Sampled localities of the Oligodon cinereus species complex used in this study, including Oligodon speleoserpens sp. nov., O. huahin, O. phangan, O. promsombuti, O. purpurascens, O. saiyok, O. joynsoni, O. inornatus, O. nagao, O. cinereus s. str., and unnamed lineages of O. “cinereus” labeled by clade. Base map created using simplemappr.net. Filled icons denote populations included in phylogenetic analyses; empty icons denote populations not included in the phylogenetic analyses; type localities are indicated by a dot at the center of an icon. Colors and locality numbers correspond to Figure 2 and Table S1. Localities. Oligodon speleoserpens sp. nov.: (1) Tham Khao Ting Cave, Trang Prov., and Tham Le Stegodon Cave, Satun Prov., Thailand; Oligodon sp.: (2) Krabi Prov., Thailand (unconfirmed locality); O. saiyok: (3) Sai Yok Distr., Kanchanaburi Prov., Thailand; O. cinereus clade 4: (4) Rakhine Yoma Mts., Rakhine St., Myanmar; O. cinereus clade 1 (sensu stricto): (5) Cat Tien NP., Dong Nai Prov., Vietnam (type locality of O. cattienensis); (6) Bu Gia Map NP, Binh Phuoc Prov., Vietnam; (7) Kirirom NP, Kampong Speu Prov., Cambodia; O. cinereus clade 2: (8) Kalaw, Shan St., Myanmar; (9) Alaungdaw Kathapa NP, Sagaing Div., Myanmar; (10) Chi Linh, Hai Duong Prov., Vietnam; (11) Tam Dao NP, Vinh Phuc Prov., Vietnam; (12) Pu Mat NP, Nghe An Prov., Vietnam; O. phangan: (13) Mingalardon, Hlawga WP, Yangon Div., Myanmar; (14) Ko Phi Phi Don Isl., Krabi Prov., Thailand; (15) Ko Pha Ngan Isl., Surat Thani Prov., Thailand; O. huahin: (16) Kaeng Krachan NP, Phetchaburi, Thailand; (17) Suan Phueng, Ratchaburi, Thailand; O. nagao and O. cinereus clade 3: (18) Hainan Isl., Hainan Prov., China; (19) Nonggang NR, Guangxi Prov., China; (20) Huu Lien, Lang Son Prov., Vietnam (type locality of O. nagao); O. joynsoni: (21) Ngao Dist., Lampang Prov., Thailand (type locality); (22) Doi Tung, Chiang Rai Prov., Thailand; (23) Xishuanhbanna NP, Yunnan Prov., China; O. promsombuti: (24) Khao Phanom Wang, Surat Thani Prov., Thailand; O. inornatus: (25) Nong Kai Ploi, Chonburi Prov., Thailand (type locality); (26) Samkos WS, Pursat Prov., Cambodia.

Phylogenetic tree of Oligodon from the analysis of 12S rRNA–16S rRNA and cyt b mitochondrial DNA gene sequences. For voucher specimen information and GenBank accession numbers see Table S1. Numbers at tree nodes correspond to ML UFBS / BI PP support values, respectively; an en-dash denotes no support. Colors of clades and locality numbers correspond to those in Figure 1.

Photographs of the holotype of Oligodon speleoserpens sp. nov. (ZMMU Re-17696, adult male) (a) dorsal and (b) ventral views in life. Photographs taken by Parinya Pawangkhanant.

Photographs of the holotype of Oligodon speleoserpens sp. nov. (ZMMU Re-17696, adult male) (a) right lateral, (b) dorsal and (c) ventral views of the head in life; (d) partially everted hemipenes; (e) venter at midbody, note the distinctly keeled ventral scales; and (f) underside of tail depicting a dark gray and white color pattern. Photographs by Parinya Pawangkhanant (a–c and e–f), and Mali Naiduangchan (d).

Photographs (a) of the holotype of Oligodon speleoserpens sp. nov. (ZMMU Re-17696, adult male) in life; (b) habitat at the type locality, Tham Le Stegodon cave, Satun Province, Thailand; (c) paratype of Oligodon speleoserpens sp. nov. (ZMMU Re-17697, adult female) in life from Tham Khao Ting cave, Trang Province, Thailand; and (d) uncollected adult male specimen of Oligodon speleoserpens sp. nov. from the same locality as the paratype. Photos by Parinya Pawangkhanant (a, b), Nikolay A. Poyarkov (c), and Harry Ward–Smith (d).

Habitat photographs (a) of the karst wall above an entrance to Tham Khao Ting Cave, Trang Province, Thailand that the uncollected specimen of Oligodon speleoserpens sp. nov. was found ~12–15 m high; (b) flooded exit of the Tham Khao Ting cave; and (c) flooded habitat 50 m past the entrance of Tham Le Stegodon cave, Satun Province, Thailand. Photographs by Harry Ward–Smith.