Leptomys arfakensis, Musser & Helgen & Lunde, 2008

Musser, Guy G., Helgen, Kristofer M. & Lunde, Darrin P., 2008, Systematic Review of New Guinea Leptomys (Muridae, Murinae) with Descriptions of Two New Species, American Museum Novitates 3624 (1), pp. 1-60 : 49-52

Leptomys arfakensis , new species

HOLOTYPE AND TYPE LOCALITY: The holotype is BMNH 29.5 .27.23, an adult female collected by Fred Shaw-Mayer (original number 11) on July 19, 1925. The specimen consists of a stuffed museum study skin, broken cranium, and mandible (figs. 6, 10). The braincase is fractured behind the maxillary portion of the zygomata (laterally), the mesopterygoid fossa (ventrally), and the anterior portion of the braincase (dorsally), but most fragments are preserved, including one large intact segment consisting of the parietals and interparietal, and another including the basioccipital, basisphenoid, intact left auditory bulla, mastoid, and occipital condyles. External, cranial, and dental measurements are listed in table 1. Dentition from the paratype is illustrated in figure 12.

The holotype and paratype were collected in the Arfak Mountains of the Vogelkop Peninsula of western New Guinea (Papua Province, Indonesia) at an altitude of 1000 m. No more specific locality data are available .

REFERRED SPECIMENS: Only one other specimen is known: BMNH 29.5.27.22, a young adult male, represented by a study skin, similarly broken cranium, and mandible, with collection details as for the holotype. Dollman (1930) originally identified the type series as ‘‘ Melomys levipes ’’ (5 Paramelomys levipes [ Thomas, 1897]); subsequent authors have referred these two specimens to Leptomys ernstmayri ( Rümmler, 1938; Mahoney, 1968; Musser and Carleton, 1993) or L. elegans ( Flannery, 1995; Aplin et al., 1999).

ETYMOLOGY: Named after the type locality.

DIAGNOSIS: Leptomys arfakensis is larger in body size than L. paulus and L. ernstmayri and slightly smaller than L. elegans and L. signatus (tables 1–5). It lacks the short buffy gray dorsal fur and white blaze on the head that is diagnostic for L. signatus and in pelage coloration resembles the remaining species of the genus (figs. 4–6). Leptomys arfakensis is morphologically most like L. elegans , and is close to that species in cranial dimensions, but differs in its somewhat smaller body size, pronounced hypothenar pad on the hindfoot, lack of cusp t7 from each second upper molar (typically present in all other species except L. paulus ), and small and narrow molars, in particular its excessively reduced upper and lower third molars or absent lower third molars (lacking in both dentaries in the holotype; fig. 22).

GEOGRAPHIC DISTRIBUTION: Leptomys arfakensis is known only by the two specimens listed above, from the Arfak Mountains , the largest mountain range on the Vogelkop Peninsula , also referred to as the Bomberai Peninsula or ‘‘ Bird’s Head’ ’ of New Guinea (fig. 2). Shaw Mayer’s specimens are marked no more precisely than ‘‘ Arfak Mountains’ ’ (see Dollman, 1930), although the altitude of collection (1000 m) is recorded on the skin tags .

DESCRIPTION AND COMPARISONS: Leptomys arfakensis is of medium body size, somewhat intermediate between L. ernstmayri and L. elegans (tables 1–5). Adult dorsal pelage is thick (up to 8–10 mm long) and velvety to the touch, similar to the texture in L. elegans . The tawny-brown upperparts are dark brown over the back and rump and buffy along sides of body, thighs, and upper arms; overhairs are mostly gray with brown to dark buffy tips (dorsal coloration is similar to all other species except L. signatus , which has pale buffy gray upperparts without the rich buff and dark brown tones of the other species). Like all other Leptomys except L. signatus , the head lacks a conspicuous white blaze (figs. 4, 6), and is similar in color to the back in the paratype, but conspicuously darker brown on the crown and face in the holotype. The pinnae are large, dark brown, and although they appear naked are actually scantily covered with minute dark hairs. The longest of the mystacial vibrissae reach 65–70 mm, extending well beyond the ear when laid against the head. A dark brown mask around the eyes extends onto the muzzle in the holotype but not in the paratype. The muzzle at the base of vibrissae appears unpigmented because only short scattered pale hairs cover it, a pattern similar to L. paulus (the muzzle is brown or grayish brown in the three other species due to dense covering of pigmented hairs), and the cheeks are white. The underparts are pure white throughout, from the chin to the anterior hind legs and inguinal region, grading into tawny brown along the lower hind legs and the base of the tail.

The tail is approximately equal to the length of head and body in L. arfakensis (table 2) and covered in annuli of very small flat and inconspicuous scales (17–18 per cm). The scale hairs are fine and laid flat against the tail so that it appears naked. The tail averages relatively shorter than in the long-tailed L. paulus and L. ernstmayri , similar in relative length to L. signatus , and slightly longer than the larger-bodied L. elegans . Coloration of the tail and its patterning are similar to other species of Leptomys . About one-third (27%– 33%; table 2) of the distal portion of tail is all white, the dorsal proximal region is brownish gray, and the ventral surface below this brownish gray segment is white with pale brown mottling.

