Integrative taxonomy reveals a new unstriped Ichthyophis Fitzinger, 1826 from Vietnam and provides new data on diagnostic osteological traits for Asian tailed caecilians (Gymnophiona: Ichthyophiidae)

405 Integrative taxonomy reveals a new unstriped Ichthyophis Fitzinger, 1826 from Vietnam and provides new data on diagnostic osteological traits for Asian tailed caecilians (Gymnophiona: Ichthyophiidae) Nikolay A. Poyarkov1,2, Dana D. Skorinova3, Andrey M. Bragin2, Veniamin V. Kolchanov3, Vladislav A. Gorin1, Alexey V. Trofimets1, Alexander P. Yuzefovich1, Dac Xuan Le2, Tan Van Nguyen4,5, Pavel P. Skutschas3 1 Department of Vertebrate Zoology, Lomonosov Moscow State University, Leninskiye Gory, GSP–1, Moscow 119991, Russia 2 Joint Vietnam – Russia Tropical Science and Technology Research Center, Nghia Do, Hanoi 122000, Vietnam 3 Department of Vertebrate Zoology, Faculty of Biology, Saint Petersburg State University, Universitetskaya nab. 7/9, Saint-Petersburg 199034, Russia 4 The School of Medicine & Pharmacy, Duy Tan University, Da Nang, 550000, Vietnam 5 Center for Entomology & Parasitology Research, Duy Tan University, Da Nang, 550000, Vietnam https://zoobank.org/9EBD5402-6D5A-4C0E-B2AE-03EBB05E2E20 Corresponding authors: Nikolay A. Poyarkov ([email protected]); Pavel P. Skutschas ([email protected]) Academic editor Deepak Veerappan | Received 27 February 2025 | Accepted 2 October 2025 | Published 29 October 2025 Citation: Poyarkov NA, Skorinova DD, Bragin AM, Kolchanov VV, Gorin VA, Trofimets AV, Yuzefovich AP, Le DX, Nguyen TV, Skutschas PP (2025) Integrative taxonomy reveals a new unstriped Ichthyophis Fitzinger, 1826 from Vietnam and provides new data on diagnostic osteological traits for Asian tailed caecilians (Gymnophiona: Ichthyophiidae). Vertebrate Zoology 75: 405–440. https://doi.org/ 10.3897/vz.75.e149399 Abstract Herein we examined the cranial osteology of 15 species of Ichthyophis (I. asplenius, I. beddomei, I. glutinosus, I. kohtaoensis, I. larutensis, I. mindanaoensis, I. multicolor, I. nguyenorum, I. nigroflavus, I. sikkimensis, I. singaporensis, I. supachaii, I. tricolor, I. weberi, and Ichthyophis sp. from northern Vietnam) with a special emphasis on the temporal region. We presented the first detailed description of the cranium and the atlas of an Ichthyophis species based on micro-CT scanning data. We discuss the implications of temporal region composition for the systematics of this group and the evolution of the cranium in Gymnophiona as a whole. We further provided comments on a jaw-closing mechanism and reported on the presence of phylogenetically basal cranial features in ichthyophiids that are also found in stem caecilians. Our detailed morphological description was based on a specimen from a previously unknown population of unstriped Ichthyophis from northern Vietnam. We consequently described this population as a new species based on morphological and molecular (3967 bp from cyt b, 12S rRNA, and 16S rRNA mitochondrial DNA genes) lines of evidence. We provide comparisons of external morphological traits of the new species with its congeners and further compare its cranial osteological features with other Ichthyophis for which skull descriptions exist. The new species differs from the morphologically similar species I. yangi and I. chaloensis by a significant divergence in cyt b and 16S rRNA mitochondrial DNA gene sequences (p = 6.5%–6.9% and p = 4.5%, respectively). The new species is currently known only from evergreen forests of Xuan Lien National Park (Thanh Hoa Province) and Pu Hoat (Nghe An Province) Nature Reserve, northern Vietnam, and was recorded at elevations of 700–800 m asl. We suggest the new species be considered Data Deficient (DD), following the IUCN’s Red List categories. Keywords Ichthyophis griseivermis sp. nov., micro-CT scanning, molecular analyses, morphology, Nghe An, osteology, Pu Hoat, taxonomy, Thanh Hoa, Xuan Lien Vertebrate Zoology 75, 2025, 405–440 | DOI 10.3897/vz.75.e149399 Copyright Nikolay A. Poyarkov et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Poyarkov N et al.: New unstriped Ichthyophis and osteological traits for Asian tailed caecilians 406 Introduction Caecilians (Gymnophiona) have a worm-like body plan lacking limbs and limb girdles; most species (at least as adults) are adapted to burrowing underground, though some members of this group have aquatic or semi-aquatic lifestyles or occasionally can be