A new species of Oligodon Fitzinger, 1826 from the Langbian Plateau, southern Vietnam, with additional information on Oligodon annamensis Leviton, 1953 (Squamata: Colubridae)

We describe a new species of Oligodon from the highlands of the Langbian Plateau, southern Truong Son Mountains, Vietnam, based on morphological and molecular phylogenetic analyses. The new species, Oligodon rostralis sp. nov is distinguished from its congeners by the following morphological characters: medium size in adults (male TL = 582 mm); small and broad head with long protruding snout; dorsal scale row formula 15-15-13; 167 ventrals, 47 subcaudals; single preocular, single postocular; loreal and presubocular absent; six supralabials, third and fourth entering orbit; six infralabials, anterior four contacting first pair of chin shields; internasals separate from prefrontals; nasal divided; single anterior and two posterior temporals; cloacal plate undivided; hemipenes short, bilobed, bifurcating in anterior one third of their length, extending to 8th subcaudal, lacking spines and papillae, with a prominent transverse flounces and distal calyces; six maxillary teeth, the posterior three enlarged; dorsal pattern consisting of 14+4 large dark-brown blotches and a bright-orange vertebral stripe on tail and dorsum; and ventral surfaces in life cream laterally with dark quadrangular spots; dark temporal streak present, edged with white. We also provide additional information on O. annamensis, including a morphological dataset of all specimens known from natural history collections and confirmation of an earlier record of O. annamensis from Cambodia. We also provide the first record of O. annamensis for Dak Lak Province. Phylogenetic analyses of mtDNA genes (3,131 bp of 12S rRNA, 16S rRNA and cyt b) suggest sister relationships of Oligodon rostralis sp. nov. and O. annamensis and place them in one clade with the O. cyclurus and O. taeniatus species groups, which is concordant with previous studies on the phylogenetic relationships of Oligodon. Our study demonstrates high level of herpetofaunal diversity and endemism of Langbian Plateau and further supports the importance of this area for conservation herpetofaunal diversity in Indochina.

One of the least known and enigmatic Oligodon species from Indochina is Oligodon annamensis Leviton, 1953 which was described based on a single female specimen collected from ''Blao, Haut Donai'' in the Langbian plateau (currently Bao Loc, Lam Dong Province, south Vietnam) (Leviton, 1953;Leviton, 1960). Leviton (1953) was puzzled by the taxonomic and phylogenetic affinities of this species, and only after examining a second male specimen he assumed that O. annamensis might be a part of the ''taeniatus-cyclurus-complex'' (Leviton, 1960). The only other existing record of this species was recently published by Neang & Hun (2013), who reported a subadult specimen identified as Oligodon annamensis from Phnom Samkos Wildlife Sanctuary of the Cardamom Mountains in southwest Cambodia; over 600 km westwards from the type locality (Neang & Hun, 2013). However, identification of the Cambodian specimen was tentative and has not been confirmed by molecular analyses; no information on the phylogenetic position of O. annamensis is available until this paper.
During our recent surveys in Lam Dong and Dak Lak provinces of southern Vietnam we collected two Oligodon specimens superficially similar in morphology with description of O. annamensis. However, after a closer examination of specimens from Vietnam and Cambodia, comparison of diagnostic morphological traits and phylogenetic analyses of 3,131 bp of mtDNA, we were able to identify the Dak Lak and Cambodian specimens as O. annamensis, while the Oligodon specimen from Lam Dong Province showed a unique Morphological analysis. Color characters and patterns were recorded during examination of specimens in life and taken from digital images of the living specimens. Morphological characters and morphometric ratios considered to be of taxonomic importance for Oligodon were used for species descriptions and followed a number of recent revisions of the genus (David, Das & Vogel, 2008;David et al., 2012;Leviton, 1953;Leviton, 1960;Neang & Hun, 2013;Nguyen et al., 2016;Nguyen et al., 2017;Vassilieva et al., 2013;Vassilieva, 2015). All body measurements, except body and tail lengths, were taken under a binocular microscope using digital slide-caliper to the nearest 0.1 mm. Body and tail lengths were measured to the nearest millimetre with a measuring tape. The right hemipenis was forcedly everted by using water injection prior the preservation of the specimen. Methodology of ventral and subcaudal scales counts followed Dowling (1951). Terminology for hemipenial structures generally followed Smith (1943) and Dowling & Savage (1960). Maxillary teeth of the specimens were counted by examining both maxillae, directly by pushing back the soft tissue with a needle under binocular microscope prior to preservation.
