Chironectes Illiger, 1811

Voss, RS & Jansa, SA, 2009, Phylogenetic Relationships And Classification Of Didelphid Marsupials, An Extant Radiation Of New World Metatherian Mammals, Bulletin of the American Museum of Natural History 2009 (322), pp. 1-177 : 113-115

Chironectes Illiger, 1811 Figure 45

CONTENTS: minimus Zimmermann, 1780

(including argyrodytes Dickey, 1928; bres-

slaui Pohle, 1927; cayennensis Turton, 1800; guianensis Kerr, 1792; gujanensis Link, 1795; langsdorffi Boitard, 1845; palmata Daudin, 1802 ; panamensis Goldman, 1914; paraguensis Kerr, 1792; sarcovienna Shaw, 1800; variegatus Olfers, 1818; and yapock Desmarest, 1820).

MORPHOLOGICAL DESCRIPTION: Combined length of adult head and body ca. 250– 320 mm ; adult weight ca. 510–790 g. Rhinarium with one ventrolateral groove on each side of median sulcus ; dark circumocular mask present, continuous with dark coronal fur (the whole top of the head except for an indistinct pale bar over each eye is blackish); pale supraocular spot absent; dark midrostral stripe absent; throat gland absent. Dorsal pelage pale silvery gray with blackish transverse bars connected by blackish middorsal stripe (when fresh; old museum skins become brownish or reddish with age) ; dorsal underfur pale gray; dorsal guard hairs short; ventral fur self-white. Manus mesaxonic (dIII.dIV); manual claws shorter than fleshy apical pads of digits; dermatoglyph-bearing manual plantar pads absent (plantar epithelium uniformly and very densely covered with minutely denticulate tubercles among which are scattered at regular intervals many smooth, hemispherical papillae); large, fleshy carpal tubercle supported internally by pisiform present in both sexes. Pedal digits webbed from base to terminal phalanges; dIV longer than other pedal digits; plantar surface of heel naked. Pouch present, opening posteriorly; mammae 2–1–2 5 5; cloaca absent (urogenital and rectal openings widely separated by furred skin). Tail longer than combined length of head and body, thick and muscular (not incrassate); body fur extends onto basal one-sixth or less of tail, to about the same extent dorsally as ventrally; naked caudal integument (beyond furry base) blackish with abruptly whitish tip (rarely all blackish); caudal scales in spiral series, each dorsal scale usually with three subequal bristlelike hairs emerging from distal margin (ventral scales often have 4–5 hairs each); ventral caudal surface not externally modified for prehension.

Premaxillary rostral process absent. Nasals short, not extending anteriorly above I1 (exposing nasal orifice in dorsal view), and 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 isM4.

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 seems probable that at least some of the many putative synonyms of Chironectes minimus will be found to represent valid taxa in future analyses of morphological and/or molecular data.