Chacodelphys formosa ( Shamel, 1930b )

VOSS, ROBERT S., GARDNER, ALFRED L. & JANSA, SHARON A., 2004, On the Relationships of ‘‘ Marmosa’ ’ formosa Shamel, 1930 (Marsupialia: Didelphidae), a Phylogenetic Puzzle from the Chaco of Northern Argentina, American Museum Novitates 3442 (1), pp. 1-18 : 2-7

Chacodelphys formosa ( Shamel, 1930b)

Marmosa muscula Shamel, 1930a: 83 . Original description.

Marmosa formosa Shamel, 1930b: 311 . Replacement name.

Marmosa (Thylamys) velutina formosa: Cabrera, 1958: 33 . New name combination.

Marmosa (Thylamys) formosa: Kirsch and Calaby, 1977: 14 . New name combination.

Thylamys formosa: Reig, Kirsch, and Marshall, 1985: 342 . New name combination, implied by raising Thylamys (sensu Kirsch and Calaby, 1977) to generic rank.

Gracilinanus agilis: Gardner and Creighton, 1989: 5 (part). New generic assignment and synonymy, based on alleged conspecificity with G. agilis Burmeister.

HOLOTYPE: A young adult male specimen in the National Museum of Natural History ( USNM 236330 ; original number 1081) collected by Alexander Wetmore on 9 August 1920 on an estancia called Linda Vista near the Riacho Pilaga´ , about 200 km northwest of Formosa, Provincia Formosa, Argentina. The type is preserved as a round skin with skull and hemimandibles, all of which are in good condition. Although Cabrera (1958) thought that Shamel’s type was a juvenile, the permanent dentition of USNM 236330 is fully erupted.

The type locality has been variously re­

TABLE 1 External and Craniodental Dimensions (mm) of the Holotype of Chacodelphys formosa a ported in the literature and merits comment. The original skin tag gives the collection locality of USNM 236330 as ‘‘ Argentina:/ Formosa: Kilometro 182’’ on one side, and ‘‘Riacho Pilaga/ 10 mi. N.W.’’ on the other, but Shamel (1930a: 83) stated that the type was collected at ‘‘Kilometer 182’’ and did not mention Riacho Pilaga´. Tate (1933: 232) correctly interpreted the type locality as ‘‘Riacho Pilaga, 10 mi. northwest of Kilom[eter]. 182.’’ Wetmore’s (1926) published itinerary (quoted at length below) states that his base of operations from 5 to 21 August 1920 was at Linda Vista, an estancia on the Riacho Pilaga´, 15 km northwest of a station known as Kilometro 182 (now Comandante Fontana) on the railroad from Formosa. According to Paynter (1995), Linda Vista is about 100 m above sea level near 25°13̍S, 59°47̍W.

DIAGNOSIS: Chacodelphys formosa is a very small didelphid—possibly the smallest living form (see table 1 for measurements)— that can be distinguished from other confamilial taxa by numerous qualitative morphological characters, of which the following provide the most conspicuous points of comparison (see Voss and Jansa [2003] for character definitions and anatomical terminology).

Eye narrowly surrounded by mask of dark fur contrasting in color with fur of cheeks and crown; pale spot above eye absent; dark midrostral stripe absent; gular gland present (distinct but perhaps not fully developed on holotype); dorsal fur brownish, somewhat darker middorsally than along flanks, but pelage not distinctly tricolored (sensu Tate, 1933); dorsal underfur gray­based; dorsal guard hairs very short and inconspicuous; ventral fur gray­based but superficially washed with buff­yellow from throat to anus (there is some self­colored buffy fur on the chin); third manual digit (dIII) longer than adjacent digits (dII and dIV); manual claws shorter than apical digital pads; central palmar surface of manus densely covered with small convex tubercles; fourth pedal digit (dIV) slightly longer than adjacent digits (dIII and dV); plantar epithelium of pes naked from heel to toes; body pelage not extending onto tail base; tail densely covered with short hairs (three per scale) and distinctly bicolored (dark above, pale below); tail scales arranged in annular series; caudal prehensile surface absent (tail tip completely hairy above and below); tail not incrassate.

