Dixonius hinchangsi, Pauwels & Das & Kunya & Sumontha & Donbundit & Pauwels & Sonet & Brecko & Meesook, 2025

Pauwels, Olivier S. G., Das, Sunandan, Kunya, Kirati, Sumontha, Montri, Donbundit, Nattasuda, Pauwels, Thomas P. C., Sonet, Gontran, Brecko, Jonathan & Meesook, Worawitoo, 2025, A new sandstone-dwelling leaf-toed gecko (Gekkonidae: Dixonius hinchangsi) from Khon Kaen Province, northeastern Thailand, Zootaxa 5584 (4), pp. 482-504 : 485-500

Dixonius hinchangsi sp. nov.

( Figures 1–9 )

Dixonius siamensis — Sumontha et al. 2017: 568

Dixonius siamensis — Pauwels et al. 2020: 108

Dixonius siamensis — Sumontha & Pauwels 2020: 176

Dixonius siamensis — Pauwels et al. 2021: 538

Holotype. RBINS 2739 (formerly under RBINS 17015 ); adult male from Ban Kaeng Sila (ca. 16°39’42.5”N, 102°36’04.4”E), W of the border with Nam Phong National Park , Ubolratana District, Khon Kaen Province, northeastern Thailand; collected by Kirati Kunya in 2000.

Paratypes (3). RBINS 2740 (formerly RBINS 17016 ), adult male, and RBINS 2741–2742 (formerly RBINS 17017–17018 ), adult females; same collector, locality and date as the holotype .

Diagnosis. Dixonius hinchangsi sp. nov. can be distinguished from all other congeners by the combination of its maximal known SVL of 50.1 mm; 12 longitudinal rows of dorsal tubercles; 30 to 34 paravertebral tubercles; 23 to 27 longitudinal rows of ventrals across the abdomen; six to eight precloacal pores in males, no precloacal pores in females; no canthal stripe; strongly barred lips; and a spotted to uniform dorsal pattern in males and females.

Description of holotype. Adult male ( Figures 1 and 2 ). SVL 47.5 mm. Head relatively long (HL/SVL ratio 0.31), wide (HW/HL ratio 0.64), not markedly depressed (HD/HL ratio 0.46), distinct from neck. Lores and interorbital region weakly inflated. Canthus rostralis relatively prominent. Snout moderately short (SnOrb/HL ratio 0.40), rounded, slightly longer than orbit diameter (OrbD/SnOrb ratio 0.53). 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. Orbit of moderate size (OrbD/HL ratio 0.21). Pupil vertical with crenelated margins. Supraciliaries short, without spines. Ear opening roundish, small (EarL/HL ratio 0.05); orbit to ear distance greater than orbit diameter. Rostral 1.43 times wider than high, dorsally divided on a third of its height by a median cleft. Two enlarged supranasals, in contact along their anterior half. Rostral in contact with supralabial I on each side, nostrils and both supranasals. Nostrils rounded, each surrounded by supranasal, rostral, supralabial I and two postnasals. Mental triangular, wider than long. Two pairs of enlarged postmentals, anteriormost approximately three of 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 three throat scales. Supralabials to mid-orbital position 5/6; enlarged supralabials to angle of jaws 7/8. Infralabials 7/6. Interorbital scales 9.

Body slender, elongate (TrunkL/SVL ratio 0.42), without ventrolateral folds. Dorsal scales small, irregular, flattened to conical, distributed among large, strongly keeled tubercles arranged in 12 regular longitudinal rows at midbody. Paravertebral tubercle rows separated by two granular scales, i.e. less than the width of a tubercle. Lower flanks covered with irregularly shaped, smooth to slightly conical scales. Gular region with relatively homogeneous, granular scales. Ventral scales smooth, imbricate, their free posterior margin rounded. Ventrals increasing in size from throat to chest to abdomen. Midbody scale rows across belly to lowest rows of tubercles 24. Eight precloacal pores in a continuous series. Pore-bearing scales smaller than scales of adjacent scale rows. No femoral pores or enlarged femoral scales.

Fore- and hind limbs short, slender (FaL/SVL ratio 0.11; TibL/SVL ratio 0.16). Digits slender, dilated distally, all bearing robust, slightly recurved claws. Basal subdigital lamellae narrow, without scansorial surfaces (6-8- 11-10-9 right manus; 8-10-11-13-12 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>II≈IV>V>I (manus), III>IV>V>II>I (pes). Tail length 45.6 mm, of which the last 2.5 mm correspond to the regenerated part of the tail. Supracaudals markedly keeled along the whole original part of the tail. Thirty subcaudals enlarged into transverse plates on the original part of the tail.

