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A Rodent Described from the Mathews Range, Central Kenya

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A Rodent Described from the Mathews Range, Central Kenya PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON 119(4):477-515. 2006. The status of Ototnys or estes dollmani Heller, 1912 (Muridae: Otomyinae), a rodent described from the Mathews Range, central Kenya Michael D. Carleton* and Ellen Schaefer Byrne (MDC) Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560-0108, U.S.A., e-mail: [email protected]; (ESB) 4209 Lawn Avenue, Western Springs, Illinois 60558, U.S.A. Abstract.•Morphological and morphometric comparisons of Otomys orestes dollmani, known only from Mount Gargues in the Mathews Range, central Kenya, indicate that the taxon is a valid species distinct from other African forms under which it has been previously synonymized (O. irroratus, O. orestes, O. tropicalis). Based also on these comparisons, the morphological recognition and distribution of O. orestes (including thomasi Osgood) are further clarified in relation to O. tropicalis (including elgonis Wroughton) in Kenyan mountains and O. typus proper in Ethiopian highlands; another taxon relegated to junior synonymy within O. typus, O. uzungwensis Lawrence and Loveridge from south-central Tanzania, is resurrected to species. Certain traditional characters used in Otomys taxonomy, in particular molar lamination, demonstrate conservative patterns of variation that complement spatial structure derived from morphometric analyses of craniometric data and that vindicate their continued utility in delimiting species. We argue that uncritical emphasis of polytypic species, applied following the biological species concept during the latter 1900s, has led to chronic underestimation of species diversity of Otomys confined to the Afromontane Biotic Region in eastern Africa, in particular those populations that inhabit afroalpine environments. Vlei or laminate-toothed rats of the Honacki et al. 1982, Corbet & Hill 1980, genus Otomys (Muroidea: Muridae) con- 1986, 1991; Musser & Carleton 1993). stitute a distinctive morphological and Musser & Carleton (2005) recently ac- phylogenetic radiation indigenous to the knowledged 23 species. In the most savannas and highlands within Subsa- considered revisionary study attempted haran Africa (Bohmann 1952, Carleton & to date, Bohmann (1952) recognized only Musser 1984, Watts & Baverstock 1995, 11 species, most of them containing from Ducroz et al. 2001, Taylor et al. 2004a). 3 to 5 well-marked subspecies and the Since the circumscription of the group as exceptionally polymorphic O. irroratus a suprageneric taxon (Otomyinae Thom- embracing 23 geographic races. Much of as, 1897), the number of Otomys {sensu the indecision over specific diversity has lato) considered to be valid species has involved populations distributed across vacillated greatly, ranging from 30 (Eller- eastern African mountains north of the man 1941) to as few as 8 (Petter 1982) but Zambezi River•from the Nyika Plateau usually around 12-14 (Misonne 1974, and Eastern Arc Mountains, through the ranges and volcanoes fronting the West- ern and Eastern Rift Valleys, to the * Corresponding author. Ethiopian Highlands. Within this region, 478 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON named forms have been variously and especially the last emerging M3. This inconsistently treated as subspecies of O. junction marks the border of the glossy irroratus, O. tropicalis, or O. typus. white enamel of the crown and dull white The present study focuses on one such cementum of the root and is visible as geographically localized population, Ot- a faint longitudinal line along the base of omys or es tes dollmani Heller (1912), de- the tooth (in lateral view). Those individ- scribed from Mount Gargues in the uals whose molars were incompletely Mathews Range, a small and relatively erupted, such that the line of the crown- low-elevation range lying to the north of root junction was visible on only the M1 Mount Kenya in central Kenya (Fig. 1). or Ml•2, were categorized as juveniles. Following its description, dollmani has Adult specimens were identified by the been sequentially referred to O. orestes visibility on all molars of the crown-root (Dollman 1915, Allen 1939, Ellerman line, which initiaUy appears approximate- 1941), O. irroratus (Bohmann 1952), or ly even with the maxillary alveoli (young O. tropicalis (Misonne 1974, Musser & adults); by this growth stage, the occlusal Carleton 1993). Early on, however. Hoh- surface of the molar row approximates lster (1919:148) had pointedly raised a horizontal plane. With advancing wear, dollmani to species until "actual inter- the crown-root line is progressively dis- grading specimens are found," a pragmat- placed above the plane of the maxillary ic viewpoint