A new species of the genus Trimeresurus Lacépède, 1804 (Squamata, Viperidae) from western Sichuan Province, China

The genus Trimeresurus Lacépède comprises over 57 species, with recent taxonomic revisions recognizing six subgenera. Despite extensive research, many regions in China remain understudied, particularly the biodiversity-rich West China Rain Zone. This study reports a new pitviper species from western Sichuan Province, China, based on comprehensive morphological and molecular analyses. Specimens collected from the West China Rain Zone were examined using detailed morphological comparisons and phylogenetic analyses of three mitochondrial genes ( 16S , cytb , and ND4 ). The results show that this population, together with T. loong , forms a distinct monophyletic clade within the subgenus Viridovipera Malhotra & Thorpe, with uncorrected genetic distances to its closest relatives exceeding 6.50% ( cytb ) and 5.77% ( ND4 ). Diagnostic morphological differences, including smooth cephalic scales, a postocular streak extending beyond the 2 nd supralabial in males, dorsal scale rows of 21–21–15, and a grass-green body color lacking bands or markings, further support its taxonomic distinctness. We propose the new species Trimeresurus lii sp. nov ., which represents the second species of Viridovipera in Sichuan Province and the 58 th species of Trimeresurus worldwide, and contributes to understanding biodiversity in the Giant Panda National Park region. This discovery highlights the importance of continued field surveys in biodiversity hotspots for documenting and conserving unique faunal assemblages.