Chacodelphys Voss et al., 2004

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 : 124-126

Chacodelphys Voss et al., 2004

CONTENTS: formosa Shamel, 1930 (including muscula Shamel, 1930).

MORPHOLOGICAL DESCRIPTION: Combined length of adult head and body 68 mm (external measurements are only available from the young adult holotype) ; adult weight unknown but probably ca. 10 g. Rhinarial morphology unknown (no fluid-preserved specimens have been examined); dark circumocular mask present but narrow and inconspicuous; pale supraocular spot absent; dark midrostral stripe absent; gular gland present. Dorsal fur unpatterned, brownish, somewhat darker middorsally than along flanks, but pelage not distinctly tricolored ; dorsal underfur gray based; dorsal guard hairs short and inconspicuous; ventral fur gray-based buffy. Manus mesaxonic (dIII. dIV); manual claws shorter than fleshy apical pads of digits; manual plantar pads present, but presence/absence of dermatoglyphs unknown; central palmar surface of manus densely covered with small convex tubercles; occurrence of carpal tubercles unknown. Pedal digits unwebbed; dIV slightly longer than other pedal digits; plantar surface of heel naked. Mammary formula, morphology of pouch (if any), and presence/absence of cloaca unknown. Tail shorter than combined length of head and body, slender and muscular (not incrassate) ; body pelage not extending onto tail base; tail densely covered with short hairs and distinctly bicolored (dark above, pale below); caudal scales arranged in annular series, each scale with three subequal bristlelike hairs emerging from distal margin; ventral caudal surface not externally modified for prehension.

Rostral process of premaxillae absent. Nasals long, extending anteriorly beyond I1 (concealing nasal orifice from dorsal view), with subparallel lateral margins (not widened posteriorly). Maxillary turbinals large and elaborately branched. Two lacrimal foramina laterally exposed on each side just anterior to orbital margin. Supraorbital margins smoothly rounded, without beads or processes; strongly marked interorbital and postorbital constrictions present. Right and left frontals and parietals separated by persistent median sutures. Parietal and alisphenoid bones in contact (no squamosal-frontal contact). Sagittal crest absent. Petrosal exposed laterally though fenestra in parietalsquamosal suture. Parietal-mastoid contact present (interparietal does not contact squamosal).

Maxillopalatine fenestrae present and very large; palatine fenestrae present but incompletely separated from maxillopalatine openings; maxillary fenestrae very small but bilaterally present near M1/M2 commissure; posterolateral palatal foramina small, not extending lingual to M4 protocones; posteri- or palate conforms to Didelphis morphotype (with prominent lateral corners, the internal choanae abruptly constricted behind). Maxillary and alisphenoid not in contact on orbital floor (separated by palatine). Transverse canal foramen present. Alisphenoid tympanic process probably smoothly globular (broken in both examined specimens), without anteromedial process or posteromedial lamina enclosing extracranial course of mandibular nerve (secondary foramen ovale absent), and probably not contacting rostral tympanic process of petrosal. Anterior limb of ectotympanic directly suspended from basicranium. Stapes triangular, perforated by large obturator foramen. Fenestra cochleae exposed, not concealed by rostral and caudal tympanic processes of petrosal. Paroccipital process small, adnate to petrosal. Dorsal margin of foramen magnum formed 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 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; second and third upper premolars (P2 and P3) subequal in height (see footnote 14, above); P3 with posterior cutting edge only; upper milk premolar (dP3) morphology unknown (no juvenile specimens examined). Upper molars highly carnassialized (postmetacristae conspicuously longer than postprotocristae); relative widths M1, M2, M3, M4; centrocrista strongly inflected labially on M1–M3; ectoflexus absent on M1, very shallow on M2, distinct only on M3; anterolabial cingulum and preprotocrista discontinuous (anterior cingulum incomplete) on M3; postprotocrista without carnassial notch. Last upper tooth to erupt is probably P3.

Lower incisors (i1–i4) with distinct lingual cusps. Lower canine (c1) semiprocumbent, with flattened bladelike apex, but simple (without a distinct posterior accessory cusp). Second lower premolar (p2) taller than p3; lower milk premolar (dp3) morphology unknown (no juvenile specimens examined). Hypoconid lingual to protoconid (not labially salient) on m3; hypoconulid twinned with entoconid on m1–m3; entoconid very small, subequal in height to hypoconulid on m1–m3.

DISTRIBUTION: Chacodelphys is currently known from just five localities in the Argentinian provinces of Chaco and Formosa (between 25–27 ° S latitude and 58–60 ° W longitude; Teta et al., 2006). However, because the humid savanna habitats that occur at these sites resemble those found elsewhere in northern Argentina, eastern Bolivia, western Paraguay, and southwestern Brazil, it would be reasonable to expect that the genus is more widely distributed.

REMARKS: In the absence of undamaged cranial material, we are unable to provide an illustration for this genus. However, photographs of two imperfect skulls were published by Voss et al. (2004a: fig. 2) and Teta et al. (2006: fig. 2).

The problematic relationships of this genus were discussed at length by Voss et al. (2004a) and seem unlikely to be resolved definitively in the absence of nuclear gene sequence data. As discussed above, the tribal membership of Chacodelphys is most compellingly supported by Bayesian analysis of a dataset that combines nonmolecular characters with nuclear-gene sequence data (fig. 36), but it is consistent with subjective assessments of similarity based on integumental and craniodental traits (e.g., by Tate, 1933). Although separate analyses of nonmolecular character data suggest that this genus may be more closely related to Thylamys and Lestodelphys than to other thylamyines (fig. 27), it seems premature to formalize such weakly supported results by subtribal nomenclature.