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Dixonius siamensis

Year: 2024
Type: other
Abstract: Results Description of the syntypes. Four specimens collected by Stanley Smyth Flower (1871–1946) preserved in ethanol, housed in the herpetological collections of the Natural History Museum, London, UK (NHMUK, previously known as the British Museum of Natural History, BMNH): one juvenile male (BMNH 97.3.31.1) and three subadult females (BMNH 97.3.31.2 and BMNH 1946.8.24.40–41) (Figures 2–7). BMNH 1946.8.24.40–41 were previously registered as BMNH 98.4.2.3–4 (P. Campbell, pers. comm. to OSGP, 2024). Measurements and scale counts for each specimen are provided in Table 1. Individual morphometric ratios are given in the following order: BMNH 97.3.31.1–2, BMNH 1946.8.24.40–41. SVL 26.8 mm, 30.6 mm, 39.7 mm and 36.4 mm. Flower (1899a: 628), based on the four specimens then known, gave the maximum total length as ‘’ 86 mm (snout to vent 42; tail 44)’’. One can assume that this corresponds to an approximate measurement of the SVL of the female type BMNH 1946.8.24.40, and that part of its tail was lost after its examination by Flower. Head relatively long (HL/SVL ratios 0.34, 0.30, 0.32, 0.31), wide (HW/HL ratios 0.59, 0.67, 0.63, 0.68), not markedly depressed (HD/HL ratios 0.43, 0.42, 0.40, 0.49), distinct from neck. Lores and interorbital region weakly inflated. Canthus rostralis relatively prominent. Snout moderately short (SnOrb/HL ratios 0.34, 0.35, 0.35, 0.42), rounded, slightly longer than orbit diameter (OrbD/SnOrb ratios 0.61, 0.67, 0.59, 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. Eye of moderate size (OrbD/HL ratios 0.21, 0.24, 0.21, 0.22). Pupil vertical with crenelated margins. Supraciliaries short, without spines. Ear opening oval, moderate (EarL/HL ratios 0.04, 0.06, 0.06, 0.06); orbit to ear distance distinctly greater than orbit diameter (OrbEar/OrbD ratios 1.37, 1.36, 1.46, 1.44). Rostral distinctly wider than high (RosW/RosH ratios 1.56, 2.00, 1.62, 1.40), dorsally incompletely divided (about 50%) by a median cleft. Two enlarged supranasals in broad contact on their whole length (BMNH 97.3.31.2), for half their length (BMNH 1946.8.24.41), or totally separated by a smaller one (BMNH 97.3.31.1 and BMNH 1946.8.24.40). Rostral in contact with supralabial I on each side, nostrils and two (BMNH 97.3.31.2 and BMNH 1946.8.24.41) or three (BMNH 97.3.31.1 and BMNH 1946.8.24.40) supranasals. Nostrils round, each surrounded by supranasal, rostral, supralabial I, and two (BMNH 97.3.31.1–2 and BMNH 1946.8.24.40) or three (BMNH 1946.8.24.41) postnasals. Mental triangular, broader than long (MenL/MenW ratios 0.73, 0.81, 0.76, 0.85). Two pairs of enlarged postmentals, anteriormost approximately three 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 (in BMNH 97.3.31.2 the left postmental also in contact by a point with infralabial II; in BMNH 1946.8.24.40 the left anterior postmental is also bordered by an additional granular scale located between the anterior and posterior postmentals and the 1 st and 2 nd IL); the pair collectively bordered posteromedially by a row of four (BMNH 1946.8.24.40–41) or five (BMNH 97.3.31.1–2) throat scales. Supralabials to mid-orbital position 6/6 (except for BMNH 97.3.31.2, which shows 6/5); enlarged supralabials to angle of jaws 7 (BMNH 97.3.31.2), 8 (BMNH 1946.8.24.40–41) or 9 (BMNH 97.3.31.1). Infralabials 6/6 (BMNH 97.3.31.2), 6/7 (BMNH 97.3.31.1) or 7/7 (BMNH 1946.8.24.40–41). Interorbital scales 10 (BMNH 97.3.31.1 and BMNH 1946.8.24.40–41) or 11 (BMNH 97.3.31.2); interciliary scales 19 (BMNH 97.3.31.1 and BMNH 1946.8.24.40–41) or 20 (BMNH 97.3.31.2). Body slender, elongate (TrunkL/SVL ratios 0.40, 0.40, 0.37, 0.42), without ventrolateral folds. Dorsal scales heterogeneous, small, irregular, flattened to conical, distributed among large, strongly keeled tubercles arranged in 12 (BMNH 97.3.31.2) or 14 (BMNH 97.3.31.1 and BMNH 1946.8.24.40–41) regular longitudinal rows at midbody. PV 33 (BMNH 1946.8.24.41), 35 (BMNH 97.3.31.2) or 38 (BMNH 97.3.31.1 and BMNH 1946.8.24.40); PV’ 23 (BMNH 97.3.31.1–2) or 24 (BMNH 1946.8.24.40–41). Paravertebral rows of tubercles separated by four dorsal scales in BMNH 97.3.31.1–2 or two or three in BMNH 1946.8.24.40–41. 