ZMMU Re-17696 (field number ISS-128), adult male collected on 20 January 2023 at the entrance of Tham Le Stegodon Cave, Satun Province, Thailand (7.158315°N, 99.802631°E, 28 meters elevation), by Parinya Pawangkhanant and Sabira S. Idiiatullina.

ZMMU Re-17697 (field number NAP-13128), adult female collected 24 September 2022 inside Tham Khao Ting Cave, Trang Province, Thailand (7.158315°N, 99.802631°E, 12 meters elevation), by Nikolay A. Poyarkov, Nikita S. Kliukin, Chatmongkon Suwannapoom and Parinya Pawangkhanant.

O. speleoserpenssp. nov. is referred to the genus Oligodon based on the presence of enlarged blade-shaped maxillary teeth without a diastema, an elongate and subcylindrical body, and the presence of a large inflated rostral scale that blocks the internasal scales from contacting anteriorly (Wall 1923; Smith 1943; David et al. 2023). It is distinguished from all other Oligodon by the following combination of morphological characters: 1) head oblong-shaped with a truncated snout and slightly inflated rostral scale; 2) 8 maxillary teeth, with the posterior three teeth enlarged and blade-like; 3) dorsal scales in 19–17–15 rows (17–17–15 rows in one specimen); 4) reduction from 19 dorsal scale rows to 17 dorsal scale rows occurring at the 28^th–30^th ventral scale; 5) reduction from 17 dorsal scale rows to 15 dorsal scale rows occurring at the 100^th–113^th ventral scale; 6) cloacal plate entire; 7) 189–193 ventral scales (189–190 in male; 193 in female), distinctly keeled; 8) 47–54 paired subcaudal scales (47–54 in male; 48 in female); 9) 238–244 total body scales; 10) relative tail length 0.136–0.139 and subcaudal ratio 0.198–0.221; 11) 8 supralabials on either side of the head, with the fourth and fifth scales in contact with the orbit; 12) 9 infralabials on either side of the head with the first four scales in contact with the first pair of chin shields; 13) one loreal and one presubocular present; 14) 1+2 temporal scales; 15) dorsal color pattern uniform gray or grayish–brown without any markings or reticulations; 16) anterior half of ventral surface white with gray–brown irregularly shaped spots, posterior half immaculate dark gray, underside of tail splashed with white markings; 17) hemipenis bilobed with broad, awn-shaped lobes, simple sulcus spermaticus and smooth calyces.

We compared Oligodon speleoserpenssp. nov. with all other members of the O. cinereus species group and use Table S3 to compare it with other Oligodon native to Thailand. Its uniform gray dorsum distinguishes most members of the O. cinereus species group that have banded, cross-barred, or blotched dorsal color patterns; namely, O. albocinctus, Oligodon ancorus (Girard, 1857), Oligodon kampucheanensis Neang, Grismer and Daltry, 2012, O. lipipengi, O. nagao, Oligodon purpurascens (Schlegel, 1837), O. saiyok, and Oligodon teyniei David, Hauser, and Vogel, 2022. Exceptions include O. “cinereus”, O. huahin, O. joynsoni, O. inornatus, and O. phangan, which have variable color patterns, or are uniform/reticulated dorsally. Between these congeners, Oligodon speleoserpenssp. nov. is most easily distinguished by usually having 19 anterior scale rows (vs. no more than 15 or 17 scale rows anywhere on the body; only one specimen of Oligodon speleoserpenssp. nov. with 17 anterior scale rows) with the reduction from 19 to 17 rows occurring between ventral scales 28 and 30.