The following measurements (all in mm) and counts were taken: snout to vent length (SVL)-measured from the tip of the snout to the vent; tail length (TaL)-measured from the vent to the tip of the tail; total length (TL)-sum of SVL and TaL; relative tail length to total length (RTL) calculated as tail length to total length ratio (TaL/TL); head length (HL) from the tip of the snout to the posterior margin of the mandible; head width (HW) measured at the widest part of the head immediately posterior to the eye; head width to head length ratio (HW/HL); snout length (SnL)-distance between the tip of the snout and anterior edge of eye; eye diameter (EyeL)-maximal horizontal length of the eye; frontal scale length/width (FrL/FrW)-length and width of the frontal scale; interorbital distance (IOD)-the shortest distance between the eyes; internarial distance (IND)-distance between the nostrils; number of maxillary teeth (DEN); dorsal scale rows at neck (ASR)-number of scale rows at one head length behind the head; midbody scale rows (MSR)-number of scale rows at midbody; dorsal scale rows anterior to the vent (PSR)-number of dorsal scale rows at one head length prior to the vent; dorsal scale rows formula (DSR)-referred to as a general scale formula in the form ''ASR-MSR-PSR'' (for number of dorsal scale rows at neck, midbody and prior to vent, respectively); first dorsal scale reduction (RED1)-the first reduction of dorsal scale rows, corresponding to a ventral scale; ventral scales (VS)-number of scales from the second ventral scale posterior to gulars to the vent excluding cloacal plate; cloacal plate (AP)-number of terminal ventral scales immediately anterior to vent; subcaudal scales (SC)-number of paired subcaudal scales excluding the terminal scute; total belly scales (Total Sc.)-sum of ventral and subcaudal scales; supralabials (SL)-number of scales on upper lip; SL-Eye-number of SL entering orbit; infralabials (IL)-number of scales on lower lip; infralabials contacting each other (IL-contact)-number of pairs of infralabial scales in contact; infralabials contacting the anterior chin shields (IL-CS)-infralabial scales contacting the upper chin shields; number of preocular scales (PrO); number of presubocular scales (PrsO); number of postocular scales (PtO); number of anterior temporals (Ate)-temporal scales which contact the postocular scales; number of posterior temporals (Pte)-temporal scales immediately contacting the anterior temporal scales; condition of loreal scale (LOR)-1-present, 0-absent, *-vestigial; condition of nasal scale (NAS)-D-vertically divided, E-entire, PD-partially divided; hemipenis shape-(1) unforked, a single organ with no lobes at apex; (2) bilobed, organ contains two lobes at its apex; hemipenis ornamentation-notes on ornamentation of organ (i.e., spinules, calyces or flounces); presence of appendages seen in situ (papillae sensu Smith, 1943); hemipenis length-length of the everted hemipenis in mm and relative to number of subcaudal scales. Symmetric characters are given in left/right order. protocol. We used the polymerase chain reaction (PCR) to amplify two fragments of mitochondrial DNA (hereafter mtDNA): the first fragment including partial sequences of 12S ribosomal RNA (rRNA), tRNA-Valine and 16 rRNA genes (total length up to 2035 bp) and a complete sequence of cytochrome b gene (1,096 bp). Primers used for both PCR and sequencing are summarized in Table 1. PCR protocol for 12S-16S rRNA mtDNA fragment in general followed Green, Orlov & Murphy (2010). For both primer pairs of 12S and 16S rRNA, we used the following PCR protocol: (1) initial denaturation step at 94 • C for 5 min; (2) 35 cycles of denaturation at 94 • C for 1 min, annealing at 55 • C for 1 min and extension at 72 • C for 1 min; (3) final extension at 72 • C for 10 min; and (4) cooling step at 4 • C for storage.