Most attributes of the hind feet and front feet agree with other species of Leptomys and are as described for L. paulus , although the hind feet appear longer (relative to length of head and body) than in L. elegans but slightly shorter than in the other three species. A pronounced hypothenar plantar pad is present in both examples of L. arfakensis .

Overall conformation of the skull and mandible of L. arfakensis resemble those of the other species of Leptomys (figs. 7–10), but some proportional differences are clearly apparent. Morphometric analyses, for example, highlight the distinctive proportional attributes of the skull of L. arfakensis in relation to the two other species of similar body size, L. elegans and L. signatus . In our discriminantfunction analysis performed on 10 variables (instead of 17, which allowed us to include the two damaged skulls of L. arfakensis ), separation of the specimen scores for L. arfakensis from those representing L. elegans and L. signatus along the first canonical illustrates especially the small molars of L. arfakensis relative to cranial size, while separation from both L. elegans and L. signatus along the third canonical axis highlights the relatively wider interorbit and zygomatic plate of L. arfakensis (figs. 13, 14; table 7)—differences confirmed by univariate comparisons (table 4).

The molars, especially the first upper and first and second lowers, are narrower in L. arfakensis relative to the other species of Leptomys . As in L. paulus , both specimens of L. arfakensis lack a cusp t7 on the second upper molar; this cusp is usually present in the other three species of Leptomys (table 6), and we consider its loss in L. arfakensis and L. paulus to be a derived trait convergently achieved in these two species that are separat- ed by such great geographic distances, one at each end of New Guinea —the ‘‘Bird’s Head’’ region in the west and the ‘‘Bird’s Tail’’ in the east. An anterolabial cingulum is not present in either specimen of L. arfakensis ; this structure is also absent in L. paulus , L. elegans , and L. signatus , but variably present in L. ernstmayri (table 6).

Of all the hydromyin genera, Leptomys has the least reduced dental formula. It is the only hydromyin genus (cf. Helgen, 2005b) that retains third molars; all others in that group possess only first and second molars (or only first molars in Pseudohydromys ellermani ). Within Leptomys , however, L. arfakensis demonstrates the greatest dental reduction. The paratype has very small third molars in each quadrant of the jaw. The holotype has small upper third molars but lacks any trace of the third molars in each dentary forming the lower jaw, as first noticed by Mahoney (1968: 68):

the molar formula of an female individual (B.M.29.5.27.23) of Leptomys ernstmayri Rümmler, 1932 from altitude 1000 meters, Arfak Mountains, west New Guinea is 3/2 [i.e., three molars in each upper quadrant, two in each lower] while that of a male L. ernstmayri (B.M.29.5.27.22) with the same locality data is 3/3 (both individuals collected by Mr. F. Shaw Mayer, July 19 th., 1928). The latter molar formula is indicated for Leptomys ernstmayri and Leptomys elegans Thomas, 1897 by Rümmler (1932 …) and again by Tate (1951, p. 222) for Leptomys elegans within which he includes Leptomys ernstmayri . The absence of M 3 in B.M.29.5.27.22 supports Tate’s contention (1951, p. 222) that a tendency to lose the last molars may well be present in Leptomys .

Loss of the third molars has not been observed in any other sample of Leptomys , and reflects a trend unique to L. arfakensis (fig. 23).

HABITAT AND BIOLOGY: No biological information is yet available for Leptomys arfakensis . Within the Arfak Mountains, the transition between tropical-lowland and lower-montane forested habitats occurs at approximately 1000 m ( Pasveer and Aplin, 1998; Stattersfield et al., 1998), which suggests that the type locality of L. arfakensis (‘‘Arfak Mountains, 1000 meters’’) lies at or near this ecotone. We are uncertain whether L. arfakensis can be characterized as occurring in tropical-lowland and lower-montane forest formations, as do L. elegans and L. signatus , or inhabits lower montane to midmontane by Tim Flannery in 1992 in the Mokwam area (specimens at AM) and by collectors from the Bernice P. Bishop Museum at the Anggi Giji Lakes in 1963 (specimens at BBM-NG and AMNH; see Helgen and Flannery, 2004). Neither endeavor recovered Leptomys , although these undertakings are unlikely to represent a comprehensive inventory of small mammals in the region ( Helgen, 2007a, 2007b). Currently, forested habitats at lower altitudes in the Vogelkop remain comparatively poorly surveyed for mammals ( Aplin et al., 1999; Helgen, 2005b, 2007a, 2007b), and we suspect that, as for L. elegans and L. signatus , it is lowland and hill forest formations and the lower montane forest ecotone that will prove to be the preferred habitats of L. arfakensis .

forests, in which L. ernstmayri and L. paulus are found. Judged from the closer anatomical resemblance of L. arfakensis to L. elegans and L. signatus , we suspect that its ecology is more likely to mirror those species, which are also characterized in part by large body size. This supposition is given circumstantial support by the little information that has been gleaned to date from small mammal trapping in the Vogelkop Peninsula. The most intensive trapping efforts in mid- to upper montane forests in the Arfaks (higher than 1500 m) are those