More specifically, O. speleoserpenssp. nov. is distinguished from O. huahin by having 189–193 ventrals (vs. 166–173 in males), 47–54 subcaudals (vs. 35–41 in males), 9 infralabials (vs. 7–8), dorsum dark gray (vs. dorsum uniform but lighter gray, tan or orange), and a dark gray venter without conspicuous dark spots or rectangular blotches on the posterior half (vs. venter plain white); from O. inornatus by having 189–193 ventrals (vs. 169–173), 47–54 subcaudals (vs. 31–43), 8 maxillary teeth (vs. 10–11), and a ventral coloration without conspicuous dark spots or rectangular blotches on the posterior half (vs. ventral surface with small square or rectangular spots); from O. joynsoni by having 54 subcaudals in females (vs. 37–41 in females), relative tail length (TailLR) 0.139 in females (vs. 0.125–0.129 in females), 8 maxillary teeth (vs. 11–12), and a uniform gray or grayish–brown dorsum without crossbars or reticulations (vs. light tan, ochre brown or gray, narrow irregular shaped crossbars present on dorsal surface of body and tail); from O. phangan by having 189–193 ventrals (vs. 163–166), 47–54 subcaudals (vs. 33–42), 8 maxillary teeth (vs. 12), 9 infralabials (vs. 8), presubocular present (vs. absent), dorsum uniform gray or grayish–brown (vs. dorsum uniform orange-gray with indistinct vertebral lines present), and a ventral coloration without conspicuous dark spots or rectangular blotches on the posterior half (vs. ventral surface plain white); and from O. promsombuti by having 189–193 ventrals (vs. 177), 47–54 subcaudals (vs. 40), 8 maxillary teeth (vs. 12), 9 infralabials (vs. 8), presubocular present (vs. absent), and a ventral coloration without conspicuous dark spots or rectangular blotches on the posterior half (vs. venter with large rectangular blotches across entire ventral surface). O. speleoserpenssp. nov. differs from all clades hitherto referred to as O. “cinereus” (including O. cinereus sensu stricto) by having 189–193 ventral scales (vs. no more than 186), 47–54 subcaudals (vs. no more than 45), 8 maxillary teeth (vs. 10–13), and 9 infralabials (vs. usually 7–8, rarely 9). Moreover, the posterior portion of the venter is dark and uniform colored in O. speleoserpenssp. nov., whereas uniform or reticulated colored populations of O. “cinereus” we examined (Appendix S1) have an immaculate venter with rectangular spots or blotches on the lateral edge of each ventral scale. The ventral underside of the tail, which is dark gray smeared with minimal white markings, is found in all specimens of O. speleoserpenssp. nov. and is absent in all uniformly patterned O. “cinereus”, as well as most other members of the O. cinereus species complex.

Five additional species of Oligodon outside of the O. cinereus species group may occur in close proximity with O. speleoserpenssp. nov. and are thus compared here. Oligodon speleoserpenssp. nov. differs from members of the O. cyclurus-taeniatus species group found in the same area of southern Thailand, namely Oligodon fasciolatus (Günther, 1864), Oligodon mouhoti (Boulenger, 1914) and O. taeniatus, by its uniform dark-olive dorsum without any conspicuous head markings (vs. ligher brown dorsum with a well-defined series of vertebral stripes [O. mouhoti and O. taeniatus] or series of large blotches [O. fasciolatus], in addition to the presence of conspicuous nuchal and temporal markings on the head). Next, O. speleoserpenssp. nov. most easily differs from Oligodon signatus (Günther, 1864), restricted to a few provinces at the southern end of Thailand (Pawangkhanant et al. 2021), by its uniform dorsum (vs. dark gray with a series of reddish-brown blotches), 8 supralabials (vs. 7) and two postoculars (vs. only one). Finally, O. speleoserpenssp. nov. differs from Oligodon jintakuneiPauwels et al., 2002, known from just one specimen allegedly from Krabi Province, by its uniform dorsum (vs. distinct black and white crossbars) and by possessing an entire cloacal plate (vs. divided). Additional characters distinguishing each species can be found in Table S3.