For cytochrome b sequences (fragment up to 1,096 bp) we used a modified PCR protocol of Dahn et al. (2018) with touchdown: (1) initial denaturation step at 94 • C for 5 min; (2) 10 cycles of denaturation at 94 • C for 1 min, annealing for 1 min with temperature decreasing from 50 • C to 45 • C (with cool-down at 0.5 • C per each cycle) and extension at 72 • C for 1 min; (3) 24 cycles of denaturation at 94 • C for 1 min, annealing at 45 • C for 1 min and extension at 72 • C for 1 min; (4) final extension at 72 • C for 10 min; and (5) cooling step at 4 • C for storage.
All PCR products were sequenced in both directions by Genomics BioSci & Tech Corp. (Taipei, Taiwan). Sequences were assembled and checked using sequencher 4.9 (GeneCodes). The obtained sequences are deposited in GenBank under the accession numbers MN395601-MN395604 and MN396762; MN396765 (Table 2).
Phylogenetic analyses. The 12S-16S rRNA datasets of Green, Orlov & Murphy (2010), Pyron et al. (2013), our newly obtained sequences and other Oligodon sequences available in GenBank were used to examine the position of the Lam Dong Oligodon sp. in the matrilineal genealogy of the genus (summarized in Table 2). In total, we analysed mtDNA sequence data for 52 specimens, including 43 samples of ca. 24 species of Oligodon, and eight outgroup sequences of other Colubrinae representatives, and sequences of Hebius vibakari (Natricinae) which were used to root the tree.
Nucleotide sequences were initially aligned in MAFFT v.6 (Katoh et al., 2002) with default parameters, and subsequently checked by eye in BioEdit 7.0.5.2 (Hall, 1999) and slightly adjusted. MODELTEST v.3.6 (Posada & Crandall, 1998) was applied to estimate optimal evolutionary models for the data set analysis. Mean uncorrected genetic distances (p-distances) were calculated in MEGA 6.0 (Tamura et al., 2013).   The matrilineal genealogy was inferred using Bayesian inference (BI) and Maximum Likelihood (ML) approaches. The best-fitting model for both BI and ML analyses for 12S-16S rRNA fragments was the GTR+G+I model as of DNA evolution suggested by the Akaike Information Criterion (AIC); for cyt b gene AIC suggested GTR+G model for first and third codon partitions, and HKY+G+I for second codon partition. BI was conducted in MrBayes 3.1.2 (Ronquist & Huelsenbeck, 2003); Metropolis-coupled Markov chain Monte Carlo (MCMCMC) analyses were performed run with one cold chain and three heated chains for twenty million generations and sampled every 2000 generations. Five independent MCMCMC run iterations were performed and 1000 trees were discarded as burn-in. The convergence of the runs were checked by exploring examining the likelihood plots in TRACER v1. 6 (Rambaut et al., 2014); the effective sample sizes (ESS) were all above 200. Nodal support was assessed by calculating posterior probabilities (BI PP).
We a priori regarded tree nodes with BI PP values over 0.95 and ML BS values 75% or greater as sufficiently resolved; while BI PP values between 0.95 and 0.90 and ML BS values between 75% and 50% were regarded as tendencies. Lower values were regarded as indicating unresolved nodes (Huelsenbeck & Hillis, 1993).

Phylogenetic relationships of Oligodon
Sequence and statistics. The final concatenated alignment of the 12S rRNA -16S rRNA fragment and cyt b gene sequences contained 3131 base pairs, of which, 1959 sites were conserved and 1049 sites were variable, of which 713 were found to be parsimonyinformative. The transition-transversion bias (R) was estimated as 1.89. Nucleotide frequencies were 38.0% (A), 22.0% (T), 24.5% (C), and 15.4% (G) (all data given for ingroup only).
