Dendrelaphis thasuni, Atthanagoda & Silva & Vogel & Udayanga & Bandara & Madawala & Grismer & Karunarathna, 2025

Dendrelaphis thasuni sp. nov.

Thasun’s Bronzeback urn:lsid:zoobank.org:act:

Holotype. An adult female (NMSL 2025.01.01), SVL 462 mm, TAL 237 mm, collected from Maragala mountain, Monaragala District , Uva Province, Sri Lanka ( 6°53’26.12” N, 81°22’52.15” E, WGS1984; elevation 741 m asl, around 1600 hrs) on 25 October 2024 by Suranjan Karunarathna and Anusha Atthanagoda. GoogleMaps

Diagnosis. The head is distinct from the slender body; two internasals, snout broad and flat, equal to eye diameter; large eye with round pupil; one preocular and one loreal; two postoculars; 1+2 temporal scales bearing a very narrow black stripe (Figs. 4 and 5); V-shaped blackish transverse dorsolateral bars on anterior half of body; distance between transverse bars in forebody is equal to the eye diameter, with 2–3 lateral scales between two transverse dorsolateral bars; 151 enlarged vertebral scales and no vertebral stripe; dorsal scale rows one head length behind neck (~ 20 mm) 13, at midbody 13, one head length anterior to vent (~ 20 mm) 9; apical pits 1–3 on dorsal scales; 153 ventral scales, very narrow between lateral keels of both sides compared to D. caudolineolatus ; anal plate undivided, large; tail long with 117 or 118 divided subcaudals; a thin blackish line between subcaudals; subcaudals with strong lateral keels. Dendrelaphis thasuni sp. nov. is most similar to D. caudolineolatus in Sri Lanka , which is its presumed closest relative. It can be easily distinguished from all species of Sri Lankan Dendrelaphis by having an undivided anal plate (Fig. 6).

Description of holotype. Adult female, SVL 462.0 mm ( Table 3); tail length 237.0 mm, nearly half of the body length (TAL 51.3% of SVL; TAL 33.9% of total length); head elongate (HL 4.2% of SVL), twice as long as wide (HW 45.4% of HL), slightly flattened, distinct from neck; snout short (ES 24.5% of HL), moderate, flattened in dorsal and lateral profiles, being square in shape, rather depressed (HD 33.7% of HL). Rostral shield large, triangular, slightly visible from above, flattened posteriorly, width greater than height (RD 44.1% of RW), with rostral groove; nostrils relatively large; nasal divided, elongate, in anterior contact with rostral and internasal dorsally, 1 st and 2 nd supralabials ventrally, posterior contact with loreal and prefrontal; internasals paired, quadrangular, slightly shorter than prefrontals; equivalent in size to loreal; prefrontal large, broader than long, and subrectangular; frontal large, subtriangular, elongate posteriorly and longer than wide, equal in size to supraocular; interorbital broad (IO 82.0% of HW), twice that of internarial distance (IN 53.4% of IO); supraoculars wide, elongate, subrectangular, posteriorly wider and pointed, equal to frontal length, equal to total length of internasal and prefrontal.

Parietals large, subrectangular in shape, elongate, shorter than snout length (PL 91.7% of SN), bordered by frontal, supraoculars, upper postocular anteriorly, anterior and posterior temporals, and five nuchal scales posteriorly; one rectangular loreal scale (each side), in contact with prefrontal dorsally, 2 nd and 3 rd supralabials ventrally, preocular posteriorly and nasals anteriorly; one preocular (each side), vertically elongate, rectangular, in contact with prefrontal and loreal anteriorly, supraocular dorsally, and 4 th supralabial ventrally; eye large (ED 21.9% of HL), greater than eye–nostril length (ED 113.2% of EN), less than snout length (ED 89.6% of ES), pupil round; two postoculars, upper postocular larger, rectangular, vertically elongate, in contact with supraocular and parietal, in point contact with anterior temporal; lower postocular rectangular in contact with 5 th and 6 th supralabials ventrally, anterior temporal posteriorly; temporals 1+2, anterior temporal elongate, broadly in contact with parietal dorsally, broadly in contact with 6 th –8 th supralabials ventrally; posterior temporals short, quadrangular and similar in size, in contact with parietal dorsally, three nuchal scales posteriorly, 8 th supralabial ventrally.