Adult male specimen in excellent condition immediately after preservation (Fig. 3). Small ventral incisions at tail base and at midbody. SVL 601 mm, TailL 95 mm (TotalL 696 mm). HeadL 16.7 mm, HeadW 11.5 mm, HeadD 8.6 mm, SnoutL 5.3 mm, EyeD 2.7 mm, EyeLip 2.6, FrontalL 5.3 mm, FrontalW 4.3, IOD 6.5, IND 4.4 mm; RostralW 4.9. TailLR 0.136, HeadW/L 0.69, SnoutL/HeadL 0.32, EyeD/SnoutL 0.51, EyeD/HeadL 0.16, FrontalL/W 1.23. Body elongated and cylindrical in cross section; head oblong, only slightly distinct from neck (Fig. 4a, b); snout narrowing in dorsal view, in lateral view slightly truncate; snout tip subterminal near mouth; eyes moderately sized relative to head, pupil round; nostrils small and subelliptical, pointed laterally; lips curving upwards posteriorly along the last supralabial; tail broad at base, tapering gradually to a blunt terminal scute (Fig. 4f).

Rostral distinctly enlarged and inflated, wider than high, triangular in dorsal view, partially separating the anterior half of internasals (Fig. 4b); posterior rostral scale suture bordering the internasals “deep-V” shaped, creating a narrow obtuse angle (~100º); internasals subrectangular, 1.7× wider than long, anterior suture of each scale rounded and concave, both scales oriented obliquely and posteromedially in dorsal profile; prefrontals subpentagonal, 1.5× wider than long, 1.4× longer than suture dividing both internasals, 1.2× wider than internasals, anterior suture bordering each internasal slightly rounded, concave; frontal pentagonal and shield shaped, longer than wide, anterior suture bordering prefrontals straightened; frontal 2.3× longer than prefrontals; eyes placed posterior to anterior edge of frontal; angle formed by sutures producing the posterior vertex of frontal a narrow obtuse angle (~99º); supraoculars subrectangular, 1.8× longer than wide, 0.7× as long as frontal; parietals subpentagonal, 1.3× longer than wide, width of each scale 1.1× longer than parietal suture; posterior suture of parietals straightened; frontal and parietal approximately equal in length; frontal 1.4× longer than parietal suture; anterior angle formed by the sutures between the parietal/frontal and the supraocular/parietal a broad obtuse angle (~130º) with the lateral ray oriented slightly posterolaterally. Nasal scale rectangular, longer than wide, fully divided (Fig. 4a); loreal 1/1, small, squared, slightly higher than long, less than half the size of nasal; supralabials 8/8; with 4^th and 5^th supralabials in contact with orbit; 6^th supralabial largest, 1^st supralabial smallest; all supralabials in broad contact; preoculars 1/1, wider than long; presubocular 1/1, less than half the size of preocular; postoculars 2/2, uppermost postocular slightly larger; anterior temporal 1/1; posterior temporals 2/2, all scales longer than wide; infralabials 9/9 in all specimens, first pair contacting each other, 4/4 infralabials in contact with anterior chin shields; 5^th infralabial largest, 2^nd and 9^th infralabials smallest; mental subtriangular, wider than long; small, indistinct mental groove present, starting from border of 1^st pair of infralabials and terminating past posterior pair of chin shields; both pairs of chin shields subrectangular, anterior pair 1.4× longer than posterior pair.

Dorsal scale rows 19–17–15, smooth throughout, without apical pits; dorsal scale reduction from 19 rows to 17 rows occurring at 30^th ventral scale on either side of body; reduction from 17 rows to 15 rows occurring at 100^th ventral scale on either side; preventrals 2 and ventral scales 189; lateral edges of each ventral scale with discrete lateral keeling; subcaudals 54, paired; total body scales 244; subcaudal ratio 0.221; cloacal plate entire. Maxillary teeth 8, posterior two teeth enlarged, laterally compressed, blade-like. The partially everted hemipenes is bilobed and weakly calyculate (Fig. 4d). The base of the organ and most of the capitulum is bulbous and covered with small smooth calycles. The sulcus spermaticus is simple, indistinct, extending from the base of the organ and terminating at the point of bifurcation. Both apical lobes are large, nude, and rounded distally, lacking any ornamentation.