MtDNA-based genealogy. Our mtDNA-based genealogy for the genus Oligodon ( Fig. 2) inferred the following set of phylogenetic relationships, which is generally consistent with the results of Green, Orlov & Murphy (2010). Several well-supported clades were recovered within Oligodon (see Fig. 2 1.0/98). 5. The newly discovered Oligodon sp. from Bidoup-Nui Ba NP is reconstructed as a sister lineage with respect to two specimens of O. annamensis from Vietnam and Cambodia (1.0/100); O. octolineatus from Sundaland tends to group with this clade, however with no node support (0.62/-). All these species are clustered together with Clade 4 with strong support (1.0/100) (see Fig. 2). Sequence divergence. The uncorrected p-distances for the 16S rRNA gene fragment among and within examined Oligodon species are presented in Table 3. Intraspecific distances varied significantly and ranged from p = 0% in a number of examined species to p = 2.3% in the O. cinereus complex and p = 2.8% in the O. cyclurus complex, what is most likely explained by incomplete taxonomy of these groups (Green, Orlov & Murphy, 2010;David, Das & Vogel, 2008;David et al., 2012); a more detailed study including topotype materials on these species complexes is required.
The interspecific genetic distances within examined Oligodon varied from p = 1.8% (between O. chinensis and O. formosanus) to p = 8.5% (between O. maculatus and O. octolineatus) ( Table 3). The newly discovered Oligodon sp. lineage from Bidoup-Nui Ba NP is highly divergent from other congeners and is most closely related to O. annamensis with Table 3 Genetic differentiation of Oligodon. Uncorrected p-distance (percentage) between the sequences of 16S rRNA mtDNA gene (below the diagonal), estimate errors (above the diagonal) and intraspecific genetic p-distance (on the diagonal) of Oligodon species included in phylogenetic analyses. p = 3.3% of sequence divergence in 16S rRNA gene between these taxa. This divergence value is notably higher than the genetic differentiation between many other recognized Oligodon species (see Table 3), thus suggesting that the divergence between Oligodon sp. and O. annamensis likely reached species status. Genetic divergence between Vietnamese and Cambodian populations of O. annamensis is minimal and comprised p = 0.9% of substitutions (Table 3).

Systematics
Our mtDNA Table 3). We thus confirm the identification of the Cambodian population as O. annamensis (previously described by Neang & Hun, 2013), and also provide a morphological analysis of all presently known specimens of O. annamensis (see Table 4). Our results are further corroborated by our morphological analysis (see below), which uncovered significant differences between Oligodon sp. from Bidoup-Nui Ba NP, O. annamensis and other congeners. These results support our hypothesis that this recently discovered lineage of Oligodon represents an undescribed species, which we describe below: Oligodon rostralis sp. nov. Diagnosis. The new species is assigned to the genus Oligodon Fitzinger, 1826 on the basis of its phylogenetic position and the following morphological attributes: posterior maxillary teeth enlarged and compressed; head short, barely distinct from neck; eye well-developed with round pupil; rostral enlarged; body cylindrical with smooth scales; ventrals rounded; subcaudals paired. Oligodon rostralis sp. nov. is distinguished from its congeners by a combination of the following morphological characters: medium size in adults (male TL = 582 mm); head small and broad with long largely protruding snout; 15 dorsal scale rows at neck and midbody and 13 rows before vent; ventrals 167, subcaudals 47 in male; single preocular, single postocular; loreal and presubocular absent; six supralabials, third and fourth entering orbit; six infralabials, anterior four contacting chin shields; internasals separate from prefrontals; nasal divided; single anterior and two posterior temporals; cloacal plate undivided; comparatively short hemipenis, forked in anterior one third of their length, extending to 8th subcaudal, lacking spines and papillae, bearing prominent   Table 4. Adult male of medium size (TL 582 mm), body robust and cylindrical (Fig. 3); SVL 468 mm; head small, comparatively short and wide (HW/HL = 73.2%), ovoid in dorsal view, faintly distinct from the poorly defined