Marmosa (Stegomarmosa) andersoni

Solari, Sergio & Pine, Ronald H., 2008, Rediscovery and redescription of Marmosa (Stegomarmosa) andersoni Pine (Mammalia: Didelphimorphia: Didelphidae), an endemic Peruvian mouse opossum, with a reassessment of its affinities, Zootaxa 1756, pp. 49-61 : 53-56

Redescription of Marmosa (Stegomarmosa) andersoni

Characters which may be taken as distinctive as compared to those of other mouse opossums, and at the subgeneric or generic level, for Stegomarmosa include the relatively enormous postorbital process of the frontal, in shape somewhat like that of Caluromys J. A. Allen , the strongly constricted interorbital area, and the exceptionally large orbits (see Pine 1972). In addition, the first lower premolar is normally in broad contact, above the level of the alveoli, with the lower canine ( Fig. 2 ). On the right side of MUSM 14154, however, this contact misses by less than a hair’s width. The lower canine is procumbent and apically laterally flattened, as characterized by Voss and Jansa (2003). The ascending ramus of the dentary forms an unusually obtuse angle with the horizontal body of the dentary ( Fig. 2 ). Most of the ventral surface of the tail is covered distally with unusually long silvery bristles that form a well-developed and characteristic fringe on each side of the midventral friction pad (see Pine 1972).

The species Marmosa (Stegomarmosa) andersoni is a medium-sized mouse opossum (28-38 g.). The throat gland is apparent in both adult males and adult females and in at least one immature male (MUSM 14151) of age class 2 as defined by Pine et al. (1985a). The apparent mammary formula is 4-1-4 with all teats being abdominal-inguinal (based on MUSM 14154). Black eye-rings are prominent, widest anterior to eyes but not prolonged into area where muzzle vibrissae originate; hair on cheeks and chin cream-colored to base; a pale buffy patch on top of muzzle just posterior to rhinarium; dorsal fur relatively long, lax, not kinky, plumbeous for most of its length but tipped with a color near Snuff Brown of Ridgway (1912); ventral surface cream-buff, with hairs plumbeous at base. Fur of mammary region short, white, and woolly. Ultravioletinduced fluorescence (presumably fugitive; see Pine et al. 1985b) is bright rose on venter of (longer-prepared) juvenile MUSM 14151, less manifest in (more recently prepared) adults, as is often the case in didelphids. Dorsally, UV light elicits a maroon-chestnut shade in these animals. Facial vibrissae mostly black, but some are white near tips. Minor vibrissae along upper lip at least sometimes white; vibrissae on wrist and throat also white. Pinnae translucent and gray-brown, appearing naked but sprinkled externally with small shiny brown hairs visible under magnification; internally, pinnae with similar but mostly whitish hairs. Tail bicolored and longer than head plus body, with no evidence of incrassation; soft-furred only at base both dorsally and ventrally (on stuffed MUSM 14154 extends onto tail until about 20 mm from anus) and then with short blackishbrown spindle-shaped spines dorsally, becoming more elongate distally; ventral surface with proximal dark spindle-shaped bristles mixed with paler bristles, bristles becoming longer, slenderer, and whitish to silvery distally, longer and denser than in mouse opossums generally. Pattern of scalation of tail largely obscured by tail bristles except where skin of tail stretched by excess stuffing, apparently ranging from annular to spiral, depending on location (regarded as “in spiral series” by Voss & Jansa 2003, and scored as such by us). Scale shape is roughly circular, in some regions with distal margin flattened (see Tate 1933, who figured this condition for other mouse opossums). Interdigital pads 2 and 3 are subequal in size, a condition deemed by Creighton (1984) to be indicative of less arboreal habits.

As noted by Voss and Jansa (2003) for andersoni and other Marmosa species, the premaxillary rostral process is prominent. The rostrum is narrow and the braincase inflated. Rear margin of flaring postorbital process continuous with the low lambdoidal crest. Palatal fenestration in holotype appears to consist of elongated maxillopalatine fenestrae extending from level of anterior border of P3 to level of middle of M3; palatine fenestrae lie at level of M4. The normal condition of the palate may be completely without fenestration, however. MUSM 14154 has the left side of the palate completely imperforate, although there is a very thin area of bone extending from the hind margin of P2 to the level of the M3. Some portions of this thin region are thinner than others and in zones corresponding to reported fenestration in the holotype. Openings in the right side of the palate appear to be artifacts of specimen preparation. An originally imperforate palate appears to also be the case in MUSM 14151 and MUSM 14155. The postorbital process of jugal is very well developed, apparently indicating a very large eye. No anteromedial bony strut of the tympanic wing of the alisphenoid present, a secondary foramen ovale is absent. Incisors 2–5 increase in size toward rear; deciduous premolars fairly large and molariform. Exposed portion of upper canine about three times as long as exposed P1 and laterally compressed. Voss and Jansa (2003) wrote (pp. 64–65): “ Tlacuatzin canescens resembles Marmosa andersoni (the type species of Stegomarmosa Pine 1972 ) in having large postorbital processes ( Pine 1972: fig. 1), but these taxa are otherwise dissimilar. Based on our examination of the Peruvian type specimen (FMNH 84252), M. andersoni differs from T. canescens by having tail scales in spiral series; a long rostral process of the premaxillae; large palatine fenestrae; no maxillary fenestrae; upper incisor crowns that increase in breadth from I2 to I5; and a procumbent, apically flattened c1…we concur with the current treatment of Stegomarmosa as a synonym or subgenus of Marmosa .”

Other characters as given for the subgenus/genus.

Figures

FIGURE 2. Dorsal (above, left) and ventral (above, right) view of the skull and lower jaw (below) of MUSM 14154, a female specimen of M. andersoni reported in the text. Some of the distinctive traits of the species are shown, including the development of the postorbital processes and the first lower premolar’s filling in the space between the lower canine and the second lower premolar. Scale bars = 5 mm.