observed actively moving on the surface (e.g., Taylor 1968; Wilkinson 2012). Sharply different from all other extant amphibians, adult caecilians are characterized by a heavily ossified cranium and lower jaw, with homologies of some of its elements with those of other amphibian orders uncertain (e.g., Wake 2003; Carroll 2007; Vitt and Caldwell 2014; Palakkool et al. 2022). Osteological and especially cranial characters have been used in phylogenetic studies of the Gynmophiona (e.g., Nussbaum 1979; Wilkinson and Nussbaum 1999; Maddin et al. 2012; Wilkinson et al. 2011, 2014), though detailed descriptions of skull morphology are available only for a few taxa. Traditional studies of caecilian skull morphology were mostly based on descriptions of skeletons obtained by destructive methods such as maceration and drying, clearing and staining, and histology, which do not allow the subsequent examination of the external morphology of the specimen (e.g., Ramaswami 1941, 1942; Taylor 1969a; Wake 1980; Schmidt and Wake 1990; Müller et al. 2005). The recent development of a non-destructive, high-resolution X-ray microcomputed tomography (micro-CT) has facilitated exploration of caecilian osteology, in particular obtaining detailed information on the skeletal anatomy of museum specimens, including some precious name-bearing types (e.g., Wilkinson et al. 2011, 2014; Maddin et al. 2012; Sherratt et al. 2014). These studies are nevertheless limited by a relatively small number of caecilian specimens available in herpetological collections around the world. Therefore, the intraand interspecific variation in shape, fenestration, and composition of skull elements in caecilians remains insufficiently understood (Taylor 1969a; Nussbaum 1977; Wake 2003; Carroll 2007; Sherratt et al. 2014; Wilkinson et al. 2014; Bardua et al. 2019; Palakkool et al. 2022). The Asian tailed caecilians of the family Ichthyophiidae Taylor, 1968, comprise terrestrial limbless amphibians with aquatic larvae that are widely distributed across South and Southeast Asia from Sri Lanka and India through southern China, mainland Indochina, and Southeast Asian islands west of Wallace’s Line (Nishikawa et al. 2012a; Geissler et al. 2015; Frost 2025). The highly incomplete knowledge of ichthyophiid taxonomy, distribution, and phylogeny in Southeast Asia is explained by their secretive fossorial life history, resulting in mostly small sample sizes available in herpetological collections around the world (Geissler et al. 2015; Rao et al. 2024), as well as by relatively low interspecific morphological variability in traditional taxonomic characters (Gower et al. 2002; Nishikawa et al. 2012b, 2021). Furthermore, in the Ichthyophiidae, as in almost all other caecilian families, the barely known intraspecific variability of external traits, due to the often insufficient sample sizes, further complicates taxonomic decisions; only a few studies were able to examine a sufficient number of Ichthyophiidae specimens (e.g., Nussbaum and Gans 1980; Kupfer 2005). Several recent studies on the taxonomy and molecular phylogeny of ichthyophiids have demonstrated that the diversity of the family is substantially underestimated (Nishikawa et al. 2012a), with several new species described in recent years (e.g., Wilkinson et al. 2014; Geissler et al. 2015; Lalremsanga et al. 2021a; Rao et al. 2024). Ichthyophiids represent an ancient lineage of extant caecilians, forming a sister group to all other families except the Rhinatrematidae Nussbaum, 1977 (Kamei et al. 2012). Therefore, this family is of particular importance for reconstructing the evolution of the caecilian skeleton, including the skull. Although Ichthyophiidae, with 58 currently recognized species, is the most speciose family of caecilians (Frost 2025), current knowledge of their skeletal morphology remains scarce. It is widely accepted that the crania of ichthyophiids are characterized by a combination of primitive and derived character states that are intermediate between those of the Rhinatrematidae and all other ‘higher’ caecilians, or Teresomata Wilkinson and Nussbaum, 2006 (Nussbaum 1977, 1979, 1983; Duellman and Trueb 1986; Wilkinson and Nussbaum 1996, 2006; Wake 2003). Compared to some more specialized burrowing caecilians, ichthyophiids are more often observed moving on the surface of the ground or in loose leaf litter (e.g., Kupfer et al. 2005) while some species are likely