neck; tail quite long (19.6% of total length), 114 mm in length; robust, abruptly tapering; eye small, comprising approximately 13.5% of the head length; snout long, protruding (SnL/HL ratio 43.6%); eye diameter much shorter than the distance between eye and nostril; pupil round; Body scalation. Dorsal scales smooth, in 15-15-13 rows, scale row reduction from 15 to 13 at ventral 113; vertebral scales similar to other dorsal scales in size and shape; outermost dorsal scales slightly enlarged; 167 ventrals; cloacal plate entire; 47 subcaudals, all paired, terminal caudal scale in a shape of sharply pointed cap (Fig. 3B). Head scalation. Details of head scalation are shown in Fig. 4. From dorsal view (Figs. 4A-4B), head scalation comprising single rostral, two internasals, two prefrontals, two supraoculars, single frontal, and two parietals. Rostral large, thick, wider than high, extending on to the dorsal surface of the snout, visible from above, pointed posteriorly and inserting deeply between internasals, with a deep crease ventrally, contacting nasals, internasals and first supralabial on both sides; the portion of rostral visible from above shorter than its distance from frontal; internasals sub-rectangular, in broad contact, shorter than prefrontals, each contacting rostral, prefrontal, internasal and paired nasals on both sides; prefrontals large, pentagonal, wider than long and larger than internasals, curving dorsolaterally into loreal region, each contacting internasal and posterior portion of nasal anteriorly, second supralabial laterally, and preocular, supraocular and frontal posteriorly; supraoculars subrectangular, elongated, widening posteriorly, approximately half as wide as long, contacting the orbit, preocular and postocular laterally, prefrontal, frontal and parietal medially; frontal large, pentagonal, longer than wide, narrowing posteriorly, posterior angle rather acute, contacting prefrontals, supraoculars and parietals on both sides; parietals irregularly trapeziform, about 1.5 time larger than frontal, anteriorly contacting frontal, supraoculars and postoculars on each side, bordered posteriorly by five small scales and laterally by the first and upper second temporals; no enlarged nuchal scales present.
In lateral view (Figs. 4C-4D), head scalation comprising a sub-rectangular nasal, vertically divided by prominent suture and pierced by large nostril, nasal on each side contacting rostral anteriorly, internasal and prefrontal dorsally, and first two supralabials ventrally; loreal and presubocular scales absent; 1/1 rectangular preocular, notably higher than wide, separated from nasal by the lateral part of the prefrontal, also contacting second and third supralabials ventrally and supraocular dorsally; 1/1 rectangular postocular, almost equal in size to preocular, contacting fourth and fifth supralabials ventrally, anterior temporal and parietal posteriorly and supraocular dorsally; six supralabials: I. the smallest, in contact with nasal, II. in contact with nasal, prefrontal and preocular, III. in contact with preocular and the orbit, IV. in contact with the orbit and postocular, V. in contact with postocular, anterior temporal and lower posterior temporal, VI. in contact with lower posterior temporal and scale dorsally, and with two smaller scales posteriorly, V. and VI. strongly enlarged; supralabial scale size formula: I<II<III=IV<V<VI; 1+2 temporals on each side, the upper ones pentagonal, elongated and narrow, upper posterior temporal slightly larger than the anterior, the lower posterior temporal rhomboid, ca. two times smaller than the upper ones, posteriorly contacting an enlarged scale of same size.
Hemipenial morphology. Right hemipenis was everted prior to preservation and is shown in Fig. 5. Hemipenis rather short, the everted organ hardly reaching 8th subcaudal; bilobed, bifurcating at distal fifth of its length; organ semi-capitate and semi-calyculate; the sulcus spermaticus is bifurcated at around the proximal one-fifth of the hemipenial body and centrolineal along both lobes (Fig. 5A). The sulcal surface of hemipenis is mostly smooth (Fig. 5A), laterally and on asulcal surface hemipenis covered with several fleshy flounces, lacking spines or papillae-like structures (Fig. 5B); distal ends of hemipenial lobes with small indistinct calyces.
