A New Green Pitviper of the Genus Trimeresurus Lacépède, 1804 (Squamata: Viperidae) from Xizang, China

Species diversity within the green pit vipers of the genus <i>Trimeresurus</i> is likely underestimated. In this study, we describe a new species of <i>Trimeresurus</i> from the Xizang Autonomous Region, China, based on both morphological and molecular evidence. The new species, <i>Trimeresurus pretiosus</i> sp. nov., is assigned to the subgenus <i>Viridovipera</i> Malhotra Thorpe. It is distinguished from its congeners by the following combination of morphological characters: (1) first supralabial completely separated from nasal scale; (2) hemipenis short and spinose, reaching the 12th subcaudal when fully everted; (3) small adult size, with a maximum known snout-vent length of 516 mm in males and 512 mm in females; (4) dorsal scales in 19-19-15 rows, weakly keeled except for the outermost rows; (5) 140-143 ventral scales; (6) 56-58 subcaudal scales in males and 54 in females, partially arranged in a single row; (7) iris reddish-brown in males, orange-yellow in females; (8) body uniformly bright grass-green; postocular streak absent or faint white in males, absent in females; (9) ventrolateral stripe consisting of red above and white below and wide in males, only white and narrow in females; (10) ventral surfaces greenish-yellow; (11) tail distinctly reddish-brown dorsally, with the colouration extending from tail base to tip. Molecular analyses based on the mitochondrial <i>16S</i>, cyt <i>b</i>, and <i>ND4</i> gene fragments indicate that the new species is genetically divergent from all congeners, with uncorrected p-distances ranging from 5.8% to 12.5% in the cyt <i>b</i> gene and from 6.7% to 11.0% in the <i>ND4</i> gene. The new species is currently known only from the southern slopes of the central Himalayas in Yadong County, and it represents the second known <i>Viridovipera</i> species from the southern Himalayas. With this discovery, the total number of snake species known from the Xizang Autonomous Region increases to 67, underscoring the importance of continued herpetofaunal surveys in this biogeographically complex region.