A new sandstone-dwelling leaf-toed gecko (Gekkonidae: Dixonius mekongensis) from the Thai-Lao border Author Pauwels, Olivier S. G. Royal Belgian Institute of Natural Sciences, Rue Vautier 29, B- 1000 Brussels, Belgium. Author Panitvong, Nonn 0000-0002-3979-0762 Siamensis Biodiversity Conservation Group, 408 / 144 Phaholyothin Place Building, 34 th floor, Phaholyothin Rd., Phayathai, Bangkok, 10400, Thailand npanitvong @ gmail. com; https: // orcid. org / 0000 - 0002 - 3979 - 0762 npanitvong@gmail.com Author Kunya, Kirati 0000-0002-7219-6239 Nakhonratchasima Zoo, 111 M. 1, Ratchasima-Pak Tongchai Rd., Chaimongkol, Muang, Nakhon Ratchasima 30000, Thailand. kkunya 2006 @ yahoo. com; https: // orcid. org / 0000 - 0002 - 7219 - 6239 kkunya2006@yahoo.com Author Sumontha, Montri 0000-0003-4829-7731 Ranong Marine Fisheries Research and Development Station, 157 Chaloem Phrakiat Rd., Paknam, Muang, Ranong 85000, Thailand. montri. sumontha @ gmail. com; https: // orcid. org / 0000 - 0003 - 4829 - 7731 montri.sumontha@gmail.com text Zootaxa 2021 2021-05-12 4969 3 526 538 journal article 6456 10.11646/zootaxa.4969.3.5 0ed6eca0-fbbb-41ea-bd68-bb0d0f34edf1 1175-5326 4751459 9F2F4EA9-2178-493E-B317-DA6DE0B59715 Dixonius mekongensis sp. nov. ( Figures 1–5 ) Holotype .AUP02007 (field no. MS 561 ),adult male caught on 2 July 2009 on a sandstone platform (ca. 15°27’19.1”N , 105°34’12.0”E ) by T. Kaewmanee , Na Pho Klang Sub-district, Khong Chiam District , Ubon Ratchathani Province , eastern Thailand . FIGURE 1. Live adult male holotype of Dixonius mekongensis sp. nov. Photo. by M. Sumontha. FIGURE 2. Preserved male holotype of Dixonius mekongensis sp. nov. A. Right profile of the head. B. Dorsal surface of the head. C. Ventral surface the head. D. Cloacal area, showing the precloacal pores. Photos. by M. Sumontha. FIGURE 3. Preserved type-series of Dixonius mekongensis sp. nov. A. Dorsal view. B. Ventral view. (MS 561 = male holotype AUP 02007; MS 562 = female paratype AUP 02008; MS 624 = male paratype PSUZC-R 736). Photos. by M. Sumontha. FIGURE 4. Live adult female paratype (AUP 02008) of Dixonius mekongensis sp. nov. Photo. by M. Sumontha. Paratypes (2). PSUZC-R 736 (field no. MS 624 ), adult male, and AUP 02008 (field no. MS 562 ), adult female. Same locality, collecting date and collector as holotype . Diagnosis. Dixonius mekongensis sp. nov. can be distinguished from all other congeneric species by the combination of its maximal known SVL of 51.2 mm ; 16 longitudinal rows of dorsal tubercles; 32 to 34 paravertebral scales; 22 to 24 longitudinal rows of ventrals across the abdomen; seven precloacal pores in males, no pores in females; a marked canthal stripe; and a uniform or spotted dorsal pattern. Description of holotype . Adult male ( Figures 1–3 ). SVL 48.6 mm . Head relatively long (HL/SVL ratio 0.32), wide (HW/HL ratio 0.67), not markedly depressed (HD/HL ratio 0.44), distinct from neck. Lores and interorbital region weakly inflated. Canthus rostralis relatively prominent. Snout moderately short (SnOrb/HL ratio 0.36), rounded, slightly longer than orbit diameter (OrbD/SnOrb ratio 0.61). Scales on snout and forehead small, hexagonal to rounded, flattened, with smooth or slightly rugose surface. Scales on snout larger than those on occipital region. Eye of moderate size (OrbD/HL ratio 0.22). Pupil vertical with crenelated margins. Supraciliaries short, without spines. Ear opening oval, moderate (EarL/HL ratio 0.06); orbit to ear distance greater than orbit diameter. Rostral about twice wider