Colouration (in life). Dorsal ground color (Fig. 6A) dark brownish-gray with dense white reticulation between scales; dorsal pattern consisting of 18 large irregular blackish butterfly-shaped blotches, of which 14 are located on body and 4 on tail, the distances between two blotches comprises ca. 8-10 dorsal scale lengths; a bright orange vertebral stripe running from the base of the head to the tail tip; vertebral stripe width comprising from one to three dorsal scale rows; some dorsal scales edged with dark-brown forming an indistinct speckled or dashed pattern between blotches, lower rows of dorsal scales fringed with white. Dorsal ground color along head is grayish-brown (Fig. 6B), a butterfly-shaped marking with rusty tint with a rounded dark spot located on frontal, three separated dark-brown chevrons (one short between the eyes, forming two dark brown streaks running across the eye to the angle of the mouth; and two longer ones running from frontal postero-ventrally to neck and posteriorly to the base of the head, respectively); throat and ventral underside pale-cream laterally with alternating quadrangular black spots scattered from throat until tail (Fig. 7A); underside of tail orange-cream.
Colouration (in preservative). (Fig. 3), after two years in alcohol, coloration faded but pattern remained unchanged; body brown, vertebral stripe became somewhat darkorange and less distinct (Fig. 3, A); dorsal blotches and head marking dark brown with blackish margins remained unchanged; throat, venter and tail underside cream-white, black quadrangular spots remained unchanged (Fig. 3B).
Distribution. At present the new species is known only from the type locality in Bidoup-Nui Ba NP, in the eastern part of Langbian Plateau, southern Vietnam (see Fig. 1, locality 1). This montane area is characterized by high levels of local endemism (Nazarov et al., 2012;Poyarkov et al., 2014;Poyarkov et al., 2015a;Poyarkov et al., 2015b;Poyarkov et al., 2017;Poyarkov et al., 2019b;Stuart et al., 2011;Rowley et al., 2016); further research is needed to clarify the distribution of the new species.
Habitat and natural history. The type specimen was collected on the steep slope close to the mountain summit (Fig. 7), at late night (23 h). The animal was found on ground in leaf litter on the edge of the mixed-pine forest (dominated by Pinus keysia Royle ex Gordon) and evergreen montane broadleaf forest (dominated with trees of the families Fabaceae, Fagaceae, and few large pine trees of Pinus keysia, with understory consisting mostly of Poaceae -different species of bamboo) (Fig. 7B). In the pine forest, understory is dominated by Fagaceae family while ground is covered mostly by grasses and receives high grazing impact by livestock from the villages nearby. In the type locality the new species was recorded in sympatry with some other species of reptiles, including Cyrtodactylus bidoupimontis Nazarov, Poyarkov, Orlov, Phung, Nguyen, Hoang & Ziegler, Scincella rufocaudata (Darevsky & Nguyen), and Pareas hamptoni (Boulenger).
Phylogenetic position. Oligodon rostralis sp. nov. is suggested as a sister species of O. annamensis (Fig. 2), from which it is genetically divergent with p-distance 3.3% in 16S rRNA gene (Table 3). Both species are clustered together with the O. cyclurus and O. taeniatus species groups (Fig. 2).
Most members of the O. cinereus species group, which all are believed to have an unforked hemipenis (vs. bilobed hemipenis in the new species; see Green, Orlov & Murphy, 2010), can be also distinguished from Oligodon rostralis sp. nov. by larger MSR: O. cinereus (Günther) (17); O. nagao David,Nguyen,Nguyen,Jiang,Chen,Teynié & Ziegler (17) Diagnostics of Oligodon rostralis sp. nov. from other mainland Southeast Asian species of Oligodon with 15 or 13 dorsal scale rows appear to be the most pertinent (as the number of MSR may vary between these two values due to the position of the dorsal scale row reduction, see David et al., 2012); it is summarized in   Table 4; color pattern of all O. annamensis specimens is remarkably similar (Figs. 8-11). The holotype of O. annamensis, USNM 90408, corresponds well to the original description by Leviton (1953) (Fig. 8), thus we do not provide its formal redescription. The type specimen is a female with several morphological characters different from the known male specimens (see Table 4): it has a relatively shorter tail, RTL 11.7% (vs. RTL 16.6-19.7% in three males), a greater number of ventrals, 170 (vs. 146-157 in males), and a lesser number of subcaudals, 30 (vs. 43-46 in males). The second already known specimen of O. annamensis, MNHN 8815, a subadult male, was described in detail by Leviton (1960) (Fig. 9). Though in general morphology of MNHN 8815 corresponds well to the description by Leviton (1960) Fig. 1, locality 3). This specimen is an adult male and has the largest total length of all known O. annamensis specimens (412 mm); in scalation and coloration characters it agrees very well with the original description (Leviton, 1953) and the description of male specimen by Leviton (1960) (see Table 4). The tail of ZMMU R-14304 was dissected for examination of hemipenial structures; in full accordance with description by Leviton (1960) this specimen had deeply bilobed hemipenes each bearing two long and thin appendages seen in situ (papillae sensu Smith, 1943), reaching the 20th subcaudal. Coloration of ZMMU R-14304 in life is shown in Fig. 10; among other features, the characteristic coral-red background coloration of the ventral surfaces and black quadrangular spots forming complete transverse bars appear to be diagnostic from Oligodon rostralis sp. nov. (vs. in life ventral surfaces cream-white, black spots do not form transverse bars in the new species).
