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P.D. JENKINS AND A. BARNETT 



for measurements). Pelage soft, dense and woolly, dark brownish 

 grey dorsally, light grey ventrally; tail greyish brown, densely 

 haired, grey brown and brown hairs predominate proximally, with a 

 proportional increase of buff and cream hairs distally, extending 

 beyond tip in a short pencil. Distal portion of muzzle light grey in 

 young adults, cream in older individuals (age based on degree of 

 dental wear); rhinarium light brown in dry specimens; philtrum 

 present; conspicuous silvery-grey mystacial vibrissae present. Pin- 

 nae small, concealed by pelage; region of more-or-less conspicuous 

 light grey hairs ventro-lateral to pinnae. Manus with three interdigital 

 and two carpal pads. Well developed fringe of stiff hairs on margin 

 of metatarsus and digits of pes; claw of fifth digit extends beyond 

 first interphalangeal joint of fourth digit; claw of first digit reaches 

 midway along first phalange of second digit. See Fig. 1 for external 

 features visible in a photograph of a live specimen. 



Skull (see Fig. 2) with moderately long nasals, overlapping nasal 

 orifice to conceal incisors in dorsal view but barely projecting 

 beyond premaxillae; rostrum short and narrow, naso-lacrymal cap- 

 sules evident in dorsal view; interorbital region moderately narrow 

 relative to braincase breadth (0.38, 0.39 (n = 2); frontals slightly 

 inflated, braincase moderately broad and long; posterior border of 

 incisive foramina between anterior roots of M 1 s, palatal foramina lie 

 between posterior roots of Mis; bullae slightly inflated; orbicular 

 apophysis of maleus present. Carotid circulation pattern 1, based on 

 osteological features (see Voss, 1988: 298). 



Upper incisors moderately narrow, anterior enamel surface pale 

 buff, slightly inclined medially. No anteroloph on Ml; small 

 posteroloph on M2; M3 small, protocone and paracone evident in 

 unworn dentition, posterior conule absent. Anteroconid of ml sim- 

 ple or with slight indication of anteromedian flexid; no anterolophid 

 on m2; small posterolophids on ml and m2, small mesolophids 

 present or absent; m3 small, with small posterior basally positioned 

 conulid. 



Metatarsal proportions: third metatarsal slightly longer than fourth, 

 fourth longer than second; all three far longer than first and fifth; 

 fifth longer than first. Configuration: III > IV > II » V > I. 



Etymology 



This species is named in honour of Professor Gustavo Orces, a 

 pioneer of Ecuadorian mammalogy. He was of great help to AB with 

 fieldwork organisation in Ecuador, and his kindness and knowledge 

 were a source of inspiration. 



Distribution and Ecology 



Known only from Las Cajas Plateau, Ecuador, where specimens 

 have been recorded from three localities: Lake Luspa, Lake Llaviucu, 

 Zorracucho Valley and Lake Torreadora. All specimens were trapped 

 in close proximity to fast-flowing streams at altitudes ranging from 

 3100m to 4000m, in high-altitude moorland vegetation (paramo) 

 (see Barnett, 1992). For more precise details of the habitat at each 

 site and notes on diet see Barnett (1997). 



Comparison with c. trichotis 



The new species is similar in external appearance to C. trichotis, 

 except that the pelage is paler and slightly harsher, and the rhinarium 

 is light brown in dry specimens of C. orcesi, black in C. trichotis. A 

 philtrum is present in C. orcesi but absent in C. trichotis. 