Rostral process of premaxillae absent; palatal process of premaxilla contacts C1 alveolus on each side; nasal tips extend anterior to I1; nasals very narrow, with subparallel lateral margins; maxillary turbinals large and elaborately branched; supraorbital margins smoothly rounded, without beads or processes; strongly marked interorbital and postorbital constrictions present; sagittal crest absent; parietal and alisphenoid bones in contact (no squamosal­frontal contact); petrosal exposed on lateral aspect of braincase through small fenestra between parietal and squamosal; maxillopalatine fenestrae very large; palatine fenestrae present but incompletely separated from maxillopalatine openings; maxillary fenestrae very small but bilaterally present near M1/M2 commissure; posterolateral foramina small, not extending lingual to M4 protocones; posterior palate with prominent lateral corners, the internal choanae abruptly constricted behind; maxillary and alisphenoid not in contact on orbital floor; transverse canal foramen bilaterally present; alisphenoid tympanic wing without anteromedial processes (secondary foramen ovale absent); ectotympanic suspension direct; fenestra cochleae laterally exposed; paroccipital process of exoccipital small, adnate to petrosal; dorsal margin of foramen magnum formed by supraoccipital and exoccipi­ tals; triangular stapes perforated by large obturator foramen; two mental foramina present on lateral aspect of mandible; angular processes apparently acute and strongly inflected (broken on both hemimandibles of holotype).

Upper incisor crowns symmetrically rhomboidal and increasing in breadth from front to back (I2 <I5); C1 without anterior or posterior accessory cusps; P1 present, smaller than posterior premolars but not vestigial; P2 distinctly taller than P3; P3 without anterior cutting edge; upper molars strongly dilambdodont and highly carnassialized, increasing in width (transverse dimension) from front to back (width M1 << width M4); ectoflexus absent on M1, very shallow on M2, distinct only on M3; anterior cingulum incomplete on M3. Lower incisors with distinct lingual cusps; c1 without posterior accessory cusp; p2 taller than p3; hypoconid lingual to protoconid (not labially salient) on m3; entoconid distinct but small, subequal to hypoconulid on m1–m3.

COMPARISONS: Chacodelphys formosa differs by a large number of external and craniodental characters from members of other ‘‘marmosine’’ genera (comprising those species formerly classified as or allied with Marmosa sensu lato) and from Monodelphis as detailed in the following paragraphs.

Chacodelphys differs from species of Gracilinanus sensu stricto 4 by its long third manual digit (versus dIII and dIV subequal); densely tuberculate (versus smooth) central palmar surface of manus; very short tail (versus tail much longer than head­and­body); absence of a caudal prehensile surface (versus tail­tip modified for prehension); absence (versus presence) of a rostral process of the premaxillae; narrow nasals with subparallel lateral margins (versus nasals conspicuously widened posteriorly); absence (versus presence) of a secondary foramen ovale; incomplete (versus complete) anterior cingulum on M3; hypoconid not labially salient on m3 (versus m3 hypoconid labially salient); and an entoconid that is subequal in height to the hypoconulid on m1–m3 (versus entoconid much taller than the hypoconulid).

Chacodelphys differs from the monotypic genus Lestodelphys by not having distinctly tricolored pelage (versus pelage distinctly tricolored); gray­based ventral fur (versus ventral fur self­white); short manual claws (versus long manual claws that extend well beyond the apical digital pads); hindfoot with naked plantar epithelium from heel to toes (versus heel covered with coarse fur); long fourth pedal digit (versus dIII longer than dIV); tail not incrassate (versus incrassate); narrow nasals with subparallel lateral margins (versus nasals conspicuously widened posteriorly); presence (versus absence) of maxillary fenestrae; small posterolateral foramina (versus foramina large and usually

4 As construed herein, Gracilinanus sensu stricto comprises the following valid species, all of which conform to Gardner and Creighton’s (1989) original generic diagnosis: aceramarcae Tate, agilis Burmeister (including beatrix Thomas, buenavistae Tate, and peruanus Thomas), dryas Thomas, emiliae Thomas (including longicaudus Hershkovitz), marica Thomas (including perijae Hershkovitz), and microtarsus Wagner. Explicitly excluded from our concept of Gracilinanus sensu stricto are five nominal taxa (currently treated as synonyms of agilis , emiliae , or microtarsus ; see Gardner, 1993) that belong to an unnamed clade to be described in a manuscript currently in preparation. These include agricolai Moojen; chacoensis Tate; guahybae Tate; ignitus Díaz, Flores, and Barquez; and unduaviensis Tate. We have not examined material of three nominal taxa (blaseri, rondoni, and herhardi) originally described by Miranda­ Ribeiro (1936) and currently assigned to Gracilinanus (sensu Gardner, 1993); their membership in one or the other of the groups mentioned above seems probable but remains to be determined.

extending lingual to M4 protocones); absence (versus presence) of a secondary foramen ovale; triangular stapes with wide obturator foramen (versus stapes subtriangular and microperforate); fenestra cochleae laterally exposed (versus fenestra concealed in a sinus formed by the rostral and caudal tympanic processes of the petrosal); P2 distinctly taller than P3 (versus P2 << P3); and p2 taller than p3 (versus p2 <p3).