Supralabials eight (each side), 4 th –8 th larger in size, 7 th being the largest; 1 st supralabial in contact with rostral anteriorly, both nasals dorsally and 2 nd supralabial posteriorly; 2 nd supralabial in contact with 1 st supralabial anteriorly, loreal and postnasal dorsally, 3 rd supralabial posteriorly; 3 rd supralabial with loreal and preocular dorsally, 4 th supralabial posteriorly; 4 th supralabial with preocular and orbit dorsally, 5 th supralabial posteriorly; 5 th supralabial with orbit and postocular dorsally; 6 th supralabial with lower postocular and anterior temporal dorsally; 7 th supralabial with anterior temporal dorsally; 8 th supralabial with anterior and posterior temporals dorsally, body scales posteriorly. Mental small, triangular, wider than long, with a very small mental groove; 1 st infralabials larger than mental and in broad medial contact, in contact with anterior chin shields posteriorly; nine infralabials (each side), 1 st –4 th in contact with anterior chin shield, 5 th infralabial in broad contact with posterior chin shield, 6 th infralabial not in contact with posterior chin shield or gular scales; two smaller anterior chin shields in broad contact, and two elongate posterior chin shields in broad contact; posterior chin shields bordered posteriorly by eight gular scales.

Body thin, elongate and subcylindrical, somewhat dorsolaterally flattened; 151 fairly large vertebrals; dorsal scale rows 13-13-9, smooth, elongate and bluntly pointed; anterior dorsals with 1–3 apical pits at the tip, slightly shorter than vertebrals; two preventral scales, 153 laterally keeled narrow ventrals (width of a ventral scale between keels ~160% of its length); preanal plate undivided, anal plate large, undivided. Tail long (TAL 51.3% of SVL), thin and thick, dorsal caudals smooth, 118/117 paired subcaudals excluding the terminal scale, strongly keeled laterally.

Coloration in life. General dorsal color is brownish (Fig. 7), but the forebody has dark patches, interstitial skin bright blue, visible when inflated (Fig. 5); 26–29 prominent V-shaped blackish transverse dorsolateral bars on the anterior one-fourth of body, afterwards fading gradually; absence of a vertebral stripe and of ventrolateral stripes on the forebody. A faded black line on dorsal part of the tail, and one on both sides of the tail that ends at the tip, thin blackish mid-ventral subcaudal stripe. Top of head grey-brown with faded dark makings, especially at the internasals and parietals, cream white ventrally with irregular dark markings; a narrow black stripe (mostly between lower temporals, 7 th and 8 th supralabials) extending from lower postocular to the 8 th supralabial, after which it disappears; 1 st –3 rd supralabials light brown, followed by scales dusted with light orange. Both sides of the neck are orange in color, and the tongue is bluish-black with tiny light blue dots.

Coloration in preservative. Dorsally greyish brown; forebody with dark patches, and dark ‘V’ shaped transverse dorsolateral bars on anterior half of body clearly visible; dorsal head olive green; a narrow dark stripe starting from lower postocular reaches the 8 th supralabial. Ventrals, subcaudals, and supralabials dirty white with a thin blackish mid-ventral subcaudal stripe.

Etymology. The specific epithet is a Latinized eponym in the masculine genitive singular, honoring Dr. Amarasinghe Achchige Thasun Amarasinghe—a renowned systematic biologist, ecologist, and conservationist—for his remarkable contributions to the field of taxonomy and systematic herpetology in Asia, especially in Sri Lanka , Indonesia, and India, and for his friendship. He has also helped to popularize wildlife conservation and management in Sri Lanka through science-based education awareness programs.