Coloration in life. Dorsal surface of head brown with small light brown patches on the snout. Supraorbital area slightly bluish. Supralabials and infralabials black with each an irregular, vertical light brown bar. No canthal stripe. Dorsum brown with a few light brown spots forming an interrupted, poorly contrasting, line. Flanks blackish brown with irregular light brown marks. Dorsal surfaces of members blackish brown with scattered lighter and darker spots. Tail blackish brown with poorly contrasted, lighter rings not encircling the tail. Throat, lower flanks and belly gray, darker on the underside of limbs and tail. In preservative the colors strongly fade and become less contrasted ( Figures 1 and 2 ).

Cranial osteology. Based on the adult female paratype RBINS 2742. Typical, relatively depressed, gekkonid cranium ( Figure 7 ). The snout complex is formed of: 1) a single premaxilla bearing 10 teeth; 2) paired, elongated nasals with an embayed rostral end and pointed posterolateral end; 3) paired, laminar septomaxillae with hooked posterolateral processes forming the roofing of the vomeronasal organ; and 4) paired laminar vomers completing the ventral encasing of the vomeronasal organ.

The palatomaxillary arch consists of: 1) paired maxillae with a high, shark fin-shaped facial process with a deep embayment at its anterior base, premaxillary process (itself divided into an anterolateral process and an anteromedial lappet), orbital process and alveolar shelf bearing 35–36 teeth; 2) paired, laminar, edentulous palatines with an anterolateral and an anteromedial process; 3) paired, edentulous, trifurcate pterygoid comprised of ectopterygoid, palatine and quadrate processes; and 4) paired, crescentic ectopterygoids.

The dermatocranial roofing and circumorbital bones consist of: 1) single, elongated frontal with dorsal lamina that widens markedly at the caudal end and orbital laminae merging along the ventral midline; 2) paired parietal with a posteromedial process; 3) paired, crescentic prefrontals; 4) paired, Y-shaped postorbitofrontal; and 5) paired, rudimentary jugals.

The chondrocranial braincase and otic region bones consist of: 1) single supraoccipital housing the posterior semicircular canal; 2) paired prootics with a prominent alar process, crista prootica laterally extended into a spina prootica and forming the housing of the anterior and horizontal semicircular canal; 3) paired otoccipitals bearing the fenestra ovalis, a pronounced paroccipital process and housing cochlea, horizontal and posterior semicircular canals; 4) single parabasisphenoid with the sella turcica and a pair of elongated basipterygoid processes; and 5) single basisphenoid forming the occipital condyle with contribution from the otoccipitals.

The suspensorial elements and palatoquadrate arch element include: 1) paired, curved, slender squamosals; 2) paired quadrates with a cephalic condyle, central column, tympanic crest (with a concavity along its posterior surface) and bicipital mandibular condyle; and 3) paired, rod-like epipterygoids.

The mandible consists of: 1) paired compound bones which themselves are a complex of articular, prearticular and surangular, with a retroarticular process and an adductor fossa; 2) paired coronoids with a triangular apex; 3) paired splenials; and 4) paired, elongated dentary with 35 teeth.

External morphological variation. 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. In RBINS 2740 , each anterior postmental is bordered anteriorly by the mental, medially by the other anterior postmental, anterolaterally by infralabial I, posterolaterally by the second postmental and a small scale in contact with infralabials I and II (left), or by the second postmental only (right); the pair collectively bordered posteromedially by three throat scales of unequal sizes ( Figure 4 ). In RBINS 2740 , 7-9 - 10-10 -9 lamellae on right manus, 6-9-12-14-11 on right pes (numbers of lamellae not noted for the two other paratypes, except for 4 th toe, see Table 1 ). Relative length of digits of RBINS 2740 : III>II≈IV>V>I (manus), IV>III>V>II>I (pes) (not noted for the two other paratypes). More than 43 widened subcaudals in RBINS 2740 (tail original, but tip missing; Figure 3 ). The male paratype RBINS 2740 shows a continuous series of six pores. Similarly to all other species of Dixonius , there are no femoral pores in both sexes, and precloacal pores are absent in females ( Figures 5 and 8 ). The dorsal pattern and color do not seem to show a sexual dimorphism. None of the types has a complete, original tail, so TailL/SVL ratio could not be established. None of the types was photographed alive, but Figures 8 and 9 show live, uncollected, adults from the type locality. The female shown on Figure 8 has an original tail on which ten light rings, not encircling the tail, are regularly disposed.

