phylogenetic relationships and classification of didelphid marsupials ...

36 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
Fig. 14. Palatal morphology of Thylamys
venustus (AMNH 261254) illustrating nomenclature
for fenestrae, foramina, and other features
described in the text. Dental loci (I1–M4) provide
convenient landmarks for defining the size and
position of palatal structures. Abbreviations: if,
incisive foramen; m, maxillary fenestra; mp, maxillopalatine
fenestra; p, palatine fenestra; plpf,
posterolateral palatal foramen.
lambdoid crest, even as juveniles. The condition
in caenolestids is uninterpretable because
no sutures persist on the posterodorsal
braincase of any examined specimen.
The didelphid interparietal participates in
two alternative patterns of contact among
bones of the posterior braincase and the
occiput. In most opossums the interparietal
does not contact the squamosal because the
parietal is in contact with the mastoid
(fig. 13A, B). However, in Chironectes, Didelphis,
Lutreolina, Philander, and in one
examined specimen of Metachirus (fig. 13C,
D) the interparietal contacts the squamosal
and prevents the parietal from contacting the
mastoid. Two species of Monodelphis (M.
brevicaudata and M. emiliae) exhibit both
conditions with approximately equal frequency
among the specimens we examined.
Most examined nondidelphid marsupials
exhibit parietal-mastoid contact, but acrobatids
and Tarsipes exhibit broad interparietalsquamosal
contact (Parker, 1890: fig. 2), and
phalangerids uniquely exhibit squamosalexoccipital
contact (Flannery et al., 1987:
fig. 4C, D).
PALATE: The bony palate is variously
perforated by foramina and fenestrae that
exhibit considerable variation in occurrence,
size, and position among marsupials. Unfortunately,
inconsistent terminology has long
inhibited clear communication about taxonomic
differences. The nomenclature for
palatal perforations adopted herein is illustrated
in figure 14 and compared with other
terminology in table 6.
The incisive foramina are prominent slots
in the anterior palate that transmit the
nasopalatine ducts (Sánchez-Villagra, 2001a)
together with nerves and blood vessels that
remain to be identified in marsupials (Wible,
2003). In didelphids, these openings extend
from the upper incisor arcade posteriorly to
or between the canines. Although didelphid
incisive foramina are always bordered posteriorly
and posterolaterally by the maxillary
bones, their dividing septum is formed
primarily by the premaxillae. By contrast,
the enormously elongated incisive foramina
of caenolestids extend posteriorly between
the premolar rows (opposite P1 or P2), and
the posterior part of the dividing septum is
formed by the maxillae (Osgood, 1921: pl.
20). The long incisive foramina of peramelemorphians
do not extend behind C1, but the
posterior part of their dividing septum is
likewise formed by the maxillae. Most other
marsupials (e.g., dasyuromorphians and Dromiciops)
have incisive foramina that essentially
resemble those of didelphids, with the
conspicuous exception of some diprotodontians
(e.g., Macropus; Wells and Tedford,
1995: fig. 9E) in which these openings are
completely contained by the premaxillae.
Among Recent didelphids, only Caluromys
and Caluromysiops consistently lack wellformed
maxillopalatine fenestrae (figs. 38,
39). Although small perforations in the
maxillary-palatine sutures occur in most
examined specimens of both genera, these
are obviously vascular openings (perhaps
homologous with the major palatine foramina
of placental mammals) that do not
resemble the nonvascular fenestrae of other
didelphids. Glironia is sometimes said to lack
palatal fenestrae (e.g., by Archer, 1982; Reig