phylogenetic relationships and classification of didelphid marsupials ...

104 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
usually unilateral variants in some species);
angular process acute and strongly inflected.
Unworn crowns of I2–I5 symmetrically
rhomboidal (‘‘premolariform’’), with subequal
anterior and posterior cutting edges, and
increasing in length (mesiodistal dimension)
from I2 to I5. Upper canine (C1) alveolus in
premaxillary-maxillary suture; C1 simple
(without accessory cusps in most species) or
with posterior accessory cusp only (in M.
lepida). First upper premolar (P1) smaller
than posterior premolars but well formed and
not vestigial; second and third upper premolars
(P2 and P3) subequal in height; P3 with
posterior cutting edge only; upper milk
premolar (dP3) large and molariform. Molars
moderately carnassialized (postmetacristae
are visibly longer than postprotocristae);
relative widths usually M1 , M2 , M3 ,
M4; centrocrista strongly inflected labially on
M1–M3; ectoflexus indistinct or absent on
M1, shallow but usually distinct on M2, and
consistently deep on M3; anterolabial cingulum
continuous with preprotocrista (complete
anterior cingulum present) on M3. Last
upper tooth to erupt is P3.
Lower incisors (i1–i4) with distinct lingual
cusps. Unworn lower canine (c1) usually
semiprocumbent, with flattened bladelike apex,
with or without distinct posterior accessory
cusp. Second lower premolar (p2) taller than
p3; lower milk premolar (dp3) trigonid
incomplete (bicuspid). Hypoconid labially
salient on m3; hypoconulid twinned with
entoconid on m1–m3; entoconid much taller
than hypoconulid on m1–m3.
DISTRIBUTION: Species of Marmosa collectively
range from the Mexican state of
Tamaulipas southward throughout most of
Central and tropical South America to
Bolivia, Paraguay, and northern Argentina
(Hall, 1981; Creighton and Gardner, 2008b;
Gardner and Creighton, 2008b). Although
most species inhabit lowland rainforests, a
few are restricted to dry habitats (e.g., M.
xerophila; Handley and Gordon, 1979) and
some species of the subgenus Micoureus
occur in montane rainforest at elevations in
excess of 2000 m (e.g., M. mapiriensis sensu
Tate, 1933: 76).
REMARKS: The monophyly of Marmosa
(including Micoureus) is supported by sequence
data from five genes analyzed sepa-
rately (figs. 28–32), in tandem (fig. 33), and
in combination with nonmolecular characters
(figs. 35, 36); generic monophyly is also
supported by a uniquely shared insertion at
the BRCA1 locus (fig. 31). Only a single
morphological character, the possession of a
rostral process of the premaxillae, optimizes
as an unambiguous generic synapomorphy
(appendix 5), but even this weak phenotypic
evidence is compromised by the absence of a
rostral process in M. xerophila, a species that
we did not score for this study.
No published phylogenetic analysis supports
the reciprocal monophyly of Marmosa
and Micoureus, both of which have been
treated as valid genera by recent authors
(e.g., Gardner, 2005; Creighton and Gardner,
2008b; Gardner and Creighton, 2008b).
Instead, species of Micoureus have consistently
been recovered as nested within a
paraphyletic group of Marmosa species
(Kirsch and Palma, 1995; Voss and Jansa,
2003; Jansa and Voss, 2005; Steiner et al.,
2005; Jansa et al., 2006; Gruber et al., 2007).
Obviously, there are several alternative taxonomic
solutions to this problem.
One solution would be to treat Micoureus
as a junior synonym of Marmosa without
recognizing any subgenera of the latter.
Another would be to recognize Micoureus
as a subgenus of Marmosa and to name new
subgenera for other monophyletic clusters of
Marmosa species. A third would be to
recognize Micoureus as a genus and to
describe new genera as needed to make
Marmosa monophyletic. Unfortunately, the
first option would result in the loss of a useful
name (Micoureus, see below), whereas the
second and third options are not currently
workable because many species of Marmosa
have not been included in any phylogenetic
analysis, and their relationships are correspondingly
obscure. Our interim solution is
to move the currently intractable problem of
paraphyly from the generic to the subgeneric
level. Although taxonomic rank is biologically
arbitrary, it affects binomial usage,
which should be conformable with phylogenetic
relationships insofar as these are
known. In effect, because the use of generic
names is obligatory under the current Linnaean
system, it is crucial that genera be
monophyletic.