phylogenetic relationships and classification of didelphid marsupials ...

54 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
near the base of the paracone; the anterolabial
cingulum does not converge toward the
preprotocrista in these taxa, but passes
obliquely dorsally such that the two crests
are discontinuous on the anterior surface of
the tooth crown (fig. 21E, F). Voss and Jansa
(2003: 39) discussed several inconsistencies in
the published literature concerning the distribution
of these traits among didelphids.
The postprotocrista also exhibits noteworthy
variation among didelphids. In most
genera, this crest decreases in height and
width as it passes posterolabially around the
base of the metacone before merging with the
posterior surface of the tooth crown, leaving
a distinct groove or gap in the posterior wall
of the trigon basin. By contrast, the postprotocrista
connects directly with the base of
the metacone, such that the posterior wall of
the trigon basin has no gap or groove, in
Chironectes, Didelphis, Lutreolina, and Philander;
instead, the unworn postprotocrista
of these taxa exhibits a distinct carnassial
notch near the base of the metacone.
Dasyurids and Dromiciops have tribosphenic
upper molars that differ from those of
didelphids in only minor details. Indeed,
some dasyurids and didelphids are indistinguishable
in published matrices of upper
molar characters (e.g., Archer, 1976b: table
1), although there is a tendency for stylar
cusp D to be larger than stylar cusp B when
both structures are present in the former
group (Wroe, 1997), and a few dasyurids
(e.g., Sarcophilus) are conspicuously divergent
in other upper molar traits. The upper
molars of Dromiciops differ from those of
most didelphids by having a linear centrocrista,
indistinct stylar cusps, and a stylar
shelf that is conspicuously reduced in width
labial to the paracone on M1. 17
17 These distinctive features are not apparent in some
published illustrations of the dentition of Dromiciops, notably
Marshall’s (1982b) figure 16b, where an occlusal view of the
upper teeth of FMNH 22671 shows ‘-shaped centrocristae and
an unreduced stylar shelf on M1. However, in another
illustration (based on FMNH 22673; op. cit.: fig. 17b), M1
is correctly shown with a reduced stylar shelf labial to the
paracone, and with a linear centrocrista. We borrowed FMNH
22671 to determine whether this specimen is unusual in any
way and found that it is not: the paracone of M1 is almost on
the labial margin of the tooth, and the centrocristae of M1–M3
are linear; therefore, Marshall’s figure 16b is inaccurate.
The upper molars of Recent peramelemorphians
differ from those of didelphids in
several respects, most notably by having a
discontinuous centrocrista: instead of forming
a ‘-shaped (labially inflected) crest with
an apex that is lingual to the stylar shelf, the
postparacrista and the premetacrista of
modern peramelemorphians terminate on
the labial margin of the tooth, where they
are separated by a small gap (Muirhead and
Filan [1995] referred to this trait as the result
of the centrocrista ‘‘breaching’’ the ectoloph).
Additionally, all modern peramelemophians
have a large posterolingual cusp on M1–M3;
in most genera (e.g., Echymipera, Perameles)
this is a hypertrophied metaconule, but in
Macrotis it is the metacone (Archer, 1976b).
Another peramelemorphian trait is that the
preparacrista of M1 is a tall crest that passes
posterolabially to a large stylar cusp at or
near the C position (Archer, 1976b). The
anterolabial cingulum is present in some
peramelemorphians (e.g., Echymipera) but
not in others (e.g., Perameles).
On caenolestid molars the principal labial
cusps are hypertrophied stylar elements in the
B and D positions (Osgood, 1921; Marshall,
1987; Goin and Candela, 2004; Goin et al.,
2007). In both Caenolestes and Rhyncholestes,
the paracone is absent (or indistinguishably
fused with stylar cusp B). The
metacone is also absent in Rhyncholestes but,
although fused to the lingual aspect of stylar
cusp D, the metacone is still clearly recognizable
in Caenolestes. Given this interpretation
of caenolestid cusp homologies, it
follows that a low crest along the labial base
of the tooth crowns in both genera is a
neomorphic cingulum rather than a vestigial
stylar shelf. The first two caenolestid upper
molars also have a well-developed posterolingual
cusp that has been interpreted either
as a hypertrophied metaconule (Marshall,
1987; Goin and Candela, 2004; Goin et al.,
2007) or as a neomorphic outgrowth of the
postprotocingulum (Hunter and Jernvall,
1995). Another distinctive trait of the upper
molars of Recent caenolestids is the complete
absence of an anterolabial cingulum.
UPPER DENTAL ERUPTION SEQUENCES: As
described by Tribe (1990), P3 is the last upper
tooth to erupt in most didelphids. By
contrast, P3 erupts before M4, which is the