In life, dorsal ground color ashy gray, margins of most dorsal scales edged with black, but distinct reticulations absent (Figs 3a, 5a); scattered small reddish–orange mottling interspaced across dorsal surface of the tail and neck. Dorsal portion of head brown (Fig. 4b); a very indistinct temporal bar on each side of the head across the supraoculars, frontal, and medial region of the parietals; a dark, hued, and indistinct nuchal chevron present at the medial region of each parietal, descending posterolaterally towards the neck; remainder of head plain, bottommost portion of supralabials light brown (Fig. 4a). Ventral surface of head white, dark brown hueing along lateralmost margins of first 5 infralabials; the anterior third of the venter is white, with long irregular gray blotches first visible along the seventh ventral scale; these blotches continue to merge until the venter is uniform and dark gray for the remaining two thirds of the body; discrete keeling present on the edges of each ventral scale light gray, forming a subtle longitudinal stripe along each side of the venter until the tail base. The underside of the tail is dark gray, with white pigment present midventrally until its posterior half, which is completely white until the last few subcaudals and the terminal scute. In preservative, coloration overall very similar, except the dorsum is darker gray.

The adult female paratype (Fig. 5c) agrees in almost all scalation and mensural aspects of the holotype, with a few exceptions. The paratype has a small anterior portion of the dorsum with past injuries, and small ventral incisions at the tail base and midbody. Measurements of the paratype are as follows: SVL 681 mm, TailL 110 mm (TotalL 791 mm). HeadL 16.5 mm, HeadW 14.0 mm, HeadD 10.6 mm, SnoutL 7.6 mm, EyeD 2.7 mm, EyeLip 2.9, FrontalL 5.6 mm, FrontalW 4.7, IOD 8.0, IND 5.7 mm; RostralW 4.9. TailLR 0.139, HeadW/L 0.85, SnoutL/HeadL 0.46, EyeD/SnoutL 0.36, EyeD/HeadL 0.16, FrontalL/W 1.19. Body elongated, cylindrical in cross section. Head oblong, only slightly distinct from neck. Snout more elongate in lateral view than the holotype; snout tip subterminal near mouth; eyes moderately sized relative to head, pupil round; nostrils small, subelliptical, pointed in lateral view; mouth with lips curving upwards posteriorly across the last supralabial; tail gradually tapering to a blunt terminal scute.

Rostral wider than high, triangular in dorsal view; posterior scale suture of rostral bordering internasals “deep-V” shaped, creating a narrow obtuse angle (~100º, same as holotype); internasals subrectangular, 2.3× wider than long, anterior sutures rounded, concave; prefrontals subpentagonal, 2.4× wider than long, 1.3× longer than internasals, anterior sutures bordering each internasal also rounded; prefrontals 1.3× wider than internasals; frontal pentagonal, shield shaped, 1.2× longer than wide, anterior suture bordering each prefrontal straightened; frontal 1.4× longer than prefrontals; eyes placed posterior relative to the anterior edge of frontal; angle formed by suture producing the posterior vertex of frontal a narrow obtuse angle (~96º); supraoculars subrectangular, 1.4× longer than wide, 1.3× shorter than frontal; parietals subpentagonal, 1.2× longer than wide, width of each scale 1.2× longer than parietal suture, posterior suture of parietals straightened; length of parietals approximately 1.1× longer than frontal; frontal 1.1× longer than parietal suture; anterior angle formed by the sutures between the parietal/frontal and supraocular/parietal a broad obtuse angle (~131º) with its lateral ray pointing somewhat posterolaterally. Nasal scale rectangular, longer than wide, fully divided; loreal 1/1, small and square shaped, 1.2× longer than high, less than half the size of nasal; supralabials 8/8; with 4^th and 5^th supralabial in contact with orbit; 6^th supralabial largest, 1^st supralabial smallest; all supralabials in broad contact; preoculars 1/1, wider than long; presubocular 1/1, less than half the size of preocular; postoculars 2/2, uppermost postocular slightly larger in size; anterior temporal 1/1; posterior temporals 2/2; infralabials 9/9 in all specimens, first pair in contact, 4/4 infralabials in contact with anterior chin shields; 5^th infralabial largest, both 2^nd and 9^th infralabials smallest; mental subtriangular, wider than long; small, indistinct mental groove present, condition identical to holotype; chin shields subrectangular, anterior pair 1.2× longer than posterior pair.

