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DIFFERENTIATION OF APODEMUS IN View in CoL CHINA AND THE ORIGIN OF A. NIGRUS

The landscape of China is a complex mosaic, characterized by high-plateau mountain steppes in the west, temperate dry steppes and deserts in the north, a complex of mountain ranges harbouring dense rainforest in the centre and subtropical, low-elevation forests on the east coast, the so-called three levels of terrain in China. Such habitat heterogeneity appears to favour the diversification of Apodemus species. The large genetic distances and significant morphological variations among these species probably resulted from a long history of differentiation. For example, the split of A. draco and A. ilex , the youngest sister species of this genus distributed in China, was dated to approximately 3.26 Mya. Based on the locations of molecular voucher specimens, A. agrarius is widely distributed in the north to the south-east of China, A. peninsulae occurs along the ‘Hu Huanyong Line’ ( Hu, 1935) or ‘the 400 mm annual precipitation line’ from north-east to south-west China, A. uralensis inhabits the north-west Qinghai – Tibet plateau and Xinjiang, and A. semotus is endemic to Taiwan ( Liu et al., 2017). The remaining species, A. draco , A. chevrieri , A. latronum , A. ilex and A. nigrus are endemic to the mountainous region of southern China. This region harbours a high diversity of species and reflects a marked heterogeneous habitat zonation along mountain slopes, in some cases mirroring tropical– temperate forest transitions. Several new mammal species were described from this zone in recent years (e.g. Fan et al., 2017; Ge et al., 2018b), suggesting a still overlooked biodiversity in China. The discovery of the new species A. nigrus from the Wuling Mountain chain in Guizhou and Congqing Provinces reinforces this trend.

The Wuling Mountains are located in central China, which is a transitional zone connecting the lowest and highest of the three levels of terrain in China, with a mean elevation of approximately 1000 m. This region constitutes one of the major components of the Jiangnan orogenic belt in southern China. The climate of the Pliocene here was characterized by a warm period from the Early to the Middle Pliocene (2–3°C higher than today) and a transition from relatively warm climates to the prevailing cooler climates of the Pleistocene. The greatest warming appears to have occurred at higher latitudes, where temperatures were often sufficiently elevated to allow species of animals and plants to exist at higher latitudes than their closest modern relatives. Historical exploration rarely paid attention to the Wuling Mountains, which is evident today by sparse material for different taxa from this region in museums.

The new species A. nigrus is found on the Fanjing and Jinfo Mountains, both belonging to the Wuling Mountain chain. Fanjing Mountain (or Fanjingshan ) is the highest mountain in the Wuling Mountains. The peak of this mountain is 2570 m above sea level. The mountain acts as an ‘isolated ecological island’ with a high degree of endemism of plants and animals. Fanjing Mountain is unique in its geological history, landforms, geographical location and climatic conditions, and all of these factors have created a terrestrial island with a specific ecological environment. The main peak of Fanjing Mountain is separated from the surrounding karst areas by its high elevation, and it was selected as a World Heritage Site in 2018. It is an ecological island that hosts the endangered Guizhou golden monkey ( Rhinopithecus brelichi Thomas, 1903 ), the Chinese dove tree ( Davidia involucrata Baill. ), various alpine rhododendrons (Rhododendron spp.) and many other rare animals and plants. With an elevation of 2238 m, Jinfo Mountain is the highest mountain in Chongqing Province, and it is famous for its high biodiversity.

The split of A.nigrus from its sister taxa likely occurred during the Middle Pliocene, approximately 4 Mya. Climate warming drove its preferred habitat upwards, which caused this species to inhabit higher elevations. However, this region was likely less affected by climatic oscillations in the Late Quaternary than other regions of the world, for example, the stable demographic history of Eothenomys melanogaster (Milne-Edwards, 1871) and the survival of five Chinese giant salamander species that diverged over four million years ago in the same region (Lv et al., 2018; Yan et al., 2018). Subsequent global cooling allowed this species to migrate slightly to lower elevations, but the dependence of the species on local vegetation and climate warming at lower latitudes likely impeded widespread dispersal.

Abbreviations for species are the same as Table 2. ‘ N ’ gives the total number of intact adult specimens included in analyses. BM = body mass, EM = external measurements, HBL = head and body length, TL = tail length, HFL = hind foot length, EL = ear length, CM = craniodental measurements, TLC = total length of the cranium, NL = nasal length, GWS = greatest width of the ‘snout’, SDO = shortest distance between orbits, ZB = zygomatic breadth, GMB = greatest mastoid breadth, BL = basal length, BSL = basilar length, PL = palatal length, IFL = incisive foramen length, WIF = width of the incisive foramen, GPB = greatest palatal breadth, LTB = length of the tympanic bulla, ULMM = length of the maxillary molars, ULMD = length of the maxillary diastema, ML = mandibular length, LLMM = length of the mandibular molars, LLMD = length of the mandibular diastema.

TAXONOMIC ACCOUNT

Order: Rodentia

Family: Muridae

Subfamily: Murinae

Apodemus Kaup, 1829 .

APODEMUS NIGRUS DEYAN GE, ANDERSON FEIJÓ &