endorsed by Musser & bone, crown height is correspondingly Carleton (2005). Here, we consolidate diminished, and dentinal cores of the morphological and morphometric evi- individual laminae progressively expand dence that decidedly supports the last in width and length (full and old adults). interpretation. Our juvenile tooth-wear stage essentially concurs with age classes 1 and 2 as Materials and Methods defined by Taylor & Kumirai (2001), and the young, full, and old adult wear Specimens reported herein consist prin- stages to their age classes 3, 4, and 5, cipally of skins with their associated respectively. skulls and are contained in the following Specimens of Otomys, especially those North American museum collections: originating from older historically impor- Carnegie Museum of Natural History, tant surveys, complicate study of their Pittsburgh (CM); Field Museum of Nat- craniometric variation because a substan- ural History, Chicago (FMNH); Museum tial percentage of skulls encountered in of Comparative Zoology, Harvard Uni- museum collections are damaged. Com- versity (MCZ); and the National Museum monly, nasal bones are dislodged, zygo- of Natural History, Smithsonian Institu- matic arches broken, or the braincase tion, Washington D.C. (USNM, formerly fractured. Hence, we have taken a robust U.S. National Museum). approach to our definition of operational Relative age was crudely assessed for taxonomic units (OTUs), identifying an- every specimen examined based on tooth- alytical samples that correspond to moun- wear stages, employing a combination of tains or mountain ranges instead of single molar eruption and degree of molar localities and precise altitudes. Further, abrasion to identify four categories (juve- rather than eliminate those variables niles, young adult, full adult, old adult). taken on frequently broken cranial parts In view of the hypsodont molars charac- in order to amplify specimen numbers for teristic of Otomys, we used the appear- multivariate analyses, we have elected to ance of the basal crown-root junction to accept smaller sample sizes to capture gauge the degree of molar eruption. more fully size and shape differences. VOLUME 119, NUMBER 4 479 Cherangani jacksotíl^ ' íES (1891) • ' 1 ^ Ó ^ % ^ dollmani <%, • (1912) ^^x L'nubilus Mt Kenya \ '1-3' (1915) malleus f thomasi ^ * (1915) .¿^t (1910) troplcalis percivali (1902) tk (1915) •^ '--'"^ •^ • squalus ^•^ (1915) > 2500 m 2000-2500 m j 1500-200Öm Fig. 1. Mountainous region of eastern Africa inhabited by populations of Otoinys addressed herein, indicating type localities of relevant taxa and principal geographic features. Only length of rostrum, a measurement nomial species-group taxa of O. tropicalis that involves the nasal tips, was conven- are intended only to convey current tionally omitted in morphometric com- synonymy, not to denote any accepted parisons. subspecific classification. Full provenance Twelve OTUs, their two-letter abbrevia- and museum registration numbers are tions, and sample sizes of crania measured provided either in the Taxonomic Sum- were identified as follows. Provisional mary or in Appendix 1. Abbreviations are species taxonomy observes the arrange- used here and throughout the text for ment of Musser & Carleton (2005); tri- mount or mountains (Mt, Mts). 480 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON Otomys dollmani: Kenya, Mt Cargues tween the lateral edge of the upper lobe of (MG, n = 6, including the holotype the exoccipital condyles; length of ros- USNM 181790). trum (LR), an oblique line measured from Otomys irroratus: South Africa, East- the posterior bevel of the right zygomatic ern Cape Province (EC, n = 21); Kwa- notch to the end of the nasal bones at Zulu-Natal Province (KZ, n = 17); their midsagittal junction; breadth of Western Cape Province (WC, n = 32). rostrum (BR), caliper points on the nasal Otomys orestes: Kenya, Aberdare Mts process of the premaxillaries just inside and Mt Kenya (OR, n =10). the dorsal zygomatic notch; postpalatal Otomys tropicalis elgonis: Kenya, Aber- length (PPL), from the anteriormost bevel dare Mts (AM, n = 24); Uasin Gishu of the mesopterygoid fossa to the mid- Plateau (UG, n = 7); Kaimosi and notch of the basioccipital; length of bony Kakamega (KA, n = 21); Kenya and palate (LBP), from the anteriormost bevel Uganda, Mt Elgon (ME, n = 19). of the mesopterygoid fossa to the poste- Otomys tropicalis tropicalis: Kenya, Mt rior end of the left incisive foramen; post Kenya (MK, n = 28). palatal breadth (PPB), distance across the Otomys typus: Ethiopia, all localities maxillary bones just behind the third (ET, n = 14). molars; length of incisive foramen (LIE), Otomys uzungwensis: Tanzania, Ud- greatest anterior-posterior expanse mea- zungwa Mts (UZ, n = 14). sured on the left side; length of diastema Sixteen
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