Typhlomys taxuansis, Balakirev & Phuong & Rozhnov, 2024

Balakirev, Alexander E, Phuong, Bui Xuan & Rozhnov, Viatcheslav V, 2024, Typhlomys taxuansis (Rodentia, Platacanthomyidae): new species of the genus from northern Vietnam with notes on conservation status and distribution, Biodiversity Data Journal 12, pp. e 133363-e 133363 : e133363-

Typhlomys taxuansis sp. nov.

Holotype: ZMMU S- 210284 skull and flat skin (field number: BY- 60), adult female (Figs. 2 and 4) collected on 25 November 2023, by Alexander E. Balakirev and Bui Xuan Phuong. The specimen deposited at the Zoological Museum of Moscow State University, Moscow, Russia, is shown in Fig. 4 .

Materials

Type status: Holotype. Occurrence: catalogNumber: ZMMU S-210284 ; recordNumber: BY- 60; recordedBy: Alexander E. Balakirev; individualCount: 1; sex: female; lifeStage: adult; occurrenceStatus: present; preparations: skin; skull; disposition: in collection; associatedSequences: GenBank: PP 987021 - PP 987022 and PP 987159; occurrenceID: D570D565-0C0F-566C-8D44-DE2C3E91708F; Taxon: scientificName: Typhlomys taxuansis ; Location: higherGeographyID: Ta Xua Nature Reserve, prov Son La, Vietnam; higherGeography: Asia; continent: Asia; country: Vietnam; countryCode: VN; stateProvince: Son La; county: Bac Yen; locality: 4 km east from Y Xoa Homestay ; verbatimLocality: 4 km east from Y Xoa Homestay; minimumElevationInMeters: 2000; maximumElevationInMeters: 2200; decimalLatitude: 21.32218; decimalLongitude: 104.495873; geodeticDatum: WGS 84

Description

Measurements of holotype (mm): BM = 21.10 g; HB = 85.0; TL = 117.0; HL = 25.0; EL = 18.0; ONL = 25.92; ZB = 13.85; IB = 5.59; LR = 8.34; BR = 3.67; BBC = 11.49; HBC = 8.55; ZBP = 1.78; LD = 7.08; LIF = 1.53; BIF = 1.87; LBP = 11.27; BBP = 4.00; PPL = 9.21; BMF = 1.78; LB = 3.42; CLM 1-3 = 4.14; BM 1 = 1.18; CLM 1-3 = 4.29; and BM 1 = 1.04.

One of the larger species within Typhlomys (HB = 85; ONL = 25.92). Vibrissae very long white; ears prominent, almost bare; eyes vestigial (Fig. 2 ). Dorsal body colouration: dark grey; entire ventral body from chin to anus, including inner side of limbs to wrists and knees; greyish due to dark grey hair base and white tip. Fingers four at fore-limbs and five on hind ones; hind feet slender and elongated (HL = 25 mm); plantar palms of all limbs light brown; fingers pale whitish; skin on dorsal surfaces of hind feet brownish, covered with slender hair. Tail long, well exceeding head and body length (TL = 117 mm), with scale rings; proximal third of the tail covered with extremely short and sparse hairs, back part is covered with longer hairs than the ventral side; and the distal half of the tail has tufts of long, dark grey hairs with no white inclusions.

The braincase is generally dome-shaped and relatively high due to its large size ( HBC = 8.55 mm). The rostrum is straight beyond the upper incisors. Tympanic bullas are small. Zygomatic plate narrow; zygomatic arch strait, not incurvate on approximately equal thickness throughout its length. The incisive openings are small, rounded and have a pointed anterior edge. The bony palate is pierced by two pairs of additional symmetrical foramina, with the first pair, located under the rostrum, being approximately half as long as the posterior pair, located between the teeth. Diagonal bone trabeculae are clearly visible deep inside them. Dental formula is usual for the genus 1.0.0.3 / 1.0.0.3 = 16. M 1, with almost equally wide anteroloph and posteroloph. M 1 antherofosette is divided into two separate parts (Fig. 4 H). The first molars of the upper jaws have six dark fossette-shaped structures; the first lower molars have only five dark fossette-shaped structures; the second upper and lower ones have four fosettes; and the third molars have two and four. The mesofossette on M 1 is open on only the lingual sides. M 2 with anterofossette, divided by a complete, well developed mesofosette; m 1 with two antherofossettids and a closed mesofossettid. Anterofossettids are present in m 2, but relatively short. m 3 mesolophid has a crescent shape due to the facet protruding from the lingual side (Fig. 4 H).

Diagnosis

The new species, morphologically, is most similar to T. daloushanensis , but can be distinguished, based on its dental and skull morphology. Based on genetic diversity, the most relative genetic lineage is T. fengjiensis . It obviously differs from geographically most adjacent species T. chapensis and T. nanus by a more flattened braincase; from all known Typhlomys species, except for T. cinereus , by zygomatic arch with deeper incurve; from T. cinereus by mesofossette on M 1 open on both buccal and lingual sides rather than open on the buccal side only; and from T. nanus by posterofossettid on M 1 present. The new species further differs from other species, except T. daloushanensis , by anterofossette on M 2 present.