We present additional morphological information (see Table 4) and photos in life (Fig.  11) of the single known Cambodian specimen of O. annamensis CBC 01899 (see Fig. 1, locality 4) described by Neang & Hun (2013). Based on relative tail length (16.6%) this specimen is identified as male. In accordance with earlier results of Neang & Hun (2013) it shows certain morphological differences from the Vietnamese specimens, namely: having 6/5 supralabials of which 3-4/2-3 touching the orbit (vs. 6/6 and 3-4/3-4 in Vietnamese specimens); infralabials I-III contacting chin shields (vs. I-IV in Vietnamese specimens); posterior temporal single (vs. two posterior temporals in Vietnamese specimens); ventral coloration in life orange red with black markings not forming transverse bars, see Fig. 11B (vs. coral-red belly getting lighter anteriorly; black markings form numerous transverse bars in Vietnamese specimen, see Fig. 10B). Distribution. To date O. annamensis is reliably known from two provinces of southern Vietnam (Lam Dong and Dak Lak), where it was recorded in montane forests of Langbian Plateau at elevations around 1,000 m a.s.l., and from similar elevations in montane forests of Phnom Samkos Mt. in the western part of the Cardamoms, Pursat, Cambodia. The record of O. annamensis from Dak Lak Province is a range extension and the first provincial record of this species.

DISCUSSION
Our study reports on a new species of Oligodon from southern Vietnam, Oligodon rostralis sp. nov., and provides new data on distribution, taxonomy and phylogenetic position of O. annamensis, including the first life photographs of this rare species and a range extension and first provincial record of O. annamensis for Dak Lak Province of Vietnam. We also confirm the previous identification of a specimen from Cardamom Mountains in Cambodia (Neang & Hun, 2013) as O. annamensis based on genetic and morphological lines of evidence. Despite the observed minor morphological differences and geographic isolation, genetic differentiation between Cambodian and Vietnamese populations of O. annamensis is quite small and corresponds to common intraspecific levels of divergence in snakes (p = 0.9%, see Table 3). Hence, O. annamensis has a disrupted range confined to Langbian Plateau in the east and to Cardamom Mountains in the west and separated by the Mekong River valley. Interestingly, a similar distribution pattern was recently reported for a number of lizard taxa inhabiting Indochina (e.g., Grismer et al., 2019;Poyarkov et al., 2019a), but was never recorded in Indochinese amphibians (Geissler et al., 2015b).
