The Central Indochina Limestone is the most extensive tracts of limestone karst habitat in Indochina (Sterling et al. 2006) which straddles the border between Vietnam (Quang Binh and Quang Tri Provinces) and Lao People's Democratic Republic (Khammouane Province). This landscape harbors an endemicity in birds and mammals which would appear to exceed that of other limestone areas of Southeast Asia (Baltzer et al. 2001, Tordoff et al. 2003). Recent studies have discovered two new rodent species: Laotian Rock Rat Laonastes aenigmamus (Jenkins et al. 2005) and Paulina's Limestone Rat Saxatilomys paulinae (Musser et al. 2005) in Khammouane Limestone (NBCA) in Khammouane Province. Laonastes aenigmamus is listed in IUCN Red List as Endangered while Saxatilomys paulinae as Data Deficient as this is a very recently described species and the limits of its disrtibution are not yet well known. There is a need for further survey work to determine whether it might occur more widely than current records suggest, and its population status (Lunde and Musser 2008). Providing that comparable environments are extant in Quang Binh province (Vietnam), we expected that these two species would inhabit also Quang Binh limestone. Thus, from 2011 to 2014, we conducted extensive rodent surveys and collected specimens of both, Laonastes aenigmamus and Saxatilomys paulinae, in the area. These are the first records of Laonastes aenigmamus and Saxatilomys paulinae in Vietnam. We have published the data on Laonastes aenigmamus in Dang et al. (2012) and Nguyen et al. (2014). This paper provides new data on morphology and habitat characteristics of Saxatilomys paulinae in Vietnam.

Quang Binh limestone habitat is characterized by very specific natural conditions of precipitous karst ridges, which rise to elevations of around 400 m. Scattered among this ridges are narrow valleys and pockets of igneous rock formations. The limestone karst is almost entirely forested, apart from steep cliff faces. The forest clearance occurs only in flat valleys within the limestone massif and in lowland area bordering it. The most widespread forest type in the landscape is limestone evergreen forest, but there are also significant areas of lowland evergreen forest distributed on non-calcareous substrate in valleys among the limestone karsts (Le et al. 2012). The climate in the area is tropical, hot and humid. The annual mean temperature ranges between 23 °C and 25 °C, with summer maximum of 41 °C and winter minimum of 6 °C. The high annual rainfall averages 2,000 - 2,500 mm, 88% falling between July and December, though there is rain in every month. The mean annual relative humidity is 84% (Le et al. 2012). Because of limestone topography, drainage is complex and there area few permanent water sources.

We used Sherman live traps and Tomahawk live traps to sample the rodent specimens. At each survey location, trap-transects were established within various habitat types and contain 30-50 traps each depending on habitat type. Traps were set at each transect for 4–6 days and checked on subsequent mornings (for specimens and for bait replacement). Baits were made of fresh manioc or sweet potato. All captured animals were anesthetized and measured for external measurements following Lunde and Le (2001): head-body length (HB), tail length (T), hind-foot length (HF), and ear length (E) (in mm), and weight (W) (in g). Captured animals were identified, sexed and the time of capture recorded. After examination, captured rodents were released immediately at capture point.

Those individuals that could not be identified to the species level in the field (this includes all specimens of Saxatilomys paulinae) were preserved in 70% ethanol for later identification using museum mammal collections. Species identification follows Musser et al. (2005)​. All voucher specimens of rodents were cataloged and stored at zoological museums of Institute of Ecology and Biological Resources in Hanoi, Vietnam (DVZ-PNKB-01, DVZ-PNKB-03, DVZ-PNKB-04. DVZ-PNKB-07). Craniodental measurements of the voucher skulls were taken according to Musser and Newcomb (1983) with vernier calipers to the nearest 0.05 mm. Following measurements were taken from each specimen: occipitonasal length (ONL), zygomatic breadth (ZB), interorbital breadth (IB), length of rostrum (LR), breadth of rostrum (BR), breadth of braincase (BBC), height of braincase (HBC), breadth of zygomatic plate (BZP), length of diastema (LD), length of incisive foramina (LIF), breadth of incisive foramina (BIF), length of bony palate (LBP), breadth across bony palate at first molars (BBP), postpalatal length (PPL), breadth of mesopterygoid fossa (BMF), length of bulla (LB), crown length of maxillary molar row (CLM1-3) and breadth of first upper molar (BM1). The craniodental measurements are taken only from adult specimens. Species identification follows Lekagul and McNeeley (1988) and Francis (2008).

Study of habitat was carried out using transect and plot techniques (White and Edwards 2000). Plots of 10 x 10 m were used for inventory of all trees with height more than 3 m, plots of 4 x 4 m were used for inventory of bush trees of height from 0.5 m to 3 m and plots of 1 x 1 m were used for inventory of herbs and tree seedlings of less than 0.5 m high. Topographic features and surface ground substrates were also noted. Threats to rodent populations and habitat were evaluated based on interviews of local villagers and direct field observation of threat signs (traps, hunters, logging, forest clearing, human encroachment) in the survey area.

Simple comparison of external and craniodental measurement of specimens from Vietnam with those of specimens from Lao indicates that specimens from Vietnam are generally larger than specimens from Lao (Table 1​). However, due to very small number of specimens examined, this needs to be verified by more studies in future.

In Lao, Saxatilomys paulinae was reported to inhabit steep rocky slopes with large limestone boulders covered in heavily degraded deciduous forest mixed with scrub and bamboo at the base of the surrounding massive karst (Musser et al. 2005). Vietnam's population of Saxatilomys paulinae was found in rocky slopes with large limestone boulders, under the limestone humid evergreen forest which is different from the deciduous forest type in Lao. This indicates Saxatilomys paulinae can tolerate different limestone forest habitats existing in the Central Indochina Limestone landscape.

Before this study, Saxatilomys paulinae was recorded only in the Phoun Hin Poun NBCA in Khammuoane province of Lao (Musser et al. 2005). Our records of Saxatilomys paulinae in Quang Binh province expand global distribution range of this species into Vietnam's part of Central Indochina Limestone for about 100 km east-ward. However, the range of the species distribution remains restricted by only five known localities (four localities in Lao and one locality in Vietnam), while trapping and habitat disturbance remain as current threats to the species survival. More surveys need to be conducted to assess the species distributional range and the status of its populations for undertaking relevant conservation measures.

It is interesting that both Saxatilomys paulinae and Laonastes aenigmamus share the same limestone forest habitat in Central Indochina Limestone. The Laonastes aenigmamus is the only surviving member of the otherwise extinct rodent family Diatomyidae, that was formerly believed to have been extinct for more than 11 million years (Dawson et al. 2006). Both species are currently known only from few localities in Central Indochina Limestone. This again indicates high importance of the Central Indochina Limestone for the global biodiversity conservation.