than high, dorsally incompletely divided by a median cleft. Two enlarged supranasals in broad contact. Rostral in contact with supralabial I on each side, nostrils and both supranasals. Nostrils round, each surrounded by supranasal, rostral, supralabial I and two postnasals. Mental triangular, about as long as deep. Two pairs of enlarged postmentals, anteriormost approximately four times larger than posterior. Each anterior postmental bordered anteriorly by mental, medially by the other anterior postmental, anterolaterally by infralabial I, posterolaterally by the second postmental; the pair collectively bordered posteromedially by a row of six throat scales. Supralabials to mid-orbital position 7/6; enlarged supralabials to angle of jaws 8/9. Infralabials 6/7. Interorbital scales eight. FIGURE 5. Live adult individuals of Dixonius mekongensis sp. nov. , uncollected, from Pha Taem National Park, Ubon Ratchathani Province, eastern Thailand. A. Male. Photo. by B. Siriphiphat. B. Adult female. Photo. by N. Panitvong. C. Adult female. Photo. by B. Siriphiphat. D. Adult pregnant female. Photo. by K. Kunya. Body slender, elongate (TrunkL/SVL ratio 0.44), without ventrolateral folds. Dorsal scales heterogeneous, small, irregular, flattened to conical, distributed among large, strongly keeled tubercles arranged in 16 regular longitudinal rows at midbody. Flanks covered with irregular, smooth to slightly conical scales. Gular region with relatively homogeneous, granular scales. Ventral scales smooth, imbricate, their free margin rounded. Ventrals increasing in size from throat to chest to abdomen. Midbody scale rows across belly to lowest rows of tubercles 24. Seven precloacal pores in a continuous series. Pore-bearing scales not enlarged relative to adjacent scale rows. No femoral pores or enlarged femoral scales. Fore- and hind limbs short, slender (FAL/SVL ratio 0.14; TibL/SVL ratio 0.15). Digits slender, dilated distally, all bearing robust, slightly recurved claws. Basal subdigital lamellae narrow, without scansorial surfaces (6- 8-10-11-10 right manus; 12-15-14-8-7 right pes); setae-bearing lamellae restricted to enlarged, distal, ‘‘leaf-like’’ scansors. Scales on palm and sole small, smooth, rounded to oval. Interdigital webbing absent. Relative length of digits: III>IV>V>II>I (manus), IV>V>III>II>I (pes). Tail length 60.8 mm of which the last 23.7 mm are regenerated. Supracaudals markedly keeled in the anterior portion of the tail. Ventral tail scales of the original portion of the tail enlarged into transverse plates. TABLE 1. Meristic and morphometric (in mm) data for the type series of Dixonius mekongensis sp. nov. Paired meristic characters are given left/right.

Holotype, AUP 02007	Paratype, PSUZC-R 736	Paratype, AUP 02008
Sex	Male	Male	Female
SVL	48.6	51.2	45.9
TailL	60.8*	52.7*	53.8
TrunkL	21.2	21.6	19.9
TailW	6.8	5.2	5.0
BW	11.3	9.0	10.5
HL	15.6	15.4	14.3
HW	10.5	9.1	8.4
HD	6.9	6.0	4.9
EarL	0.9	1.0	0.8
TibL	7.1	6.7	6.6
FAL	6.6	6.1	5.6
OrbD	3.4	3.6	3.0
NosOrb	4.1	3.6	3.8
SnOrb	5.6	5.2	4.9
OrbEar	4.8	4.4	3.6
InterN	2.2	2.2	1.7
InterOrb	2.2	2.2	2.2
Ven	24	22	22
DTR	16	16	16
PV	32	34	32
PV’	22	24	20
SubLT4	14/14	15/15	12/12
InterOrbS	8	9	8
ICS	25	26	27
SL	8/9	8/9	9/7
IL	6/7	7/7	6/6
SLMOrb	7/6	7/7	6/5
PrePo	7	7	0
FemPo	0/0	0/0	0/0

*partly regenerated. TABLE 2. Comparison of selected diagnostic characters of Dixonius spp. Ba = banded; Bl = blotched; Sp = spotted; St = striped; U = uniform. NA = not available. Bolded values are diagnostic differences from Dixonius mekongensis sp. nov .

