phylogenetic relationships and classification of didelphid marsupials ...

2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 57
Fig. 23. Labial views of unworn anterior
mandibular dentitions illustrating taxonomic differences
in lower canine (c1) morphology. A,
Lestodelphys halli (MMNH 17171) with erect,
acutely pointed, and simple c1. B, Marmosa
mexicana (AMNH 17136) with semiprocumbent
and flat-crowned but simple c1. C, Marmosops
pinheiroi (AMNH 267342) with fully premolariform
c1 (procumbent and flat crowned with
posterior accessory cusp).
tendencies are minimally indicated on the
unworn teeth of Chacodelphys and Metachirus
(which are flattened along their anterior
margin but have no posterior cingulum),
they are increasingly apparent in Cryptonanus,
Gracilinanus, Marmosa (fig. 23B), and
Thylamys. Premolarization of the lower
canine is maximally developed in the smaller
species of Marmosops (e.g., M. parvidens and
M. pinheiroi), where c1 and p1–3 form a
series of four strikingly similar teeth (fig.
23C). Generally speaking, taxa with welldeveloped
accessory cusps on the upper
canine also have a well-developed posterior
accessory cusp on c1, so variation in these
occluding teeth is at least partially correlated.
Large dasyurids (e.g., Dasyurus) have erect,
unicuspid lower canines, but many small
dasyurids (e.g., Sminthopsis) have procumbent,
premolariform teeth. The lower canines
are small and blunt in peramelemorphians. In
Dromiciops, c1 is procumbent and more or
less premolariform (Marshall, 1982b: fig. 14).
LOWER PREMOLARS: Didelphid lower premolars
are unremarkable teeth, each having a
single dominant cusp and a well-developed
distal cingulum that often produces a posterior
accessory cusp. Small diastemata are
sometimes present between p1 and c1,
between p1 and p2, and (rarely) between p2
and p3. The lower first premolar (p1) is
vestigial and occlusally featureless in Caluromys
and Caluromysiops, but it is well
developed in all of the other genera.
The second lower premolar is distinctly
taller than p3 in most didelphids, but p2 and
p3 are subequal in Marmosops incanus and
most examined species of Monodelphis and
Thylamys. In Lestodelphys and in certain
other species of Monodelphis (e.g., M. emiliae),
however, p3 is distinctly taller than p2.
Most other plesiomorphic marsupials have
three lower premolars that are essentially
similar to those of didelphids, although some
dasyurids (e.g., Dasyurus) have only two.
Among the taxa we examined, p3 is taller
than p2 in caenolestids, Dromiciops (see
Marshall, 1982b: fig. 14), some dasyurids
(e.g., Murexia), and some peramelemorphians
(e.g., Echymipera), but p2 and p3 are
subequal in height or p2 is the taller tooth in
other dasyurids (e.g., Sminthopsis) and peramelemorphians
(e.g., Macrotis).