phylogenetic relationships and classification of didelphid marsupials ...

48 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
Fig. 18. Unworn premaxillary dentitions of
Marmosops pinheiroi (A, AMNH 267341) and
Lutreolina crassicaudata (B, AMNH 210422)
illustrating taxonomic differences in the shape of
the incisor crowns. In Marmosops and many other
small didelphines, the crowns of I2–I4 are symmetrically
rhomboidal, with subequal anterior
(mesial) and posterior (distal) cutting edges that
converge to form a sharp central apex; a distinct
posterior corner (distostyle) is always present on
I5. By contrast, in Lutreolina (and certain other
taxa), the crowns of I2–I4 are conspicuously
asymmetrical, with longer anterior than posterior
cutting edges; a distinct distostyle is usually absent
on I5. Scale bars 5 1mm.
in most of these taxa, but I1 is a laterally
compressed, distinctly chisellike tooth in
caenolestids, and it is a labiolingually compressed,
flat-crowned tooth in peramelemorphians.
The unworn posterior incisors have
approximately rhomboidal crowns with subequal
anterior and posterior cutting edges
that converge to a sharp apex in most
dasyurids and in Dromiciops, but I2–4 in
caenolestids have strikingly asymmetrical
crowns (appropriately described as ‘‘hatchet-shaped’’
by Osgood, 1921: 117), the
posterior margins of which are deeply
notched in Rhyncholestes (see Osgood, 1924:
figs. 3b, c; Bublitz, 1987: fig. 13). In
peramelemorphians, I2–4 are flat crowned
(like I1), but I5 (when present) is caniniform.
All of the upper incisors are single rooted in
dasyurids, caenolestids, Dromiciops, and
many peramelemorphians, but I5 is double
rooted in Perameles gunnii.
UPPER CANINE: The didelphid upper canine
(C1) is invariably separated from the
posteriormost upper incisor by a wide
diastema that may or may not contain a
distinct paracanine fossa for the reception of
the lower canine. In most didelphids, C1
occupies the premaxillary-maxillary suture,
despite the fact that the maxilla always
extends anterolaterally beyond this tooth,
sometimes reaching the alveolus of I5;
therefore, the premaxilla only contacts the
C1 alveolus medially (fig. 14). In Caluromys
and Caluromysiops, however, the upper
canine alveolus is entirely contained within
the maxilla (figs. 38, 39).
The upper canine is a simple unicuspid
tooth in most didelphids, but some species
(e.g., Gracilinanus emiliae; Voss et al., 2001:
fig. 12A) have a small posterior accessory
cusp, and others (e.g., Marmosops pinheiroi;
fig. 53) have both anterior and posterior
accessory cusps. Some care is needed in
determining canine crown morphology, however,
because accessory cusps are sometimes
obliterated by wear (as inferred from their
absence in old adults of species that uniformly
exhibit such structures as juveniles and
subadults). Alternatively, a false posterior
accessory cusp is sometimes formed when the
trailing edge of C1 is notched by occlusion
with p1. Unlike other teeth whose size does
not increase ontogenetically after the crown
is fully erupted, didelphid canines often
appear much larger in older adults than in
younger animals because the root is continuously
extruded from the alveolus throughout
life.
Most other marsupials have single-rooted
unicuspid upper canines like those of didelphids,
but accessory cusps are present in
some peramelids (e.g., Echymipera) and
dasyurids (e.g., Antechinomys; Archer,
1976b). Additionally, the canine is double
rooted and provided with accessory cusps in
female Caenolestes and Rhyncholestes and in