Character
D. aaronbaueri	D. dulayaphitakorum	D. hangseesom	D. kaweesaki	D. lao	D. mekongensis s p. nov.	D. melanostictus	D. minhlei	D. pawangkhananti	D. siamensis	D. somchanhae	D. taoi	D. vietnamensis
Max. SVL	38.6	47.8	42.1	41.6	55.4	51.2	50.0	47.5	42.6	57.0	47.1	43.9	42.4
Ven	18–19		22	22–26	24	23–24		22–24		22	20–23		16	18–25		23–26	21–23		15–21
DTR	11	22	12–14	12–13		20–23		16	10–11		14–15		16	10–14		19–21	11–12		13–17
PV	45–50		33–35		NA	NA	40–43		32–34		NA	38–44		30–32		NA	35–40	31–42		36
PV	29–32		20–25		NA	NA	24–25		20–24		NA	23–26		18–21		NA	19–27	18–25		NA
SubLT4	13–15		12–15		NA	15	15	12–15		15	12–15		14–15		12–16		13–15	12–14		12–15
InterOrbS	8–10	9–10	10	6–7	8–9	8–9	NA	7–10		7	NA	7–8	7–10		8–10
SL	8–9	8	8	10–11		8–10	7–9	9	7–9	7–8	7–8	7–8	7–8	7
SLMOrb	6–7	6	6	7–8	7–8	5–7	7	5–6	5–6	NA	6–7	5–6	5–6
IL	6–8	7	8	6–8	7–8	6–8	7	6–7	6–7	6–7	5–6	5–7	6–7
PrePo	5	6–7	6–8	9–11	8	7	9	7–8	6	6–7	5–6	5–6	5–7
FemPo	0	0	0	0	0	0	0	0	0	0	0	0	0
Marked canthal stripe	1	0	1	1	0	1	1	1	1	0	1	1	1
Dorsal pattern	U	Sp	Ba / Bl		St	U	U / Sp		St	Sp	Ba / Bl		Sp	Sp	Bl	Sp

FIGURE 6. Map showing the position of the type-locality of Dixonius mekongensis sp. nov. in Ubon Ratchathani Province, eastern Thailand. Map by W. Sodoab. FIGURE 7. Biotope of Dixonius mekongensis sp. nov. at Soi Sawan Waterfall in Pha Taem National Park, Ubon Ratchathani Province, eastern Thailand. Photo. by M. Sumontha. Coloration in life. Dorsal surface of head gray with numerous small and irregular black blotches. On each side of the head a black canthal stripe runs from the nostril through the eye and extends to the ear; it is bordered below and above by a light gray thin area. After an interruption at the level of the tympanum, the black stripe continues till above the shoulder. On the snout, at about mid-length between the eyes and the tip of the snout, a transversal bars links the left and right canthal stripes. The supralabials and infralabials are whitish. Similarly to the dorsal surface of the head, the neck, the dorsum and the dorsal surface of the original portion of the tail show a gray background color with numerous, irregularly disposed, black spots ( Figure 1 ). Flanks lighter than the dorsum and less punctuated. Dorsal surfaces of members gray with small black spots. Ventral surfaces of head, body, members and tail whitish. In preservative the colors strongly fade and become less contrasted ( Figures 2 and 3 ). Variation. The main morphometric and meristic characters of the type series are provided in Table 1 . Morphological characters of the paratypes agree in most respects with the holotype . The last 22.1 mm of the tail of the male paratype are regenerated. The female paratype has an original tail and shows a TailL/SVL ratio of 1.17. Similarly to the holotype , the male paratype has a continuous series of pores. Precloacal pores are absent in the female. Depending on the individuals, the background color of the body is gray to brown, and the pattern varies from spotted to uniform ( Figures 1 , 4 and 5 ). The dorsal pattern does not seem to show a clear sexual dimorphism, although all fully spotted individuals observed were males, and totally uniform patterns were found only in females. Distribution and natural history. All individuals of the new species were found active at night on large sandstone platforms and boulders at the type-locality. We also observed numerous individuals in Pha Taem National Park at Soi Sawan Waterfall ( 15°27’38.4”N , 105°34’40.5”E ) and surrounding sandstone outcrops ( Figures 6 and 7 ). In each of these locations the vegetation is sparse, and includes patches of grass, bamboos and shrubs. Dixonius mekongensis sp. nov. is locally abundant. We found a single other squamate species in strict syntopy in all sites, Gekko petricolus Taylor , another sandstone obligate gecko. Etymology. The specific epithet refers to the