Comparison. The new species, Dendrelaphis thasuni sp. nov., is most similar to D. caudolineolatus (Figs. 3 and 8), however, it is distinguished from the latter by having an undivided anal plate ( vs divided), ventral scales very narrow between keels ( vs wider), snout broad and flat ( vs narrow and pointed), a narrow temporal stripe ( vs broad), distance between lateral transverse bars in forebody is equal to the eye diameter ( vs larger), 2–3 lateral scales between two transverse dorsolateral bars ( vs 5–6), rostral much wider than mental ( vs subequal), and anterior chin shield very small and its length about 50% of posterior chin shield ( vs large and 83–91% of posterior chin shield). Dendrelaphis thasuni sp. nov. is distinguished from all the species of the genus Dendrelaphis except for D. proarchos ( Wall, 1909) and D. walli Vogel and van Rooijen, 2011 ( Table 1) by having an undivided anal plate ( vs. divided anal plate); it is distinguished from D. proarchos by having costals in 13 rows at mid body ( vs 15), 153 ventrals ( vs 173–198), and 117 or 118 subcaudals ( vs 138–156); from D. walli by having 153 ventrals ( vs 203–213) and 117 or 118 subcaudals ( vs 145–163). Dendrelaphis thasuni sp. nov. further differs from the other Sri Lankan species as follows: from D. bifrenalis by having one loreal scale ( vs two), costals in 13 rows at mid body ( vs 15), eight supralabials with 4 th and 5 th touching the eye (nine supralabials with 5 th and 6 th touching the eye) and 117 or 118 subcaudals ( vs 137–166); from D. effrenis by having one loreal scale ( vs no loreal), temporal stripe present ( vs absent), undivided anal plate ( vs. divided), 153 ventrals ( vs 174–175), and 117 or 118 subcaudals ( vs 129–139); from D. oliveri by having one loreal scale ( vs no loreal), eight supralabials with 4 th and 5 th touching the eye (nine supralabials with 5 th and 6 th touching the eye), costals in 13 rows at mid body ( vs 15), 153 ventrals ( vs 173), 117 or 118 subcaudals ( vs 134), and undivided anal plate ( vs. divided); from D. schokari by having costals in 13 rows at mid body ( vs 15), undivided anal plate ( vs. divided), both dorsal and ventrolateral stripes absent on the body ( vs distinct dorsal and ventrolateral stripes present), and eight supralabials with 4 th and 5 th touching the eye (nine supralabials with 5 th and 6 th touching the eye); from D. tristis by having costals in 13 rows at mid body ( vs 15), undivided anal plate ( vs. divided), 153 ventrals ( vs 178–198), eight supralabials with 4 th and 5 th touching the eye (nine supralabials with 5 th and 6 th touching the eye), no interparietal bright spot ( vs present), both dorsal and ventrolateral stripes absent on the body ( vs distinct dorsal and ventrolateral stripes present); from D. wickrorum by having one loreal scale ( vs two), costals in 13 rows at mid body ( vs 15), 153 ventrals ( vs 162–174), and 117 or 118 subcaudals ( vs 137–157), undivided anal plate ( vs divided), and eight supralabials with 4 th and 5 th touching the eye (nine supralabials with 5 th and 6 th touching the eye).

Habitat and ecology. In the Maragala isolated mountain ranges (6.842525°– 6.922392° N 81.336822°– 81.413719° E; altitude ranges from 350–850 m), the upper elevations are characterized by dry tropical mixed semi-evergreen forest ( Gunatilleke and Gunatilleke 1990) and the lower elevations contain mixed cultivation with anthropogenic habitats of tea, coffee, cocoa, and rubber plantations (Fig. 9). It is approximately 4,100 ha in size, situated in Monaragala District, Uva Province, within the lowland intermediatebioclimaticzoneinSriLanka ( Karunarathna et al. 2019). The mean annual rainfall varies, ranging from 1,500 –2,500 mm, which comes mostly during the northeast monsoon season (November–February). The mean annual temperature is 26.8–28.9 °C. The new species was found near the summit at 741 m asl, at approximately 1600 hrs. The microhabitat was riparian, near a stream with closed canopy cover. The surrounding environment at the top of the mountain was undisturbed with no human habitation, but it had the characteristic of being a very cool lowland wet zone forest ( Herath and Patabandi 2023). This snake was adept at moving fast

Fig. 9. View of Maragala Mountain in Monaragala, Sri Lanka , taken (A) from Kotiyagala site and (B) from Kahambana site. Photos by Ashan Geeganage and Nayana Sanjeewa.

through the trees, but its movement shifted awkwardly when on the forest floor. When inflated, the bright blue interstitial skin appeared on the anterior one-fourth of the body (Fig. 6), and the expanded V-shaped, blackish, transverse, dorsolateral bars could be seen.