The genus Oligodon is traditionally classified in informal species groups on the basis of the hemipenial morphology, number of dorsal scale rows and other characters (Smith, 1943;David, Das & Vogel, 2008;David et al., 2012;Vassilieva et al., 2013;Vassilieva, 2015). The role of hemipenial morphology in delimiting clades within Oligodon was also partially confirmed based on phylogenetic analysis by Green, Orlov & Murphy (2010). Among the species with available data on hemipenial morphology, only the species groups of O. taeniatus and O. cyclurus have bilobed hemipenes, while in other groups copulative organs are unilobed (Green, Orlov & Murphy, 2010). Oligodon rostralis sp. nov. shows a significant morphological similarity to O. annamensis-a species with unclear phylogenetic position. Leviton (1960), describing hemipenial morphology of the only known male specimen, showed that O. annamensis has deeply bilobed hemipenis with papillae, basing on what he proposed that this species may be a part of the ''taeniatus-cyclurus-complex'' (Leviton, 1953;Leviton, 1960). Our observations on additional specimens of O. annamensis (see above) confirm the presence of deeply bifurcated hemipenes with papillae in this species. Oligodon rostralis sp. nov. also showed a forked hemipenis morphology, though lacking papillae. Our phylogenetic analysis suggests sister relationships between Oligodon rostralis sp. nov. and O. annamensis and places these two species in one clade with the members of the ''taeniatus-cyclurus-complex'', therefore confirming earlier hypothesis of Leviton (1953); Leviton (1960). Finally, our phylogeny also suggests that O. lacroixi is a sister species of O. eberhardti and is not closely related to O. annamensis or Oligodon rostralis sp. nov. despite certain morphological similarity between these species .
The description of Oligodon rostralis sp. nov. brings the number of Oligodon species known for Vietnam to 24, thus making the country a local center of Oligodon diversity in Southeast Asia. Our work provides further evidence on high herpetofaunal diversity and endemism in Langbian Plateau, which mostly has been discovered only recently (e.g., Chen et al., 2018;Duong et al., 2018;Geissler et al., 2015a;Geissler et al., 2015b;Hartmann et al., 2013;Nazarov et al., 2012;Orlov, Nguyen & Ho, 2008;Orlov et al., 2012;Pauwels et al., 2018;Poyarkov et al., 2014;Poyarkov et al., 2015a;Poyarkov et al., 2015b;Poyarkov et al., 2017;Poyarkov et al., 2019a;Poyarkov et al., 2019b;Poyarkov & Vasilieva, 2011;Rowley et al., 2010;Rowley et al., 2011;Rowley et al., 2016;Stuart et al., 2011;Vassilieva et al., 2014). Despite the impressive increase in species discoveries in the recent years, many isolated montane areas of the Truong Son Mountains, such as the Langbian Plateau, still remain insufficiently studied and likely cradle even more unknown biodiversity. The need for further biodiversity exploration in southern Indochina is urgent given the ongoing loss of natural habitats due to such intensifying threats as logging, agricultural pressure, road construction and other anthropogenic activities (De Koninck, 1999;Laurance, 2007;Meyfroidt & Lambin, 2008;Kuznetsov & Kuznetsova, 2011). Further studies on herpetofaunal biodiversity in this region are immediately required for elaboration of effective conservation measures.

CONCLUSIONS
Here, we present new molecular sequence data and an updated mtDNA genealogy for the genus Oligodon, one of the most species rich groups of Asian snakes. We confirm the presence of four main clades within the genus Oligodon, and for the first time report on the phylogenetic placement of several poorly known Oligodon species, including O. annamensis and O. lacroixi. We analyze all available collection material of O. annamensis from southern Vietnam and Cambodia and confirm the earlier assignation of Cambodian population from Cardamom Mountains to this species based on both morphological and molecular lines of evidence. Finally, we report on a new species of Oligodon from southern Vietnam, known from a single male specimen. Oligodon rostralis sp. nov. is distinct from all other congeners in a number of morphological diagnostic characters and is morphologically and phylogenetically most closely related to O. annamensis, from which it can be easily distinguished in scalation, coloration and mtDNA sequences. We analyze available morphological data on Oligodon species with 15 or 13 dorsal scale rows occurring in the mainland Asia, and discuss phylogenetic relationships among them. We provide further evidence for an unprecedented herpetofaunal diversity and endemism in Langbian Plateau, Southern Vietnam.

Field Study Permissions
The following information was supplied relating to field study approvals (i.e., approving body and any reference numbers): The fieldwork in Bidoup-Nui Ba NP was conducted under scope of the contract between Sustainable Nature Resource Management Project (

DNA Deposition
The following information was supplied regarding the deposition of DNA sequences: 12S rRNA-16S rRNA mtDNA fragment and cytochrome b genes sequences are available at GenBank: MN395601-MN395604 and MN396762-MN396765.