phylogenetic relationships and classification of didelphid marsupials ...

46 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
er, has only a single mental foramen, which is
very large and occupies the same position as
the anterior foramen in those taxa with two
lateral mandibular perforations.
Alternative states of the form and orientation
of the marsupial angular process were
defined by Sánchez-Villagra and Smith
(1997), but we are unable to consistently
recognize the distinction between the ‘‘rodlike’’
and ‘‘intermediate’’ conditions they
scored for didelphid exemplars. Instead, the
common didelphid condition seems more
appropriately characterized as ‘‘acute and
strongly inflected’’ (Voss and Jansa, 2003:
34). Caluromys and Caluromysiops are the
only didelphids with bluntly rounded and
weakly inflected mandibular angles.
Most other plesiomorphic marsupials have
mandibles that grossly resemble the common
didelphid morphotype, but Dromiciops has
only one mental foramen and some peramelemorphians
(e.g., Perameles gunnii) often
have three mental foramina. Additionally,
Caenolestes has a small foramen in the
retromolar space that Sánchez-Villagra et
al. (2000) called the retrodental canal (after
Hoffstetter and Villaroel, 1974), and it has a
small masseteric foramen near the ventral
margin of the masseteric fossa (Osgood,
1921), the lumen of which communicates
with that of the much larger mandibular
foramen on the medial surface of the jaw.
Dentition
All didelphids normally have 50 teeth that
most authors now recognize as including I1–
5, C1, P1–3, and M1–4 in the upper dentition
and i1–4, c1, p1–3, and m1–4 in the lower
dentition. As in other metatherians, only the
third upper and lower premolars have
deciduous precursors (Flower, 1867; Luckett,
1993; Luo et al., 2004; van Nievelt and Smith,
2005a, 2005b). Despite this prevailing consensus,
marsupial tooth homologies have
long been controversial, with the result that
several alternative systems have been used to
identify didelphid dental loci (table 7).
Asfaraswehavebeenabletodetermine
without dissections or X-rays (i.e., from
external inspection of teeth in situ, and from
the alveoli of specimens with loose teeth), all
didelphids have single-rooted incisors and
canines, double-rooted premolars and lower
molars, and three-rooted upper molars; apparently,
dP3 always has three roots and dp3 has
two. Most of the following remarks therefore
concern crown morphologies, which afford
greater scope for taxonomic comparisons.
UPPER INCISORS: The didelphid first upper
incisor (I1) is a more or less styliform,
somewhat procumbent, hypsodont tooth that
is conspicuously unlike I2–I5, from which it
is always separated by a small diastema.
Although taxonomic variation among opossums
in I1 hypsodonty and diastema formation
has been suggested by authors (e.g.,
Takahashi, 1974; Archer, 1976b; Creighton,
1984; Wroe et al., 2000), diagnostically useful
differences are more easily recognized among
the posterior teeth of this series.
The crowns of I2–I5 in most didelphids are
approximately symmetrical, labiolingually
compressed rhomboids with subequal anterior
(mesial) and posterior (distal) cutting edges
that converge to form a sharp central apex;
usually, a distinct anterior angle (mesiostyle)
and a posterior angle (distostyle) can be seen
on unworn teeth (fig. 18A). This is the
morphology that Takahashi (1974: 414)
termed ‘‘premolariform’’, as exemplified by
Monodelphis, Marmosa, and Metachirus in
her study. In many taxa with posterior upper
incisors of this type, the tooth crowns increase
in length (anteroposterior or mesiodistal
dimension) from front to back, such that I2
appears visibly smaller than I5 in labial view.
This tendency is very pronounced in some
taxa with premolariform incisors (e.g., Marmosops,
Metachirus) but seems to grade
imperceptibly (via intermediate morphologies)
to the essentially uniform toothrows of
other taxa (e.g., Lestodelphys, Tlacuatzin) in
which I2 and I5 are subequal in crown length.
An alternative morphology, in which the
crowns of I2–I5 are conspicuously asymmetrical,
with much longer anterior (mesial) than
posterior (distal) cutting edges, occurs in
Caluromys, Caluromysiops, Didelphis, Glironia,
Hyladelphys, Lutreolina, andPhilander.
On unworn teeth of these taxa, the anterior
angle (mesiostyle) is more frequently distinct
than is the posterior angle (distostyle), which
is often entirely lacking on I5 (fig. 18B). This
is the morphology that Takahashi (1974)
labeled ‘‘incisiform,’’ which she recorded for