phylogenetic relationships and classification of didelphid marsupials ...

2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 133
and unfurred except at base; caudal integument
bicolored (dark above, distinctly paler
below); tail scales difficult to distinguish but
apparently in annular series; caudal hairs all
subequal in length and thickness; ventral
caudal surface not modified for prehension.
Premaxillary rostral process absent. Nasals
long, extending anterior to I1 (concealing
nasal orifice in dorsal view), and conspicuously
widened posteriorly near premaxillarymaxillary
suture. Maxillary turbinals elaborately
branched. Two lacrimal foramina
present on each side, exposed in lateral view
anterior to orbital margin. Supraorbital
margins smoothly rounded, without beads
or crests; postorbital frontal processes usually
absent (indistinct processes are occasionally
developed in old adult males). Left and
right frontals and parietals separated by
persistent median sutures. Parietal and alisphenoid
bones in contact on lateral surface
of braincase (no frontal-squamosal contact).
Sagittal crest usually absent but weakly
developed along midparietal suture (not
extending anteriorly to frontals) in some
large specimens. 31 Petrosal exposed through
fenestra in parietal-squamosal suture in some
individuals but not in others. Parietal-mastoid
contact present (interparietal does not
contact squamosal).
Maxillopalatine fenestrae present but
sometimes quite small; palatine fenestrae
variable but usually present (Martin, 2005:
fig. 2); maxillary fenestrae absent; posterolateral
palatal foramina very long, usually
extending anteriorly lingual to M4 protocones;
posterior palatal morphology conforms to
Didelphis morphotype (with strongly produced
posterolateral corners, the choanae
constricted behind). Maxillary and alisphenoid
not in contact (separated by palatine) on
floor of orbit. Transverse canal foramen
present. Alisphenoid tympanic process
smoothly globular, with well-developed anteromedial
process enclosing extracranial
course of mandibular nerve (secondary foramen
ovale present), and not contacting
rostral tympanic process of petrosal. Anterior
limb of ectotympanic directly suspended
from basicranium. Stapes usually columelli-
31 Among the specimens we examined, a small sagittal crest is
best developed in UWZM 22422.
form and microperforate or imperforate.
Fenestra cochleae concealed in sinus formed
by rostral and caudal tympanic processes of
petrosal. Paroccipital process small, rounded,
and adnate to petrosal. Dorsal margin of
foramen magnum bordered by supraoccipital
and exoccipitals, incisura occipitalis present.
Two mental foramina usually present on
lateral surface of each hemimandible (three
foramina are present unilaterally on two
specimens examined); angular process acute
and strongly inflected.
Unworn crowns of I2–I5 symmetrically
rhomboidal (‘‘premolariform’’), with subequal
anterior and posterior cutting edges,
slightly increasing in length (mesiodistal
dimension) from I2 to I5. Upper canine
(C1) alveolus in premaxillary-maxillary suture;
C1 simple, without accessory cusps.
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 strongly carnassialized (postmetacristae
much longer than postprotocrista);
relative widths M1 , M2 , M3 ,
M4; centrocrista strongly inflected labially on
M1–M3; ectoflexus shallow or absent on M1,
consistently present and distinct on M2,
consistently deep on M3; anterolabial cingulum
and preprotocrista discontinuous (anterior
cingulum incomplete) on M3; postprotocrista
without carnassial notch. Last upper
tooth to erupt is P3.
Lower incisors (i1–i4) with distinct lingual
cusps. Lower canine (c1) erect, acutely
pointed, and simple (without a posterior
accessory cusp). Third lower premolar (p3)
taller than p2; lower milk premolar (dp3)
large, but trigonid incomplete (uni- or
bicuspid). Hypoconid lingual to protoconid
(not labially salient) on m3; hypoconulid
twinned with entoconid on m1–m3; entoconid
taller than hypoconulid on m1–m3.
DISTRIBUTION: Most known specimens of
Lestodelphys have been collected in semidesert
shrubland and steppe habitats in Patagonian
Argentina between 41u and 47uS latitude,
but there are two outlying records from
the Monte desert (between 32u and 38uS) that
may represent a relictual population (Sauthier
et al., 2007); all reported elevations