Dorsal scale rows 19–17–15, scale ornamentation like holotype; dorsal scale reduction from 19 rows to 17 rows occurring at the 28^th ventral scale; reduction from 17 rows to 15 rows occurring at the 113^th ventral scale; preventrals 2, ventral scales 193, distinctly keeled; subcaudals 48, paired; total body scales 242; subcaudal ratio 0.198; cloacal plate entire. Maxillary teeth 8, posterior two teeth enlarged and blade-like.

In life, dorsal ground color dark olive–gray (Fig. 5c), plain, edges of each dorsal scale slightly darker, distinct reticulations absent; a very indistinct pair of vertebral stripes present from nape to vent; scattered, small reddish–orange mottling interspaced across dorsal surface of tail; tail tip dark gray. Dorsal portion of head olive–brown; indistinct ocular and temporal bar present on each side of head, both subdued, dark brown, present across the supraoculars, frontal and medial portion of the parietals; remainder of head plain olive–brown, bottommost portion of supralabials light gray. Ventral surface of head also light gray, darker hueing along lateralmost margins of the first four infralabials; anterior third of ventral surface white with small irregularly-shaped olive–gray mottling; posterior two thirds of the venter plain dark gray; keel along ventral scales forming a narrow light-gray stripe present until the vent. Ventral surface of tail dark gray, less white pigment than holotype, anterior half similar to ventral surface of body, posterior half with small white spotting at midline until tail tip. After preservation, coloration very similar but head markings completely subdued and dorsal coloration darker gray, matching the coloration of the ventral surface.

Morphological data of another male individual from the same locality as the paratype were collected by two of us (HWS and RGL; Fig. 5d) shortly after capture (see Table 1 for more information). The uncollected specimen from Tham Khao Ting cave agrees with the paratype in most features except it has 17 anterior dorsal scale rows compared to the 19 rows found in both type specimens (total dorsal scale row formula: 17–17–15). The dorsum is ashy gray with dark edges on each dorsal scale, as observed in the type series. The indistinct temporal bar and nuchal chevron present in the holotype are extremely subdued and essentially absent. The venter is white anteriorly with small gray and irregularly shaped markings along the anterior third of the body before becoming plain and dark gray along its remaining posterior two thirds. The ventral underside of the tail resembles the paratype and is entirely dark gray except for a few subdued white spots present near the posterior end of the tail.

The species epithet “speleoserpens” is a compound name combining the Latinzed Greek noun “spēlēum”, meaning “cave” and the Latin noun “serpens [= serpentis]”, the present active participle of “serpō” meaning “to crawl or creep”, often used in reference to snakes. This roughly translates to “cave crawler” or “cave serpent”, an allusion to both the type locality and the discovery of the paratype. We recommend the English common name “Cave Kukri Snake” and the Thai common name “ปี่แก้วควนหิน” (Ngu Pi Kaew Kuan Hin) for this species.

