phylogenetic relationships and classification of didelphid marsupials ...

2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 115
conspicuously widened posteriorly near maxillary-frontal
suture. Maxillary turbinals
elaborately branched. One lacrimal foramen
usually present on each side just outside
anterior orbital margin. Blunt, hornlike
postorbital frontal processes present in
adults. Right and left frontals co-ossified,
midfrontal suture incomplete or absent; left
and right parietals separated by persistent
midparietal suture. Parietal and alisphenoid
in contact on lateral braincase (no frontalsquamosal
contact). Sagittal crest present,
well developed on parietals, and extending
anteriorly onto frontals. Petrosal not laterally
exposed through fenestra in parietal-squamosal
suture (fenestra absent). Parietal-mastoid
contact absent (interparietal narrowly
contacts squamosal).
Maxillopalatine fenestrae present; palatine
and maxillary fenestrae absent; posterolateral
palatal foramina not extending anteriorly
between M4 protocones; posterior palatal
morphology conforms to Didelphis morphotype
(with well-developed lateral corners, the
choanae constricted behind). Maxillary and
alisphenoid usually not in contact on floor of
orbit (uni- or bilateral contacts were observed
as rare variants). Transverse canal foramen
present. Alisphenoid tympanic process small
and uninflated, usually with medial lamina
enclosing extracranial course of mandibular
nerve (secondary foramen ovale usually
present), and not in contact with rostral
tympanic process of petrosal. Anterior limb
of ectotympanic suspended indirectly from
basicranium (by malleus). Stapes triangular,
with large obturator foramen. Fenestra
cochleae exposed, not concealed by rostral
and caudal tympanic processes of petrosal.
Paroccipital process large and erect, not
adnate to petrosal. Dorsal margin of foramen
magnum bordered by supraoccipital and
exoccipitals, incisura occipitalis present but
narrow.
One mental foramen 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 subequal 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. Upper
molars highly carnassialized (postmetacristae
much longer than postprotocristae); relative
widths M1 , M2 , M3 . M4 or M1 , M2
, M3 , M4; centrocrista only weakly
inflected labially on M1–M3; ectoflexus
shallow but distinct on M1 and M2, deepest
on M3; anterolabial cingulum and preprotocrista
discontinuous (anterior cingulum
incomplete) on M3; postprotocrista with
carnassial notch. Last upper tooth to erupt
is M4.
Lower incisors (i1–i4) without distinct
lingual cusps. Lower canine (c1) erect,
acutely pointed, and simple (without a
posterior accessory cusp). Second lower
premolar (p2) taller than p3; lower milk
premolar (dp3) large and molariform with a
complete (tricuspid) trigonid; hypoconid labially
salient on m3; hypoconulid twinned
with entoconid on m1–m3; entoconid much
taller than hypoconulid on m1–m3.
DISTRIBUTION: Chironectes occurs along
streams in moist lowland and lower montane
forests (the highest elevational record appears
to be about 1900 m; Handley, 1976)
from the Mexican state of Oaxaca throughout
most of Central America and tropical
South America to northeastern Argentina
(Misiones) and Uruguay (González and
Fregueiro, 1998). As mapped by Marshall
(1978d), the geographic range of this genus is
strikingly disjunct, but collecting localities
subsequently reported by Anderson (1997),
Linares (1998), and Brown (2004) have closed
many distributional gaps. As mapped by
Stein and Patton (2008a), Chironectes appears
to be absent from central Amazonia,
but an unvouchered record from the lower
Tapajos (George et al., 1988) suggests that
this hiatus may be an artifact of inadequate
collecting. Although water opossums are
known to inhabit gallery-forested streams in
the Cerrado (Mares et al., 1989), there are no
published records from the Caatinga and
Chaco.
REMARKS: Given the very broad geographic
distribution of this unrevised genus, it