114 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 Fig. 45. Chironectes minimus (based on AMNH 96759, an adult male from Ilha do Taiuna, Pará, Brazil).
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 <strong>and</strong> left frontals co-ossified, midfrontal suture incomplete or absent; left <strong>and</strong> right parietals separated by persistent midparietal suture. Parietal <strong>and</strong> alisphenoid in contact on lateral braincase (no frontalsquamosal contact). Sagittal crest present, well developed on parietals, <strong>and</strong> 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 <strong>and</strong> 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 <strong>and</strong> alisphenoid usually not in contact on floor <strong>of</strong> orbit (uni- or bilateral contacts were observed as rare variants). Transverse canal foramen present. Alisphenoid tympanic process small <strong>and</strong> uninflated, usually with medial lamina enclosing extracranial course <strong>of</strong> m<strong>and</strong>ibular nerve (secondary foramen ovale usually present), <strong>and</strong> not in contact with rostral tympanic process <strong>of</strong> petrosal. Anterior limb <strong>of</strong> ectotympanic suspended indirectly from basicranium (by malleus). Stapes triangular, with large obturator foramen. Fenestra cochleae exposed, not concealed by rostral <strong>and</strong> caudal tympanic processes <strong>of</strong> petrosal. Paroccipital process large <strong>and</strong> erect, not adnate to petrosal. Dorsal margin <strong>of</strong> foramen magnum bordered by supraoccipital <strong>and</strong> exoccipitals, incisura occipitalis present but narrow. One mental foramen present on lateral surface <strong>of</strong> each hemim<strong>and</strong>ible; angular process acute <strong>and</strong> strongly inflected. Unworn crowns <strong>of</strong> I2–I5 symmetrically rhomboidal (‘‘premolariform’’), with subequal anterior <strong>and</strong> posterior cutting edges, <strong>and</strong> 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 <strong>and</strong> not vestigial; third upper premolar (P3) taller than P2; P3 with posterior cutting edge only; upper milk premolar (dP3) large <strong>and</strong> 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; ect<strong>of</strong>lexus shallow but distinct on M1 <strong>and</strong> M2, deepest on M3; anterolabial cingulum <strong>and</strong> 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, <strong>and</strong> simple (without a posterior accessory cusp). Second lower premolar (p2) taller than p3; lower milk premolar (dp3) large <strong>and</strong> 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 lowl<strong>and</strong> <strong>and</strong> lower montane forests (the highest elevational record appears to be about 1900 m; H<strong>and</strong>ley, 1976) from the Mexican state <strong>of</strong> Oaxaca throughout most <strong>of</strong> Central America <strong>and</strong> tropical South America to northeastern Argentina (Misiones) <strong>and</strong> Uruguay (González <strong>and</strong> Fregueiro, 1998). As mapped by Marshall (1978d), the geographic range <strong>of</strong> this genus is strikingly disjunct, but collecting localities subsequently reported by Anderson (1997), Linares (1998), <strong>and</strong> Brown (2004) have closed many distributional gaps. As mapped by Stein <strong>and</strong> 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 <strong>of</strong> 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 <strong>and</strong> Chaco. REMARKS: Given the very broad geographic distribution <strong>of</strong> this unrevised genus, it
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