Oligodon speleoserpenssp. nov. is currently known from only two localities in southeastern Peninsular Thailand: Tham Le Stegodon, Satun Province and Tham Khao Ting cave, Trang Province (see locality 1, Fig. 1). Both locations are cave formations that form part of a larger complex of limestone karst massifs along the border between Satun and Trang provinces. The holotype was discovered crawling along a vertical limestone wall just outside the entrance of Tham Le Stegodon (Fig. 5b) at 1950 hrs (local ITC time), whereas the paratype was discovered at 2200 hrs in-situ coiled within a crevice in Tham Khao Ting cave, approximately 10 meters past the entrance. The third, uncollected specimen from Tham Khao Ting cave was observed by two of us (HWS and RGL) on 19 March 2023 at 2100 hrs. It was first observed ~12–15 meters up a karst wall (Fig. 6a) with its head peeking into a small crack. It proceeded into this crack, emerging 5 minutes later with no signs of finding prey. Both of these specimens were adult males, whereas the paratype collected earlier in September 2022 was an adult female. Due to the proximity of Satun and Trang Provinces to the equator, sunset varies very little through the year, from 1810 hrs at the winter solstice to 1837 hrs at the summer solstice. All specimens were observed after sunset (1900–2300 hrs), but we assume this species is cathemeral (including both diurnal and nocturnal surface activity), as is consistent with other members of the O. cinereus species group.

The diet of most Oligodon is presumed to consist primarily of reptile eggs and frogs (Wall 1923; Bringsøe et al. 2020; David et al. 2023), but Meggitt (1931) and Pope (1935) both recorded several arachnid and arthropod species in the stomachs of specimens in the O. cinereus species complex (Savitzky 1983; as O. cinereus). Whether O. speleoserpenssp. nov. has a similar reptile egg-rich diet remains an open question. We observed three highly abundant gecko species present in sympatry with O. speleoserpenssp. nov. including Cnemaspis niyomwanaeGrismer et al., 2010, Cyrtodactylus cf. lekaguliGrismer et al., 2012 and Gekko gecko (Linnaeus, 1758). All three lizards occupy the same karstic surfaces and utilize crevices as egg laying sites. The exploratory behavior observed in the uncollected specimen could be related to foraging for these species and their eggs. We also observed five additional generalist and widespread gecko species at the same locality, including Cyrtodactylus quadrivirgatus Taylor, 1962, Cyrtodactylus zebraicus Taylor, 1962, Gekko tokehos (Grismer et al., 2019), Gekko kuhli (Stejneger, 1902) and Hemidactylus frenatus Duméril and Bibron, 1836. Other snake species found in sympatry with O. speleoserpenssp. nov. include Ahaetulla mycterizans (Linnaeus, 1758), Ahaetulla prasina (Boie, 1827), Boiga melanota (Boulenger, 1896) (recognized at species-level fide Weinell et al. 2021), Bungarus flaviceps Reinhardt, 1843, Elaphe taeniura ridleyi (Butler, 1889), Lycodon capucinus Boie, 1827, Lycodon (Dryocalamus) davisonii (Blanford, 1878), Malayopython reticulatus (Schneider, 1801), Pareas carinatus Wagler, 1830, Trimeresurus ciliarisIdiiatullina et al., 2023, Tropidolaemus wagleri (Boie, 1827), and Xenochrophis trianguligerus (Boie, 1827). The diversity and abundance of geckos and other squamate reptiles in this karstic habitat provides numerous potential prey items for Oligodon speleoserpenssp. nov., especially in the form of eggs. No other information on its natural history or behavior is known.

Only three specimens of Oligodon speleoserpenssp. nov. have been documented so far, all within a span of six months. The surrounding limestone karst massif where the new species has been collected spans a total area of approximately 10.7 square kilometers. Both sites are offered protection from human development. Tham Le Stegodon is part of the Satun UNESCO Global Geopark and Tham Khao Ting is owned by the Liphang Subdistrict Administrative Organization of Trang Province. The underground river networks of both cave systems are popular tourist attractions for swimming and kayaking. We believe the current level of recreation poses minimal impact on the caves and surrounding karst habitat and is thus unlikely to threaten the conservation of Oligodon speleoserpenssp. nov. at these sites. However, future work on the ecology and behavior of this species is needed to understand whether any other human activities in this region might act as conservation threats. Owing to a lack of ecological information on this species, we suggest classifying Oligodon speleoserpenssp. nov. as “Data Deficient” based on the criteria adopted by the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. This classification is based on the fact that little data exists for the new species, and that more information is needed to understand its distributional limits within southern Thailand.