phylogenetic relationships and classification of didelphid marsupials ...

2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 59
cingulid (precingulid) is always present, but a
posterior cingulid (postcingulid) is just as
consistently absent. 20 The anterior cingulid
never extends all the way to the base of the
paraconid because it abuts on the hypoconulid
of the preceding tooth (dp3 in the case of
m1), resulting in a prominent concavity in the
anterolingual occlusal outline (the ‘‘hypoconulid
notch’’ of Archer, 1976b). The entocristid
is a short crest that approximately
parallels the lingual margin of the tooth.
Didelphid lower molars exhibit taxonomic
shape variation that is correlated with
carnassialization as previously described for
the upper teeth. Thus, taxa with uncarnassialized
molars have relatively small trigonids
and large talonids, whereas opossums with
highly carnassialized molars have relatively
large trigonids and small talonids. Trigonid
enlargement is invariably accompanied by an
increase in occlusal relief, the protoconid and
metacristid in particular tending to become
very tall in the most carnassialized forms
(e.g., Lestodelphys and Lutreolina). The width
gradient previously noted for the upper
molars is also seen in the lowers, m2 being
the widest tooth in uncarnassialized dentitions
whereas m3 or m4 is widest in
progressively more carnassialized forms. As
for the upper teeth, the highly divergent
endpoints of this conspicuous gradient of
dental shape variation are connected by an
almost continuous taxonomic series of morphological
intermediates.
Only a few lower molar traits that vary
among didelphids seem useful for the purposes
of taxon diagnoses and phylogenetic
analysis. Among these, the labial prominence
of the hypoconid tends to vary along the
toothrow, so we standardized comparisons at
the third molar locus. In most opossums, the
hypoconid is labially salient (projecting
beyond the protoconid or level with it) on
m3, but the hypoconid is lingual to the
protoconid in Chacodelphys, Lestodelphys,
Lutreolina, and Monodelphis.
Most didelphids have a well-developed
entoconid that is about as tall as the
20 A posterior cingulid (‘‘postcingulum’’) was illustrated by
Goin and Candela (2004: fig. 2B) on a diagram representing the
occlusal features of a ‘‘didelphimorphian opossum,’’ but no
living didelphimorphian has such a structure.
hypoconid and much exceeds the adjacent
hypoconulid in height and breadth on m1–m3
(fig. 25, top). In Chacodelphys and Monodelphis,
however, the entoconid is very small,
never more than subequal to the hypoconulid,
and often smaller than that cusp (fig. 25,
bottom); it becomes indistinct or is obliterated
by wear in most adult specimens. Discrepant
observations in the literature about
didelphid entoconid size variation were discussed
by Voss and Jansa (2003).
In most didelphids, the hypoconulid is
much closer to the entoconid than it is to the
hypoconid, and is said to be ‘‘twinned’’ with
the former cusp. Although twinning of the
hypoconulid and entoconid is often stated (or
implied) to be universally present among
tribosphenic marsupials (e.g., by Clemens,
1979; Kielan-Jaworowska et al., 2004), it is
not. The hypoconulid and entoconid are not
closely approximated in Caluromysiops, and
in some specimens (e.g., FMNH 121522) the
hypoconulid is at the center of the posterior
talonid margin, equidistant to the hypoconid
and the entoconid.
Most other plesiomorphic marsupials have
lower molars that resemble those of didelphids
in common tribosphenic features, but several
differences are noteworthy. Dasyurids, for
example, have a well-developed posterior
cingulid on m1–m3, the anterior cingulid of
peramelemorphian lower molars lacks a hypoconulid
notch, the anterior cingulid is
vestigial in Dromiciops (which lacks a hypoconulid
notch on m1), and an entocristid is
absent in some dasyurids (e.g., Sminthopsis)
and peramelemorphians (e.g., Perameles).
As they are in so many other character
complexes, caenolestids are the most divergent
basal marsupials in lower molar morphology.
In both Caenolestes and Rhyncholestes
the paracristid is unnotched and
decreases gradually in height from the apex
of the protoconid to the anterolingual corner
of the tooth; the paraconid is therefore
indistinct. Whereas the paracristid runs in
an uninterrupted arc from the protoconid to
the anterolingual corner of the tooth on m1,
the paracristid on m2 and m3 abuts the
hypoconulid of the preceding tooth (m1 and
m2, respectively), at which point it is abruptly
deflected posterolingually. The only distinct
trigonid cusp on m4 is the metaconid, from