Dixonius luuhoangorum sp. nov.

Suggested common names: English—Southern Mekong Leaf-toed Gecko

Khmer— ជ*ងចក់m្លឹកហុងចុង (Chingchorksloek-hongchong)

Vietnamese—thằn lằn chân lá Hòn Chông. (English transcription: Southern Mekong Leaf-toed Gecko)

Figures 3 and 4

Chersonomy

Dixonius siamensis Grismer et al. 2007:224; 2008:22

Dixonius sp. Grismer et al. 2025:8

Dixonius cf. siamensis Yodthong et al. 2025:553

Holotype. Adult male LSUHC 9262 from Hon Chong, Kien Giang, Province, Vienam (10.145788°N 104.601846°E; 16 m) collected from 1930–2200 h on 26 June 2009 collected by Ngo Van Tri, L. Lee Grismer, and Jesse L. Grismer.

Paratypes. Vietnam: Hon Chong, adult males LSUHC 9260 and 9263 and adult females LSUHC 9259, 9264–65, 9268 bear the same data as the holotype. Adult females LSUHC 8739, 8741, and 9261 from Ba Trai (10.147782°N 104.628770°E; 31 m) collected from 1900–2100 h on 2 July 20 by Ngo Van Tri, L. Lee Grismer, and Jesse L. Grismer. Cambodia, Pursat Province: adult female LSUHC 8521 and adult male LSUHC 8523 from the Baktra Pagoda (12.314101°N 103.514518°E; 234 m) by Neang Thy, Chav Thou, Seri Narin, L. Lee Grismer, and Jesse L. Grismer. Adult females LSUHC 8449, 8487, 8491, 8522, and adult male LSUHC 8420 from O’Lakmeas (12.323912°N 103.510066°E; 66 m) collected from 1900–2330 h on 30 June 2007 by Neang Thy, Chav Thou, Seri Narin, L. Lee Grismer, and Jesse L. Grismer. Adult male LSUHC 8401 from Phum Chruk Pul (12.545874°N 103.064749°E; 312 m) collected from 1830–1900 on 26 June 2007 by Neang Thy, Chav Thou, Seri Narin, L. Lee Grismer, and Jesse L. Grismer. Adult female LUSHC 9284 and adult male 9289 from Phnom Lok (12.367842°N 104.781245°E; 156 m) collected from 1730–1800 on 4 July 2009 by Neang Thy, Chav Thou, L. Lee Grismer, and Jesse L. Grismer. Adult male LSUHC 7378 and juvenile 7328 from Phnom Aural (12.019325°N 104.128507°E; 549 m) collected from 1800–2400 on 6 August 2005 by Neang Thy, Chav Thou, L. Lee Grismer, Hinrich Kaiser, Armando Ponce, Perry L. Wood Jr., and Jesse L. Grismer.

Diagnosis. Dixonius luuhoangorum sp. nov. can be diagnosed from all other species of Dixonius by having a combination of a maximum adult SVL of 48.5 mm (male) and 50.0 mm (female); 7–9 supralabials; 5–7 supralabials to the midorbital position; six or seven infralabials; 7–12 interorbital scales; 24–40 interciliary scales; 14–22 ventral scales; 13–22 longitudinal rows of dorsal tubercles; 23–28 paravertebral tubercles; 12–17 subdigital lamellae on the 4 th toe; 9–13 subdigital lamellae on the 4 th finger; 5–7 precloacal pores in males; 7–11 enlarged precloacal scales; no femoral pores; dark-colored canthal stripe present; dark-colored labial bars present (Cambodia); dark-colored irregularly shaped chevron-like marking in occiput; and a row of pale colored tubercles on flanks (Tables 4, 5). Statistically significant meristic and size-corrected morphometric characters are presented in Table 3.

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Description of the holotype. Adult male, SVL 48.5 mm; TaL 50.0 mm, axilla-groin length 21.7 mm; head longer than wide, depressed, distinct from neck; 13.6 mm in length, 8.6 mm in width; eye moderate in size, 2.7 mm diameter; ear opening oblique, 1.7 mm in length; orbit-snout 5.0 mm; internarial distance 1.6 mm; supralabials seven (six in midorbital position); nares contacting rostral, first supralabial, supranasal, two postnasals on each side; supranasals in broad contact; no intersupranasals; snout flat, covered with granular weakly keeled scales; scales on top of head granular weakly keeled, intermixed with small keeled tubercles; pupil vertical; ear opening obliquely oriented, oval, not bordered by enlarged scales; infralabials six, decreasing gradually in size posteriorly; two enlarged postmentals, first pair contact each other for approximately 60% of their length, each bordered laterally by a single enlarged sublabial; gular scales flat, subimbricate grading posteriorly into larger imbricate pectorals and larger imbricate ventrals; body somewhat dorsoventrally flattened; dorsal tubercle rows at midbody ~16, keeled, paravertebral tubercles separated from each other by 3–5 flat smaller scales; 25 paravertebral scales between limb insertions; ventral scales larger than dorsal scales, in ~14 rows at midbody; dorsal surface of forelimbs bearing flat to weakly keeled scales; underside of forelimbs bearing small granular scales; dorsal surface of hind limbs covered with large keeled scales; underside of hind limbs bearing enlarged, flat, imbricate scales abruptly transitioning into granular postfemoral scales; digits relatively short; all subdigital lamellae transversely expanded; distalmost lamellar divided, slightly enlarged forming small toe pad; 13 subdigital lamellae on fourth toe; 10 subdigital lamellae on fourth finger; eight contiguous pore-bearing precloacal scales in an angular series; two slightly enlarged postcloacal tubercles on each side at base of tail; dorsal caudals flat, very weekly keeled anteriorly; and subcaudals flat, midventral row transversely expanded.