Molecular analyses. The sequence obtained here for the paratype RBINS 2742 included the complete NADH dehydrogenase subunit 2 (ND2) gene of 1041 bp and was based on an average sequencing depth of 1230. It contained no stop codons. From the 37 ND2 sequences labelled as Dixonius already available in GenBank, we excluded HQ646374 , which showed a similarity ≥99.7% with records from Heteronotia binoei (Gray) , EU054298 , which originates from the same specimen than KP979732 ), and EU054297 , which was responsible for 37 singletons (unevenly distributed in the sequence) out of the 139 singletons in the alignment of the ingroup) ( Table 2 ). Proportions of nucleotide substitutions between species ranged from 3.11 to 18.92%. Dixonius hinchangsi sp. nov. showed divergences ranging from 7.40 to 18.11% compared to all other Dixonius species ( Table 3 ). These interspecific distances were all larger than the intraspecific distances measured for species sequenced for more than one specimen (0–0.99%). The maximum likelihood phylogenetic tree was based on an alignment of 1041 bp. The best-fit substitution models determined by IQ-TREE were TVM+F+G4, TPM3u+F+G4 and TVM+F+I+G4 for the partitions corresponding to the codon positions 1, 2 and 3, respectively. In this tree Dixonius hinchangsi sp. nov. belongs to the clade comprising D. cf. siamensis (VU023), D. cf. siamensis (AUP-01724) and D. sp. from Thailand, and D. lao Nguyen, Sitthivong, Ngo, Luu, Nguyen, Le & Ziegler, 2020 and D. muangfuangensis Luu, Nguyen, Le, Grismer, Ha, Sitthivong, Hoang & Grismer, 2023 from Laos ( Figure 10 ). In Table 2 and Figure 10 , MT468896 , EU054299 and KP979732 , originally labelled in GenBank as Dixonius siamensis , were relabelled as D. cf. siamensis , because they originate from areas distant from the type locality and likely represent two other species ( Pauwels et al. 2024); and HM997153 , identified in GenBank as D. melanostictus was changed to D. cf. melanostictus because its geographical origin within Thailand is unknown.

Distribution and natural history. The type series was found active at night on sandstone in a cultivated area along the 4014 Rural Road in the Ubolratana District of Khon Kaen Province, Isan Region on the Khorat Plateau, northeastern Thailand ( Figure 11 ). We also observed numerous individuals within Nam Phong National Park, at Hin Chang Si View Point in Ban Fang District (16°39’17.2”N 102°36’33.0”E and surroundings, see Figures 12 and 13 ). The two locations where the new species was found, at less than two km from each other, are characterized by a sandstone substrate ( Singtuen et al. 2022), with which it seems associated. On the same rocks we (KK, ND and TPCP) observed Gehyra lacerata (Taylor) , Gekko gecko (Linnaeus) and numerous Hemidactylus frenatus Duméril & Bibron and H. platyurus (Schneider) ( Gekkonidae ) in strict syntopy, and we encountered Calotes versicolor (Daudin) ( Agamidae ), Eutropis longicaudata (Hallowell) and E. multifasciata (Kuhl) , Lygosoma koratense Smith , Scincella melanosticta (Boulenger) ( Scincidae ), Ahaetulla nasuta (Lacepède) , Coelognathus radiatus (Boie) , Lycodon capucinus Boie and L. laoensis Günther , Oligodon fasciolatus (Günther) ( Colubridae ), Naja siamensis Laurenti and Ophiophagus hannah (Cantor) ( Elapidae ) in the immediate surroundings. Some aspects of the vegetation at Hin Chang Si were described by Thummawongsa et al. (2014).

Etymology. The specific epithet refers to Hin Chang Si (“Elephant-Rubbing Rock”), a remarkably beautiful sandstone formation ( Figure 12 ) in Ban Fang District within Nam Phong National Park, where the new species is common. It is a noun in apposition, invariable.

We suggest the following common names: จิ้งจกดินหินช้างสี (Djing-djok din Hin Chang Si, Thai), Hin Chang Si leaf-toed gecko (English) , and Dixonius de Hin Chang Si (French) .