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 20 (BMNH 1946.8.24.41), 25 (BMNH 97.3.31.1 and BMNH 1946.8.24.40) or 26 (BMNH 97.3.31.2). Six poorly developed precloacal pores in a continuous series in the male (difficult to see to the naked eye and possibly overlooked by Boulenger, they appear clearly under a careful examination through the binocular). Pore-bearing scales not enlarged relative to adjacent scale rows. No pits or pores in the females. No femoral pores or enlarged femoral scales. Fore- and hind limbs short, slender (FaL/SVL ratios 0.14, 0.12, 0.14, 0.13; TibL/SVL ratios 0.15, 0.16, 0.18, 0.17). Digits slender, dilated distally, all bearing robust, slightly recurved claws. Basal subdigital lamellae narrow, without scansorial surfaces (8-10-10-10-8 on right manus and 7-10-13-14-11 on right pes of BMNH 97.3.31.1; 8- 9-9-8-8 on right manus and 7-9-11-12-11 on right pes of BMNH 97.3.31.2; 6-9-10-10-9 on right manus and 8-10- 12-15-13 on right pes of BMNH 1946.8.24.40; and 7-9-10-9-9 on right manus and 8-10-12-14-13 on right pes of BMNH 1946.8.24.41); 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 (manus): III≈IV>V≈II>I (BMNH 97.3.31.1), III≈IV>II>V>I (BMNH 97.3.31.2) or III≈IV>II>V>I (BMNH 1946.8.24.40–41). Relative length of toes (pes): IV>V>III>II>I (BMNH 97.3.31.1 and BMNH 1946.8.24.40–41) or III>IV>V>II>I (BMNH 97.3.31.2). Tail complete and original only in the male, BMNH 97.3.31.1 (TailL 30.5 mm; TailL/SVL ratio 1.14), showing 57 subcaudals enlarged into transverse plates (counted from the first widened at the base of the tail). In the three other types a part of the tail is missing, and the part remaining is original, not healed or regenerated. Supracaudals markedly keeled in the anterior portion of the tail. Coloration in preservative: In the female BMNH 97.3.31.2 the pattern has totally faded or was already poorly contrasted or absent in life; the whole dorsal surface is light brown, except the tubercles which are beige, and a discontinuous series of white tubercles on the upper flanks from the nape to the anterior part of the tail; the throat and belly are uniformly beige, and the underside of the tail is light brown. In the three other types, the dorsal surface of the head is light brown with irregular dark brown blotches (nearly faded away in BMNH 97.3.31.1 or already poorly contrasted when it was alive). On each side of the head of BMNH 1946.8.24.40–41 a dark brown canthal stripe runs from the nostril through the eye and extends to the level of the occiput; this stripe is not visible in BMNH 97.3.31.1–2. The supralabials and infralabials are light brown, strongly barred vertically with dark brown in all four specimens. In BMNH 1946.8.24.40–41, similarly to the dorsal surface of the head, the neck and the dorsum show a light brown background color with irregularly disposed dark brown spots, roundish in BMNH 1946.8.24.41 or more transversely elongate in BMNH 1946.8.24.40 (Figures 4–5); the remaining part of the tail of BMNH 1946.8.24.40 shows more or less regularly disposed dark and light rings. Poorly contrasted dorsal spots and well contrasted tail rings are visible in BMNH 97.3.31.1. A discontinuous series of white tubercles is perceptible on the upper flanks and from nape to tail in BMNH 97.3.31.1–2 and, with less contrast, in BMNH 1946.8.24.40; these white tubercles are not visible in BMNH 1946.8.24.41. Dorsal surfaces of members light brown with dark brown spots, well visible in BMNH 1946.8.24.40–41, poorly contrasted in BMNH 97.3.31.1 and absent or no longer visible in BMNH 97.3.31.2. The tail of BMNH 97.3.31.1, complete and original, shows ten light rights irregularly disposed along the entire length of the tail, the anterior ones poorly contrasted. Infralabials colored as supralabials. Throat of BMNH 1946.8.24.40–41, which is more contrasted than in the two other specimens, light brown, mottled with dark brown. Ventral surfaces of body, members and tail light brown, punctuated with dark brown dots in BMNH 1946.8.24.40–41 (dots not visible in BMNH 97.3.31.1–2). Flower (1899a: 628) gave a brief description of the coloration in life of the types: ‘‘Above brown, spotted very strongly with black. Below grey, mottled with purple. Underneath of head brown. Labials marked with dark purplish brown and pale yellowish brown.’’ It is, however, not clear on which specimens Flower’s description is based. Cranial osteology. Based on the subadult female BMNH 1946.8.24.40. Snout and palatomaxillary bones.— The premaxilla is single (Figure 7A–B). The alveolar shelf bears 11 pleurodont teeth. The posterior margin of the alveolar shelf projects out caudally into two triangular palatal processes. The ascending process runs posterodorsally and articulates with the nasals. Nasals are paired and parallelogram-shaped, with an anterior process from the anteromedial end and a posterior process jutting out from the posterolateral end (Figure 7A). Though there is a slight embayment medial to the anterior process, there is no anterolateral process as seen in some gekkonids (Evans 2008; Villa et al. 2018). The nasal articulates with the maxillary facial process late
DOI: 10.5281/zenodo.14508344
Language: English
Open Access status: green
OpenAlex ID: W6930786785
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