Bull. nat. Hist. Mux. Land. (Zool.) 63(2): 123-128 



Issued 28 November 1997 



A new species of water mouse, of the genus 

 Chibchanomys (Rodentia, Muridae, 

 Sigmodontinae) from Ecuador 



PAULINA D. JENKINS^ 



Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K. 



ADRIAN A. BARNETT 



School of Life Sciences, Roehampton Institute, West Hill, London SW15 5SN, U.K. 



fifeU ZQ&^Cf 



SYNOPSIS. A new species of the rodent genus Chibchanomys is described from Ecuador on the basis of external and cranial 

 morphology. A phylogenetic analysis is used to assess the relationship of the new species to other ichthyomyine taxa. 



INTRODUCTION 



The New World rodents currently placed in the large and complex 

 subfamily Sigmodontinae (sensu Carleton & Musser, 1984) include 

 a morphologically and ecologically distinctive group of semi-aquatic 

 South American genera (Voss, 1988), for which it is convenient here 

 to use the name ichthyomyines. In a monograph of ichthyomyine 

 rodents, Voss (1988) included five genera: lchthyomys Thomas, 

 1893 (four species), Anotomys Thomas, 1906a(monotypic), Rheomys 

 Thomas, 1906b (four species), Neusticomys Anthony, 1921 (four 

 species) and a new genus, Chibchanomys Voss, 1988 for an enig- 

 matic species of uncertain generic affinity. Chibchanomys trichotis 

 (Thomas, 1897) was originally placed in lchthyomys but was subse- 

 quently assigned to Rheomys (see Tate, 1932; Cabrera, 1961 ), then 

 to Anotomys (see Handley, 1976). Subsequent to the revision by 

 Voss, an additional ichthyomyine species, Neusticomys mussoi Ochoa 

 & Soriano, 1991, has been described. 



During the course of several zoological surveys of Las Cajas 

 Plateau, Ecuador from 1981 to 1987, five specimens of an undescribed 

 species of ichthyomyine rodent were captured. Observations were 

 made on two of these animals, which subsequently escaped, while 

 three specimens were donated to The Natural History Museum. 

 Another specimen was filmed for the 1992 BBC National Geo- 

 graphic wildlife film 'Avenue of the Volcanoes' (Jim and Theresa 

 Clare, personal communication). On the basis of external and 

 craniodental characters the study specimens agree most closely with 

 the generic diagnosis of Chibchanomys but are also sufficiently 

 similar in some features to Neusticomys to warrant a phylogenetic 

 analysis. 



MATERIALS AND METHODS 



The description of the new species is based on conventional morpho- 

 logical characteristics and the terminology used follows Voss (1988). 

 Specimens were measured using dial calipers, with all measure- 

 ments provided in millimetres. The skeletal elements remaining in 

 the skins were observed by means of X-rays. 



A parsimony analysis (PAUP Version 3.0) was carried out to 

 determine the position of the new species relative to other 

 ichthyomyines. Details of the eighteen characters used, listed 

 below, are given more fully in Voss (1988 pages 440^442); the 



hypothesised primitive state, using the criteria defined by Voss, is 

 scored as '0'. 



1. Pelage: glossy (0); dull (1). 



2. Ventral pelage countershaded: absent (0); present (1). 



3. Tail: unicolored (0); bicolored (1). 



4. Philtrum: present (0); absent (1). 



5. Pinnae: large, visible above fur (0); small, concealed in fur (1). 



6. Superciliary vibrissae: present (0); absent (1). 



7. Plantar pads of manus: hypothenar pad separate, not fused with 

 third interdigital pad (0); hypothenar and third interdigital pads 

 fused (1); hypothenar and thenar pads fused respectively with 

 adjacent third and first interdigital pads (2). 



8. Fringing hairs on pes: weakly developed (0); well developed ( 1 ). 



9. Lower third molar: entoconid-hypoconid cusp pair distinct (0): 

 m3 peglike, entoconid-hypoconid cusp pair absent or reduced to 

 a small conule (1). 



10. Nasal bones: long, produced anteriorly beyond premaxillae (0); 

 short, truncated behind premaxillae (1). 



11. Supraorbital foramina: on the lateral surface of the frontals, 

 within orbital fossae (0); on the dorsal surface of the frontals 

 between the orbital fossae (1). 



12. Carotid arterial supply (see Voss, 1988 page 296): pattern 1 (0); 

 pattern 2(1); pattern 3 (2). 



13. Orbicular apophysis of maleus: present (0); absent (1). 



14. Metatarsal configuration: III > IV > II » V > 1 (0); IV > III > II, 

 V>I(1). 



15. Omohyoid muscle: present (0); absent (1). 



16. Gastric glandular epithelium: present (0); restricted (1). 



17. Gall bladder: present (0); absent (1). 



18. Bacular cartilage: tridigitate, medial digit lacking a calcified 

 centre (0); single digit (1); tridigitate, medial digit grossly 

 swollen with calcified core (2). 



The character states for the new species were assessed by PJ but the 

 character state assessments for the other taxa were taken directly 

 from Voss (1988 Table 45, page 441). Characters of the visceral and 

 reproductive systems (characters 15-18 above) were unobservable 

 in the new taxon because of the lack of whole bodies. In an initial 

 analysis, all character states were unordered; in a second analysis, 

 multistate characters were ordered (as by Voss): — > 1 — > 2 for 

 characters 7, 12 but also for character 18. For character 18, the order 

 recommended by Voss (1988) was — » 1; — > 2, a sequence not 

 readily handled by the analysis and affecting only one generic group 



© The Natural History Museum, 1997 