Comparison to other species. The main diagnostic morphological and chromatical characters of Dixonius species are presented in Table 4 . Dixonius hinchangsi sp. nov. is readily distinguished from the southern Vietnamese D. aaronbaueri Ngo & Ziegler, 2009 by its distinctly higher SVL (50.1 versus 38.6 mm), distinctly higher Ven (24–27 versus 18 or 19), slightly higher DTR (12 versus 11), distinctly lower PV (30–34 versus 45–50) and lower PV’ (21–25 versus 29–32), higher PrePo (6–8 versus 5), its lack (versus presence) of a marked canthal stripe, and the lack (versus possession) of an orange tail; from the western Thai D. chotjuckdikuli Donbundit, Sumontha, Suthanthangjai, Suthanthangjai & Pauwels, 2024 by a distinctly higher Ven (24–27 versus 16 or 18), distinctly lower DTR (12 versus 16 or 18), strongly barred (versus not barred) lips, absence (versus presence) of a marked canthal stripe, a uniform or spotted (versus blotched) dorsal pattern, and the lack (versus possession) of an orange tail; from the southern Thai D. dulayaphitakorum Sumontha & Pauwels, 2020 by a higher Ven (24–27 versus 22) and a much lower DTR (12 versus 22).

Dixonius hinchangsi sp. nov. can be separated from the central Vietnamese D. fulbrighti Luu, Grismer, Hoang, Murdoch & Grismer, 2023 by a distinctly lower DTR (12 versus 16–20), strongly barred (versus not barred) lips, and the absence (versus presence) of a marked canthal stripe; from the central Vietnamese D. gialaiensis Luu, Nguyen, Le, Grismer, Ha, Sitthivong, Hoang & Grismer, 2023 by a higher Ven (24–27 versus 19–21), distinctly lower DTR (12 versus 19), higher InterOrbS (8 or 9 versus 7), and absence (versus presence) of a marked canthal stripe; from the western Thai D. hangseesom Bauer, Sumontha, Grossmann, Pauwels & Vogel, 2004 by a distinctly higher SVL (50.1 versus 42.1 mm), lower InterOrbS (8 or 9 versus 10), lower IL (6 or 7 versus 8), absence (versus presence) of a marked canthal stripe, a uniform or spotted (versus banded/blotched) dorsal pattern, and the lack (versus possession) of an orange tail; from the peninsular Thai D. kaweesaki Sumontha, Chomngam, Phanamphon, Pawangkhanant, Viriyapanon, Thanaprayotsak & Pauwels, 2017 by a distinctly higher SVL (50.1 versus 41.6 mm), higher InterOrbS (8 or 9 versus 6–7), lower SL (6–9 versus 10 or 11), lower SLMOrb (5 th or 6 th versus 7 th or 8 th), lower PrePo (6–8 versus 9–11), uniform or spotted (versus striped) dorsal pattern, absence (versus presence) of a marked canthal stripe, and lack (versus possession) of an orange tail; from the central Laotian D. lao by a distinctly lower DTR (12 versus 20–23), distinctly lower PV (30–34 versus 40–43), lower SLMOrb (5 th or 6 th versus 7 th or 8 th), and strongly barred (versus not barred) lips; from the eastern Thai D. mekongensis Pauwels, Panitvong, Kunya & Sumontha, 2021 by a generally higher Ven (24–27 versus 22–24), a distinctly lower DTR (12 versus 16), and absence (versus presence) of a marked canthal stripe.

Dixonius hinchangsi sp. nov. can be distinguished from the central Thai D. melanostictus by a higher Ven (24–27 versus 22), higher DTR (12 versus 10 or 11), lower SLMOrb (5 th or 6 th versus 7 th), lower PrePo (6–8 versus 9), absence (versus presence) of a marked canthal stripe, strongly barred (versus not barred) lips, a uniform or spotted (versus striped) dorsal pattern, and lack (versus possession) of an orange tail; from the southern Vietnamese D. minhlei Ziegler, Botov, Nguyen, Bauer, Brennan, Ngo & Nguyen, 2016 by a higher Ven (24–27 versus 20–23), lower DTR (12 versus 14 or 15), lower PV (30–34 versus 38–44), absence (versus presence) of a marked canthal stripe, and lips strongly (versus not strongly) barred; from the northern Laotian D. muangfuangensis by a higher Ven (24–27 versus 20 or 21), distinctly lower DTR (12 versus 21–23), distinctly lower PV (30–34 versus 45–48) and higher InterOrbS (8 or 9 versus 7); from the peninsular Thai D. pawangkhananti Pauwels, Chomngam, Larsen & Sumontha, 2020 by a distinctly higher SVL (50.1 versus 42.6 mm), distinctly higher Ven (24–27 versus 16), distinctly lower DTR (12 versus 16), higher InterOrbS (8 or 9 versus 7), absence (versus presence) of a marked canthal stripe, uniform or spotted (versus banded/blotched) dorsal pattern, and lack (versus possession) of an orange tail; from the central Thai D. siamensis by a lower DTR (12 versus 14), generally lower PV (30–34 versus 33–38), lower InterOrbS (8 or 9 versus 10 or 11), and absence (versus presence) of a marked canthal stripe; from the Laotian D. somchanhae Nguyen, Luu, Sitthivong, Ngo, Nguyen, Le & Ziegler, 2021 by a distinctly lower DTR (12 versus 19–21), a lower PV (30–34 versus 35–40), and the absence (versus presence) of a marked canthal stripe; from the southern Vietnamese, insular D. taoi Botov, Phung, Nguyen, Bauer, Brennan & Ziegler, 2015 by a higher Ven (24– 27 versus 21–23) and the absence (versus presence) of a marked canthal stripe; and from the southern Vietnamese D. vietnamensis Das, 2004 by a distinctly higher SVL (50.1 versus 42.4 mm), higher Ven (24–27 versus 15–21), lower DTR (12 versus 13–17), lower PV (30–34 versus 36), absence (versus presence) of a marked canthal stripe, and strongly (versus not strongly) barred lips. Dixonius hinchangsi sp. nov. is also distinct from the Myanmar Dixonius burmanicus by its distinctly higher SubLT4 (13–15 versus 8 or 9 according to the original description).

