phylogenetic relationships and classification of didelphid marsupials ...

2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 107
cealing nasal orifice from dorsal view), and
conspicuously widened posteriorly near maxillary-frontal
suture. Maxillary turbinals
(viewed through the nasal orifice) simple or
sparsely ornamented scrolls, not elaborately
branched. Lacrimal foramina (usually two on
each side) prominently exposed on orbital
margin or on face anterior to orbit. Orbits
small, interorbital region more or less parallel
sided (usually without conspicuous constrictions);
supraorbital margins smoothly rounded,
without beads or distinct postorbital
processes (but blunt, indistinct processes
occasionally developed in large specimens of
some species; e.g., M. emiliae). Parietal and
alisphenoid in contact on lateral braincase
(no frontal-squamosal contact). Sagittal crest
absent (e.g., in M. theresa) or small (usually
not extending to frontals; e.g., in M. brevicaudata).
Petrosal not exposed laterally
through fenestra in parietal-squamosal suture
(fenestra absent). Parietal-mastoid contact
usually present (interparietal seldom contacts
squamosal).
Maxillopalatine fenestrae present; palatine
fenestrae usually absent; maxillary fenestrae
absent; posterolateral palatal foramina small,
not extending anteriorly between M4 protocones;
posterior palatal morphology conforms
to Didelphis morphotype (with moderately
well-developed lateral corners, the
choanae somewhat constricted behind).
Maxillary and alisphenoid in contact on floor
of orbit (not separated by palatine). Transverse
canal foramen present. Alisphenoid
tympanic process smoothly globular; posteromedial
lamina forming secondary foramen
ovale present in some species (e.g., M.
theresa) or lamina and secondary foramen
ovale absent (e.g., in M. brevicaudata).
Anterior limb of ectotympanic suspended
directly from basicranium. Stapes triangular
with large obturator foramen (e.g., in M.
brevicaudata), or columellar and microperforate
or imperforate (e.g., in M. peruviana).
Fenestra cochleae exposed in most species,
but fenestra concealed in sinus formed by
rostral and caudal tympanic processes of
petrosal in M. emiliae. Paroccipital process
small, rounded, and adnate to petrosal.
Dorsal margin of foramen magnum bordered
by supraoccipital and exoccipitals, incisura
occipitalis present.
Two mental foramina present on lateral
surface of each hemimandible; angular process
acute and strongly inflected.
Unworn crowns of I2–I5 symmetrically
rhomboidal (‘‘premolariform’’), with subequal
anterior and posterior cutting edges,
and usually increasing in length (mesiodistal
dimension) from I2 to I5. Upper canine (C1)
alveolus in premaxillary-maxillary suture; C1
usually simple (without accessory cusps), but
small posterior accessory cusp sometimes
present (e.g., in M. peruviana). First upper
premolar (P1) smaller than posterior premolars
but well formed and not vestigial; third
upper premolar (P3) taller than P2; P3 with
posterior cutting edge only; upper milk
premolar (dP3) large and molariform. Molars
highly carnassialized (postmetacristae
much longer than postprotocristae); relative
widths consistently M1 , M2 , M3 , M4;
centrocrista strongly inflected labially on M1–
M3; ectoflexus shallow on M1, deeper on M2,
and consistently deep on M3; anterolabial
cingulum and preprotocrista discontinuous
(anterior cingulum incomplete) on M3. Last
upper tooth to erupt is P3 in some species (e.g.,
M. peruviana), or P3 and M4 erupt simultaneously
(e.g., in M. brevicaudata).
Lower incisors (i1–i4) with distinct lingual
cusps. Second lower premolar (p2) subequal
in height to p3 (e.g., in M. brevicaudata), or p3
taller than p2 (e.g., in M. emiliae); lower milk
premolar (dp3) trigonid usually complete
(tricuspid). Hypoconid lingual to protoconid
(not labially salient) on m3; hypoconulid
twinned with entoconid on m1–m3; entoconid
subequal to or smaller than hypoconulid on
m1–m3.
DISTRIBUTION: Species of Monodelphis
occur in lowland and montane rain forests
and dry forests from eastern Panama
throughout most of tropical and subtropical
South America to about 37uS in eastern
Argentina (see range maps in Pine and
Handley, 2008); recorded elevations range
from sea level to at least 2500 m (on the
eastern slopes of the tropical Andes). The
apparent absence of Monodelphis from the
trans-Andean lowlands of western South
America is noteworthy, but this is possibly
an artifact of inadequate collecting; a representative
of the M. adusta complex is
rumored to occur along the Pacific littoral