Mekong River and the Greater Mekong Subregion. A cliff bordering the type-locality offers a spectacular view of this majestic river. We suggest the following common names: จิ้งจกดินแม่โขง ( Djing-djok din Mekong , Thai), Mekong leaf-toed gecko (English), Dixonius du Mékong (French) , and Mekong Blattfingergecko (German). Comparison to other species. The main diagnostic morphological and chromatical characters of Dixonius species are presented in Table 2 . Dixonius mekongensis sp. nov. is distinguished from the Vietnamese D. aaronbaueri Ngo & Ziegler, 2009 based on its sensibly larger SVL (51.2 vs. 38.6 mm ), higher Ven number (22–24 vs. 18 or 19), much higher DTR number (16 vs. 11), much lower PV number (32–34 vs. 45–50), much lower PV’ number (20–24 vs. 29–32), and its higher PrePo number (7 vs. 5). It differs from the southern Thai Dixonius dulayaphitakorum Sumontha & Pauwels, 2020 by its much lower DTR number (16 vs. 22), its lower ICS number (25–27 vs. 30–33), its marked (vs. unmarked) canthal stripe, and a very distinct dorsal pattern with smaller dorsal spots. It can be separated from the western Thai Dixonius hangseesom Bauer, Sumontha, Grossmann, Pauwels & Vogel, 2004 by its larger SVL (51.2 vs. 42.1 mm ), higher DTR number (16 vs. 12–14), lower InterOrbS number (8 or 9 vs. 10) and a distinct dorsal pattern (U/Sp vs. Ba/Bl). It differs from the peninsular Thai Dixonius kaweesaki Sumontha, Chomngam, Phanamphon, Pawangkhanant, Viriyapanon, Thanaprayotsak & Pauwels, 2017 by its larger SVL (51.2 vs. 41.6 mm ), shorter original tail in females (TailL/SVL ratio 1.17 vs. 1.36–1.55), higher DTR number (16 vs. 12 or 13), higher InterOrbS number (8 or 9 vs. 6 or 7), lower SL number (7–9 vs. 10 or 11), lower PrePo number (7 vs. 9–11) and by its dorsal pattern (Sp or U vs. St). It differs from the Laotian Dixonius lao Nguyen, Sitthivong, Ngo, Luu, Nguyen, Le & Ziegler, 2020 by its much lower DTR number (16 vs. 20–23), much lower PV number (32–34 vs. 40–43), lower PrePo number (7 vs. 8), and by its marked (vs. unmarked) canthal stripe. It can be separated from Dixonius melanostictus Taylor, 1962 by its much higher DTR number (16 vs. 10 or 11), lower PrePo number (7 vs. 9), and by its dorsal pattern (U/Sp vs. St). It is distinguished from the Vietnamese Dixonius minhlei Ziegler, Botov, Nguyen, Bauer, Brennan, Ngo & Nguyen, 2016 by its higher DTR number (16 vs. 14 or 15), lower PV number (32–34 vs. 38–44), and smaller black dorsal spots. It differs from the peninsular Thai Dixonius pawangkhananti Pauwels, Chomngam, Larsen & Sumontha, 2020 by its larger SVL (51.2 vs. 42.6 mm ), shorter original tail in females (TailL/SVL ratio 1.17 vs. 1.33), much higher Ven number (22–24 vs. 16), higher InterOrbS number (8 or 9 vs. 7), higher PrePo number (7 vs. 6), its continuous series of pores in males (vs. a series medially interrupted by a poreless scale), and by its dorsal pattern (U/Sp vs. Ba/Bl). It differs from Dixonius siamensis by its smaller SVL (51.2 vs. 57.0 mm), its higher DTR number (16 vs. 10–14) and its marked (vs. unmarked) canthal stripe. Dixonius mekongensis sp. nov. can be distinguished from the Laotian D. somchanhae Nguyen, Luu, Sitthivong, Ngo, Nguyen, Le & Ziegler by its lower DTR number (16 vs. 19–21), lower PV number (32–34 vs. 35–40), and higher PrePo number (7 vs. 5 or 6). It can be separated from the Vietnamese Dixonius taoi Botov, Phung, Nguyen, Bauer, Brennan & Ziegler, 2015 by its larger SVL (51.2 vs. 43.9 mm ), much higher DTR number (16 vs. 11 or 12), higher PrePo number (7 vs. 5 or 6), and by its dorsal pattern (U/Sp vs. Bl). From Dixonius vietnamensis Das, 2004 , it differs by its larger SVL (51.2 vs. 42.4 mm ), higher Ven number (22–24 vs. 15–21), and its lower PV number (32–34 vs. 36). The type-localities of all currently recognized Dixonius species recorded in Thailand are mapped on Figure 8 .