Coloration and pattern in preservation (Fig. 3). Ground color of the dorsal surfaces of the head, body, limbs, and tail grey to light-brown; no distinct dark-colored baring on lips; a dark-colored canthal stripe extends from naris to orbit; iris gold bearing reddish and black variegations; top of head bearing faint dark-colored irregularly shaped, poorly defined markings with a faint chevron-shaped marking near occipital region; paired dark-colored markings on nape, followed by two paravertebral markings; dorsum overlain with irregularly shaped poorly defined dark-colored markings of varying sizes, those on flanks somewhat U-shaped; dorsal markings terminate in sacral region but extend onto tail as a series broken, semi-transverse markings; series of five pale colored evenly spaced keeled scales on flanks edged ventrally with dark-colored markings beginning in axillary region extending down length of tail, terminating in five larger markings; forelimbs and hind limbs bearing irregularly spaced dark-colored stippling; gular region and chest beige, faintly stippled; more distinct stippling on underside of limbs and abdomen; and subcaudal region grayish.

Variation. Most specimens from the other populations are very similar to the holotype in coloration and pattern (Fig. 4). There is a general lack of distinct baring on the lips in Vietnam specimens which is more pronounced in Cambodian specimens. Variation in meristics, raw morphometrics, and color pattern are presented in Table 4.

Distribution. Dixonius luuhoangorum sp. nov. ranges at least through the Cardamom Mountains of Cambodia from Pchoek Chrum, Pursat Province in the north to Bokor, Kampot Province in the south and on into Ba Trai Mountain and Hon Chong (karst), Kien Luong District, Kien Giang Province, Vietnam in the southern Mekong Delta (Fig. 1). Dixonius luuhoangorum sp. nov. is expected to be found in other areas of the southern Mekong Delta.

Etymology. The specific epithet “ luuhoangorum ” honors the husband-and-wife team Vinh Quang Luu and Tuoi Thi Hoang for their contributions to the herpetology of Vietnam, Vietnamese students, and many students from other countries, including those from La Sierra University, Riverside, California, USA. The close working relationship and friendship developed among LLG and JLG and Vinh Quang Luu and Tuoi Thi Hoang during the formers Fulbright tenure at La Sierra University in 2023–2024 epitomizes the scholarly benefit of integrating research and friendship—a phenomenon noted by Brandt (2017).

Natural History (Fig. 5). Dixonius luuhoangorum sp. nov. is a habitat generalist equally abundant through a wide range of habitats in both disturbed and undisturbed forest with or without rocks. All specimens from the Cardamon Mountains were found in dry dipterocarp forests below 500 m in elevation. Lizards from Phnom Aural were collected during the late afternoon among shaded rocks near a small rocky stream in undisturbed forest. Specimens from Che Teal Chrum village were found during the day beneath rocks along an unimproved dirt road in disturbed forest leading to the village. The Pchoek Chrum specimen was found in leaf litter beneath a camping tarp in secondary forest. Lizards from O’Lakmeas were found at night among and on rocks along a small stream in disturbed forest. The Bokor specimen was found at night at the very southern end of the Bokor Plateau in undisturbed lowland forest. The specimen was running through the grass on a gravel substrate along an unimproved road. At the type locality in Hon Chong, all specimens were found among tower karst formations on the ground in disturbed forest bordering a small agricultural grove. One specimen was found on a low rock wall. Specimens LSUHC 8741 (SVL 44.3 mm) from Ba Trai and LSUHC 9264 (SVL 48.3 mm) from Hon Chong are gravid females collected on 26 June 2009, while the gravid LSUHC 8521 (SVL 37.1 mm) from the Baktra Pagoda near O’Lakmeas in the Cardamom Mountains was collected on 2 July 2007.

Comparisons. Dixonius luuhoangorum sp. nov. is statistically different all from other species of Dixonius examined for all characters across varying combinations of species (Table 2) but from its sister species D. sambupura, it differs in having significantly fewer ventral scales (x̄ =17.7 versus x̄ =25.5) and a smaller size-corrected interorbital distance (x̄ =0.47 versus 0.57) (Table 5). Dixonius luuhoangorum sp. nov. is also statistically different from D. sambupura in morphospace (p-adjusted=0.00013) as well as from other species (Tables 3 and 5).

Remarks. Although the first published record of Dixonius in Cambodia was that of Grismer et al. (2007) from the base of Phnom Aural, the genus was first reported in unpublished survey reports by Fauna & Flora International, Phnom Penh, Cambodia by Daltry & Chheang (2000) and Long et al. (2002) as D. siamensis.