phylogenetic relationships and classification of didelphid marsupials ...

50 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
the tooth is rounded, and only the posterior
cutting edge is well developed. Small anterior
blades are variably present near the base of
P3 in Chironectes and Philander, but the apex
of the tooth is always rounded anteriorly as
in the other taxa with single-bladed P3s.
Most other plesiomorphic marsupials have
three upper premolars, but some dasyurids
(e.g., Dasyurus) have only two. In addition to
the usual diastema between P1 and P2, some
adult dasyurids (e.g., Murexia) and all adult
peramelemorphians have a diastema between
P2 and P3. The third upper premolar (P3) is
taller than P2 in caenolestids, Dromiciops,
and some dasyurids (e.g., Murexia, Sminthopsis),
but P2 is taller than P3 in other dasyurids
(e.g., Myoictis). An anterior cutting edge is
apparently absent from P3 in all nondidelphid
marsupials.
UPPER MILK PREMOLAR: Although the
deciduous upper premolar (dP3) of didelphids
has consistently been described as large
and molariform (Flower, 1867; Thomas,
1888; Bensley, 1903; Tate, 1948b; Archer,
1976b), there is noteworthy taxonomic variation
in the morphology of this tooth. In
most opossums dP3 is, indeed, a sizable
tooth: its crown area ranges from about 42%
to 96% of the crown area of M1, the tooth
immediately behind it (Voss et al., 2001: table
5). An obviously functional element of the
upper toothrow, dP3 occludes with both the
deciduous lower premolar (dp3) and with m1.
Hyladelphys, however, has a very small upper
milk premolar (,10% of the crown area of
M1; op. cit.) that appears to be functionally
vestigial because it does not occlude with any
lower tooth.
Although molariform, the didelphid upper
milk premolar does not always match the
teeth behind it in all occlusal details. In most
specimens, the paracone of dP3 is located on
the labial margin of the crown, and the stylar
shelf is correspondingly incomplete, whereas
the paracone is lingual to a continuous stylar
shelf on all didelphid molars (see below).
Among those didelphids whose milk premolars
we examined, only Caluromys has a dP3
in which the paracone is usually lingual to a
continuous stylar shelf.
The morphology of milk premolars remains
to be widely surveyed in Marsupialia,
but of those taxa that we examined, only
Dromiciops has a large, molariform dP3
resembling the common didelphid condition
(Marshall, 1982b: fig. 17; Hershkovitz, 1999:
fig. 32). By contrast, dP3 is much smaller,
more or less vestigial, and structurally
simplified in caenolestids, dasyurids, and
peramelemorphians (Tate, 1948a; Archer,
1976b; Luckett and Hong, 2000).
UPPER MOLARS: Didelphid upper molars
conform to the basic tribosphenic bauplan
(Simpson, 1936) in having three principal
cusps—paracone, protocone, and metacone—connected
by the usual crests in a
more or less triangular array (fig. 20). A
broad stylar shelf and an anterolabial cingulum
are invariably present; the centrocrista
(postparacrista + premetacrista) and the
ectoloph (preparacrista + centrocrista +
postmetacrista) are uninterupted by gaps;
the para- and metaconules are indistinct or
absent; 15 and there is no posterolingual talon.
In addition, most didelphids have several
(usually five or six) small cusps on the stylar
shelf, for which most authors employ alphabetical
labels (after Bensley, 1906; Simpson,
1929; Clemens, 1966). Of these, stylar cusp B
(labial to the paracone) is more consistently
recognizable than the others, but a stylar
cusp in the D position (labial to the
metacone) is often subequal to it in size.
Didelphids differ conspicuously in the
relative width (transverse or labial-lingual
dimension) of successive molars within toothrows.
In some taxa, the anterior molars tend
to be wide in proportion to more posterior
teeth, but in others the posterior molars are
relatively wider (fig. 21). The ratio obtained
by dividing the width of M4 by the width of
M1 (M4/M1; table 8) conveniently indexes
this size-independent shape variation and
ranges from a minimal value of 0.83 (in
15 The literature is inconsistent on this point, with some
authors claiming to have observed significant taxonomic
variation among Recent didelphids in the occurrence of conules
(see Voss and Jansa, 2003: appendix 4). Indeed, careful
examination of unworn teeth usually reveals a tiny enameled
chevron on the postprotocrista that is presumably homologous
with the metaconule of stem metatherians; corresponding
structures on the preprotocrista, presumably vestigial paraconules,
are much less frequently observed. The fact that conules
have been scored as present and absent in the same taxon (e.g.,
Didelphis) by different authors (e.g., Reig et al. [1987] versus
Wroe et al. [2000]) sufficiently illustrates the ambiguous
interpretation of such indistinct features.