Chacodelphys differs from species of Marmosa and Micoureus by its long third manual digit (versus dIII and dIV subequal); densely tuberculate (versus smooth) central palmar surface of manus; very short tail (versus tail much longer than head­and­body); caudal scales in annular (versus spiral) series; absence of a caudal prehensile surface (versus tail­tip modified for prehension); absence (versus presence) of a rostral premaxillary process; narrow nasals with subparallel lateral margins (versus nasals conspicuously widened posteriorly); absence (versus presence) of postorbital processes; petrosal exposed laterally through a small fenestra between the parietal and squamosal (versus petrosal not so exposed); presence (versus absence) of maxillary fenestrae; P2 distinctly taller than P3 (versus P2 and P3 subequal); anterior cingulum on M3 incomplete (versus complete); hypoconid lingual to protoconid on m3 (versus m3 hypoconid labially salient); and entoconid subequal in height to hypoconulid on m1–m3 (versus entoconid much taller than hypoconulid).

Chacodelphys differs from species of Marmosops by its densely tuberculate (versus smooth) central palmar surface of manus; very short tail (versus tail longer than headand­body); caudal scales in annular (versus spiral) series; caudal prehensile surface absent (versus tail­tip modified for prehension); presence (versus absence) of maxillary palatal vacuities; absence (versus presence) of a secondary foramen ovale; P2 distinctly taller than P3 (versus P2 and P3 subequal); hypoconid lingual to protoconid on m3 (versus m3 hypoconid labially salient); and entoconid subequal in height to hypoconulid on m1–m3 (versus entoconid much taller than the hypoconulid).

Chacodelphys differs from species of Monodelphis by having a dark mask surrounding the eyes (versus circumocular mask absent); short manual claws (versus long manual claws that extend beyond the apical digital pads); densely tuberculate (versus smooth or sparsely tuberculate) central palmar surface of manus; long fourth pedal digit (versus dIII longer than dIV); narrow nasals with subparallel lateral margins (versus nasals conspicuously widened posteriorly); maxilloturbinals large and elaborately branched (versus maxilloturbinals small and unbranched); petrosal exposed laterally through a small fenestra between the parietal and squamosal (versus petrosal not so exposed); presence (versus absence) of palatine fenestrae; presence (versus absence) of maxillary fenestrae; maxillary and alisphenoid separate (versus contacting on orbital floor); and P2 distinctly taller than P3 (versus P2 <P3).

Chacodelphys differs from species of Thylamys in not having distinctly tricolored pelage (versus pelage distinctly tricolored); hindfoot with naked plantar epithelium from heel to toes (versus heel covered with coarse fur); absence of a caudal prehensile surface (versus tail­tip modified for prehension; tail not incrassate (versus incrassate); posterolateral palatal foramina small (versus foramina large and usually extending lingual to M4 protocones); absence (versus presence) of a secondary foramen ovale; fenestra cochleae exposed laterally (versus concealed in a sinus formed by the rostral and caudal tympanic processes of the petrosal); P2 distinctly taller than P3 (versus P2 <P3); p2 taller than p3 (versus p2 subequal to p3 or p2 <p3); hypoconid lingual to protoconid on m3 (versus m3 hypoconid labially salient); and entoconid subequal in height to hypoconulid on m1–m3 (versus entoconid much taller than hypoconulid).

Chacodelphys differs from the monotypic genus Tlacuatzin by its possession of a gular gland (versus gular gland absent in Tlacuatzin ); long third manual digit (versus dIII and dIV subequal); densely tuberculate (versus smooth) central palmar surface of manus; very short tail (versus tail longer than headand­body); absence of a caudal prehensile surface (versus tail­tip modified for prehension); narrow nasals with subparallel lateral margins (versus nasals conspicuously widened posteriorly); absence (versus presence) of postorbital processes; petrosal exposed laterally through a small fenestra between the parietal and squamosal (versus petrosal not so exposed); presence (versus absence) of palatine fenestrae; second through fifth upper incisor crowns increasing in breadth from front to back (versus I2–I5 crowns subequal in breadth); P2 distinctly taller than P3 (versus P2 and P3 subequal in height); anterior cingulum of M3 incomplete (versus complete); hypoconid lingual to protoconid on m3 (versus m3 hypoconid labially salient); and entoconid subequal in height to hypoconulid on m1–m3 (versus entoconid much taller than hypoconulid).

SPECIMENS EXAMINED: The holotype is the only known specimen.