Etymology

The specific Latin name taxuansis composed as an adjective refers to its type locality in Ta Xua Nature Reserve, Son La Province, Bac Yen District, Vietnam. Due to the sampling location being the southernmost location currently known for this genus, we suggest “ southern blind tree mouse ” as the English common name.

Distribution

The new species is currently obtained only from the type locality, but may also be distributed in the adjacent mountainous areas in the northern-western part of Vietnam, north of the Da River. A close genetic similarity between the specimens and the Yunnan findings was found. Additionally, the genetic relationship with the cinereus group species, widespread in central and eastern China, suggests that the range of this species may extend eastwards to the provinces of north-eastern Vietnam, as well as the Chinese Province of Guanxi. Based on the ecological characteristics of habitats, it may also be distributed southwards, for example, at the Annamite Range, both on the Laotian and Vietnamese sides, but there are still no notes on its findings to the south from the Da River.

Ecology

The specimen investigated was captured in moist, misty mountainous forests at mid-altitudes (2000–2200 m a. s. l.). Sympatric species include Neotetracus sinensis , Eothenomys miletus , Dremomys ornatus , Dremomys gularis , Niviventer lotipes , Niviventer fulvescens , Mus pahari and Leopoldamys edwardsi , these being mostly the species of the Chinese mountain faunistic complex.

Conservation

The genus Typhlomys has recently been assessed for the IUCN Red List of Threatened Species in 2016. The only species recognised there to date, Typhlomys cinereus , is listed as Least Concern ( Smith 2016). This is obviously outdated information and does not reflect current taxonomy improvements in this group. In fact, data on the conservation status of this rather exotic group of rodents is almost non-existent for most species. Here, we will try to close this gap to some extent, relying on modern data.

Based on literature sources and original data, we can compose a distribution map for the genus Typhlomys as shown in Fig. 1 . The range area of the species, estimated from an ellipse containing all reliable finds, may range from about 1.27 mln km 2 for T. cinereus with T. huangshanensis , 330000 km 2 for T. daloushanensis , 60000–70000 for T. chapensis and T. nanus , about 12000 km 2 for T. taxuansis to only 80–100 km 2 for T. fenjiensis . At the same time, the real area of mountain forest within this zone is 10–20 times smaller. This circumstance is obviously of the utmost importance for environmental protection.

In agreement with IUCN rules ( Anonymous 2001, Anonymous 2012), there are five quantitative criteria that are used to determine whether a taxon is threatened or not and, if threatened, to which category of threat it belongs (Critically Endangered, Endangered or Vulnerable). These five criteria are: population size reduction (past, present and / or projected); geographic range size and fragmentation, few locations, decline or fluctuations; small and declining population size and fragmentation, fluctuations or a few subpopulations; a very small population or very restricted distribution; and quantitative analysis of extinction risk.

Of the five circumstances given, only geographic range size and fragmentation match these species’ situation. The data currently available does not suggest either a very small population or any decline or reduction in population. For T. cinereus proper and T. daloushanensis , their natural ranges and the number of localities where the animals are listed allow for confirmation of their Least Concern ( LC) status. For another, more profound inspection of needs. In agreement with IUCN rules, to qualify for criterion geographic range size and fragmentation, the general distributional threshold must first be met for one of the categories of threat, either in terms of extent of occurrence ( EOO) or area of occupancy ( AOO). The taxon must then meet at least two of the three options listed for this criterion. The options are: (a) severely fragmented or known to exist in no more than “ X ” locations; (b) continuing decline; or (c) extreme fluctuation ( IUCN Anonymous 2001, IUCN Anonymous 2012). Of these criteria, only B 1 a is obviously applicable for all other species — fragmentation of the area due to the natural fragmentation of landscapes. It should be noticed that, within the category, T. nanus , T. taxuansis and T. fengjiensis could formally be classified as VU (Vulnerable) according to criterion B 1 (Extent of Occurrence EOO) and, according to subcategory B 2 a (Area of Occupancy AOO). For T. fengjiensis , it is possible even to be Endangered due to the less than ten closely situated localities currently identified. However, there is still a little information about the natural situation to date.

For T. taxuansis , the range area of the species, estimated from an ellipse containing known finds, including closely-related Chinese samples, covers about 12,000 km 2. At the same time, the real area of cloud forest vegetation within this zone is 10–20 times smaller. On the other hand, there is every reason to believe that the species is distributed much more widely, out of cloud forests and may well be in karst vegetation covering many hundreds of square kilometres in the regions of northern Indochina and southern China. It should also be noted that, in accordance with the IUCN rules, in the absence of any plausible threat to the taxon, the term " location " cannot be used and the subcriteria that refer to the number of locations will not be met. As far as is known to date, all the species apparently show neither a noticeable decrease in abundance (b) nor significant fluctuations (c) in relation to the size of the range or the number of individuals.