 Chibchanomys orcesi is smaller in external size and averages smaller 

 in cranial size than all known specimens of C. trichotis, with the 

 exception of the single specimen from Peru (see below for com- 

 ments on the status of this specimen). Both species of Chibchanomys 

 are similar in external proportions, except that the hindfoot is 

 proportionately shorter in C. orcesi (see Table 1). The metatarsal 



configuration differs in the two species: IV > III > II = V > I in C. 

 trichotis; III > IV > II » V > I in C. orcesi. The two species differ in 

 the following cranial features: while the nasals of both species are of 

 comparable length, those of C. orcesi are slightly broader and barely 

 project anterior to the premaxillae, unlike those of C. trichotis. 

 which project anteriorly and conceal the incisors and nasal orifice in 

 dorsal view. In lateral view, the globose braincase of C. trichotis 

 rises abruptly in the frontal region, unlike the narrower and less 

 inflated braincase of C. orcesi; the braincase is slightly broader and 

 the breadth across occipital condyles is greater in C. trichotis (see 

 Table 1). The orbicular apophysis of the maleus is present in C. 

 orcesi but absent in C. trichotis. The upper incisors of C. orcesi are 

 less delicate and slightly broader than those of C. trichotis (see Table 

 1 ), the anterior enamel surface of C. trichotis is cream coloured and 

 not medially inclined unlike C. orcesi. The third upper molar is 

 smaller relative to Ml and M2, and m3 is smaller relative to ml and 

 m2 in C. orcesi than in C. trichotis. The anteromedian flexid is 

 absent or barely indicated on the anteroconid of m 1 in C. orcesi but 

 present in C. trichotis, dividing the anteroconid into small but 

 distinct lingual and labial conulids. The posterior conulid of m3 is 

 positioned more basally in C. orcesi than in C. trichotis (when 

 present). 



According to Voss (1988) the young specimen that he identified as 

 C. trichotis from Peru differs from the northern specimens of C. 

 trichotis in several features: the braincase is much less inflated, the 

 occipital condyles are slightly broader, the bullae are somewhat 

 smaller and an indistinct philtrum is indicated; features that resem- 

 ble those of the new species. Voss mentioned that these differences 

 might indicate that southern populations of Chibchanomys are 

 phenotypically distinctive from their northern counterparts. Unfor- 

 tunately it has not proved possible to examine the Peruvian specimen, 

 although information on it was kindly provided by Mark Hafner 

 (personal communication). It is possible that the Ecuadorian and 

 Peruvian specimens are conspecific but additional material and 

 more extensive comparisons are required to elucidate the status of 

 the latter specimen. 



Comparison with other ichthyomine genera 

 Chibchanomys is readily distinguished from Anotomys, Ichthyomys 

 and Rheomys (see Voss, 1988). Chibchanomys and Neusticomys 

 differ from other ichthyomyines in showing carotid arterial circula- 

 tion pattern 1 and in the distribution of the glandular epithelium 

 around the stomach. Chibchanomys differs from Neusticomys in 

 having small pinnae concealed in the pelage (pinnae obvious in 

 Neusticomys); ventral countershading present (absent in 

 Neusticomys); tail longer than head and body (tail shorter than head 

 and body in Neusticomys); the hindfoot is broader with longer digits 

 and the fringing hairs are well developed (narrower with shorter 

 digits and less developed fringing hairs in Neusticomys). The new 

 species does however share several features withNeusticomys which 

 are not exhibited by C. trichotis, such as the similar metatarsal 

 configuration and presence of a philtrum, while the orbicular apo- 

 physis of the malleus is also present in some species of Neusticomys. 



Results of the phylogenetic analysis 



There is evidence in support of the ichthyomyines as a monophyletic 

 group of the subfamily Sigmodontinae (sensu Carleton & Musser, 

 1984) (see Voss, 1988). In contrast, evidence in support of the 

 monophyly of the Sigmodontinae is lacking and a tribal level 

 classification of this subfamily, while convenient in many respects, 

 is unsatisfactory from a phylogenetic point of view, making difficult 

 the choice of satisfactory outgroups for phylogenetic analyses (see 

 Voss, 1988: 436-438, 1991: 33-37; Carleton & Musser, 1989: 53- 

 55; Voss & Carleton, 1993: 21-22). The necessity of making such a 



