INTRODUCTION
They affirme that there are trees of suche
byggenes, that xvi men ioyninge handes togyther
and standinge in coompasse, can scarsely embrase
sum of them. Emonge these trees is fownde
that monstrous beaste with a snowte lyke a foxe,
a tayle lyke a marmasette, eares lyke a batte,
handes lyke a man, and feete lyke an ape, bearing
her whelpes abowte with her in an owtwarde
bellye much lyke vnto a greate bagge or purse. —
Richard Eden (1555), describing the first
European encounter with a marsupial, by Vicente
Yáñez Pinzón and his men on the coast of
Brazil in the year 1500
The oldest known metatherian mammals
occur in Cretaceous sediments of eastern
5
Asia and western North America (Rougier et
al., 1998; Luo et al., 2003; Kielan-Jaworowska
et al., 2004), and other early metatherians
are present in the Tertiary record of
Europe and Africa (Crochet, 1980; Kurz,
2007; Hooker et al., 2008). The metatherian
crown group Marsupialia, however, probably
evolved in South America and subsequently
dispersed across Antarctica to Australia and
New Guinea (Muizon et al., 1997; Springer et
al., 1997a; Rougier et al., 1998). Living
marsupials comprise seven major groups,
currently ranked as orders in the Linnaean
hierarchy, each of which is endemic to either
the New World or the Old World (table 1).
Although the extant New World marsupial
fauna consists of three orders, two of these