The single examined cranium of Dixonius hinchangsi sp. nov. differs from the single examined cranium of the lectotype (BMNH 1946.8.24.40) of D. siamensis in having a higher number of maxillary teeth (35 or 36 versus 32 or 33) and an obtusely pointed anterior end of the frontal (versus a more rounded anterior end). The cranial osteology of the other Dixonius species has not yet been studied.

Figures

FIGURE 1. Preserved adult male holotype (RBINS 2739) of Dixonius hinchangsi sp. nov. General dorsal (A) and ventral (B) views. Photos. by J. Venderickx.

FIGURE 2. Preserved adult male holotype (RBINS 2739) of Dixonius hinchangsi sp. nov. A. Right profile of the head. B. Dorsal surface of the head. C. Ventral surface the head. D. Cloacal area. Note the eight precloacal pores and the partly everted right hemipenis. Photos. by J. Venderickx.

FIGURE 3. Preserved adult male paratype (RBINS 2740) of Dixonius hinchangsi sp. nov. General dorsal (A) and ventral (B) views. Photos. by J. Venderickx.

FIGURE 4. Head of the preserved adult male paratype (RBINS 2740) of Dixonius hinchangsi sp. nov. A. Right profile. B. Dorsal surface. C. Frontal view. D. Ventral surface. Photos. by J. Venderickx.

FIGURE 5. Preserved adult female paratype (RBINS 2741) of Dixonius hinchangsi sp. nov. General dorsal (A) and ventral (B) views. Photos. by J. Venderickx.

FIGURE 6. General dorsal view of the preserved adult female paratype (RBINS 2742) of Dixonius hinchangsi sp. nov. Photo. by J. Venderickx.

FIGURE 7. Cranium of the adult female paratype RBINS 2742 of Dixonius hinchangsi sp. nov. in dorsal (A), lateral (B) and ventral (C) views. Scans and plate by S. Das and J. Brecko.

FIGURE 8. Dorsal (A) and ventral (B) views of a live adult female individual (not preserved) of Dixonius hinchangsi sp. nov. at the type-locality. Tail original. Skin of dorsum damaged during capture. Photo. by K. Kunya.

FIGURE 9. Live adult individual (not preserved) of Dixonius hinchangsi sp. nov. at the type-locality. Tail partly regenerated. Photo. by K. Kunya.

FIGURE 10. Phylogenetic tree inferred for the Dixonius specimens analysed here in, based on 1041 base pairs of the NADH dehydrogenase subunit 2 (ND2) gene using the maximum likelihood method with 1000 ultrafast bootstrap replicates (bootstrap support values>95 given on the branches).

FIGURE 11. Map showing the positions of the type-localities of the currently recognized Dixonius species inhabiting Thailand. Map by W. Sodob.

FIGURE 12. Habitat of Dixonius hinchangsi sp. nov. at Hin Chang Si, Khon Kaen Province, northeastern Thailand. Photo. by T. P. C. Pauwels.

FIGURE 13. Sandstone microhabitat of Dixonius hinchangsi sp. nov. at its type locality in Khon Kaen Province, northeastern Thailand. Photo. by N. Donbundit.