Thus, we believe that the category Least Concern may be applied to T. cinereus and T. daloushanensis , along with Near Threatened B 1 a + 2 a and that the current population trend is stable for T. chapensis , T. nanus , T. taxuansis and T. fengjiensis species. The main threats to its conservation are not primarily linked to direct impacts and population reduction, but rather to its association with specific and highly- specialised habitat types, such as high-level mountain and cloud forest vegetation, which may limit the potential distribution for many species.

Taxon discussion

The molecular dating analyses suggested that divergences within Typhlomys started during the middle or late Miocene. Divergence in the Miocene is usually considered a genus-level diversification ( Jansa et al. 2009). In China, remains of several species of blind tree mice have been found in the Upper Miocene ( Qiu 1989, Qiu and Ni 2019), Pliopleistocene ( Qiu Z and Jin C-Z 2017), Lower Pleistocene ( Zheng 1993, Fejfar and Kalthoff 1999, Zheng 2004, Jin et al. 2009, Wang et al. 2014) and lower Upper Pleistocene. This timing is congruent with fossil records of Platacanthomyidae ( Qiu 1989) . A special pattern of tooth morphology was also found in four congeneric fossil species from the late Miocene to the Pleistocene in China ( Qiu 1989, Zheng 1993). However, the recent species available only differed from each other by appearance, size and tender features of pelage colour. The patterns and structures of their molar teeth for many species are rather similar. Considerable differences in tooth size and tooth crown height were observed in fossil species, but the patterns and structures of teeth have remained stable since the early Pleistocene. However, as we can see, in a number of cases, quite distinct morphological features of the structure of the chewing surface can be detected, marking the species.

The palaeontological history of Typhlomys in Vietnam has been largely unknown until recently, but this year a fossil of Typhlomys stegodontis sp. nov. was described, based on a maxillary fragment and isolated teeth from the Middle Pleistocene Tham Hai cave locality in northern Vietnam (Lang Son Province, about 200 km NE from Ta Xua). This first finding of the fossil Platacanthomyidae in Vietnam fills the Middle Pleistocene gap in the palaeontological record of the family ( Lopatin 2024). Interestingly, the parafossette M 1 in the holotype of the fossil T. stegodontis is obviously bifurcated, with an anterolingual branch reaching the enamel wall of the tooth. This structure is largely similar to the completely isolated additional fossette region observed in M 1 T. taxuansis , constituting one of its unique features. This similarity, as well as the geographical location of the finding, suggests an evolutionary relationship between these taxa. Unfortunately, the third upper molar of the fossil species has not yet been discovered, but surveys continue.

The distribution of the different Typhlomys species demonstrates a distinct geographic pattern, which could be partly due to the complex topography and low dispersal ability of many animals ( Fu and Zeng 2008, Zhou et al. 2012, Cheng et al. 2017, Hinckley et al. 2020). On the other hand, in Yunnan, scattered mountain ranges with elevations over 3000 m., such as the Wuyi and Huangshan Mountains, form patches of “ sky islands ” favoured allopatry in isolated areas ( McCormack et al. 2009). It should be noted that, taking into account new data, these islands of high-mountain habitats are occupied by representatives of the chapensis group and the cinereus group, which are mainly distributed in the east and often occupy lower-lying habitats. In addition, mountains provide a wide altitudinal range that helps buffer climate changes and has provided initially continuously suitable habitats for Typhlomys since the early Late Miocene ( Wu et al. 2013, He et al. 2019). This may indicate that, during the Late Miocene, when the most rapid speciation within the genus Typhlomys occurred, this taxon may have been a less pronounced montane group. Some of the originally temperate montane populations could have been locked within a lower montane region and were forced to adapt to an increasingly high-altitude climate as the mountains rose and montane vegetation belts moved in the Pleistocene. This may explain the proximity of ranges with true allopatry of T. nanus , T. cheapens , T. taxuansis and several as yet undescribed species and forms in the mountains of Yunnan and northern Vietnam. Lately, the complex topography of Yunnan mountains may facilitate allopatric speciation by physical isolation, eventually resulting in a series of endemic species with narrow isolated areas. Up till now, there are still a number of populations without genetic attribution from many regions ( Pu et al. 2022). Thus, the number of species of the genus Typhlomys in China and Vietnam still requires further profound biodiversity surveys, taxonomic and phylogenetic studies, which may result in many interesting findings.

Figures

Figure 1. Distribution map for genetically investigated Typhlomys species and lineages.

Figure 2. A new species of Typhlomys from Ta Xua Nature Reserve, specimen BY- 60, adult female, external view. Photo of Alexander E. Balakirev. A Dorsal view; B Ventral view; C Lateral view; D Belly, enlarged scale; E Head and backside, enlarged scale; F Distal half of tail with brush; G Hind foot, dorsal view.

Figure 4. The holotype of Typhlomys taxuansis, skull, ZMMU S- 210284 (field number: BY- 60, adult female). A Dorsal view; B Ventral view; C Lateral view; D Lower jaw, dorsal view; E Lower jaw, lateral view; H M 1 - M 3 upper molars, occlusal pattern, enlarged scale; I m 1 - m 3 lower molars, occlusal pattern, enlarged scale.