associated with streams (Geissler et al. 2015). All known Ichthyophiidae have a biphasic reproductive cycle with aquatic larvae hatching from eggs laid in burrows or underground chambers near the water (Duellman and Trueb 1986; Wilkinson and Nussbaum 2006). The skulls of the ichthyophiids studied thus far have more ossified elements than any other caecilian family and usually exhibit a compact well-ossified stegokrotaphic morphology (Taylor 1969a; Duellman and Trueb 1986; Wake 2003; Gower et al. 2010), though Nussbaum (1977) noted that some ichthyophiids have a ‘weakly stegokrotaphic’ skull morphology with comparatively large upper temporal fossae (e.g., Wilkinson et al. 2014; Bardua et al. 2019). Current knowledge of Ichthyophiidae osteology, cranial morphology and head musculature and innervation is based mostly on studies of Uraeotyphlus narayani Seshachar, 1939 (Ramaswami 1941; Nussbaum 1979) and several species of the genus Ichthyophis Fitzinger, 1826 (e.g., Müller 1835; Wiedersheim 1879; Sarasin and Sarasin 1887–1890; Burckhardt 1891; Peter 1898; Visser 1963; Taylor 1969a; Nussbaum 1977; Dünker et al. 2004; Kleinteich and Haas 2007; Kleinteich et al. 2008), but only a few of them were based on micro-CT data (Kleinteich et al. 2012; Wilkinson et al. 2014; Gower et al. 2017; Lowie et al. 2022; McGrath-Blaser et al. 2025; Santos et al. 2025). The most speciose genus of Ichthyophiidae is Ich thyophis, which currently comprises 50 nominal species, 12 of which occur in Indochina (including Vietnam, Cambodia, Vertebrate Zoology 75, 2025, 405–440 407 Laos, and Thailand) and southern China (Frost 2025). Ichthyophis species can be partitioned into two major coloration types: striped species with a pair of light-colored (yellow or cream) lateral stripes running from the head along the body flanks, and unstriped species with uniform brown, grayish, or blackish coloration (Taylor 1968; Geissler et al. 2015). Although it was demonstrated that the striped and unstriped species of Ichthyophis do not form monophyletic groups, and coloration types likely evolved several times independently in this genus (e.g., Gower et al. 2002; Nishikawa et al. 2012a), the presence or absence of lateral stripes remained a interspecifically diagnostic tool important for species identification in Ichthyophis (e.g., Taylor 1968; Geissler et al. 2015), though a substantial variation in the degree of lateral stripe development was reported for some species (Nussbaum and Gans 1980). Among the 12 species of Ichthyophis known to occur in Indochina and southern China, seven species are unstriped, i.e., Ichthyophis acuminatus Taylor, 1960; I. cardamomensis Geissler et al., 2015; I. catlocensis Geissler et al., 2015; I. chaloensis Geissler et al., 2015; I. laosensis Taylor, 1969; I. youngorum Taylor, 1960; and I. yangi Rao et al., 2024. It is noteworthy that most of these species are known from single or very few specimens, and four of them were described within the last decade based on concordant evidence from molecular and morphological data (Geissler et al. 2015; Rao et al. 2024). Presently, four named Ichthyophis species are recorded from Vietnam: I. catlocensis, I. chaloensis, I. kohtaoensis Taylor, 1960 (including I. bannanicus as its junior synonym following Nishikawa et al. 2021), and I. nguyenorum Nishikawa, Matsui & Orlov, 2012 (Poyarkov et al. 2021a; Frost 2025). During our recent field surveys in the evergreen montane forests of Xuan Lien National Park in Thanh Hoa Province and Pu Hoat Nature Reserve in Nghe An Province in northern Vietnam, we encountered two unstriped caecilians specimens assigned to the genus Ichthyophis on the basis of their having a combination of a tertiary annular system and tentacle apertures distant from the eye (Wilkinson and Nussbaum 2006; Wilkinson et al. 2014). Subsequent phylogenetic analyses based on three mitochondrial DNA (hereafter, mtDNA) genes (cyt b, 12S rRNA, and 16S rRNA) confirmed its placement within the genus Ichthyophis and suggested that the Xuan Lien and Pu Hoat specimens represent a divergent lineage closely related to the recently described I. yangi from Yunnan Province, China, and I. chaloensis from central Vietnam. Closer examination of the external morphology, coloration, and osteological characteristics of the Xuan Lien and Pu Hoat specimens demonstrated clear differences from the other unstriped Ichthyophis of Indochina as well as from all other congeners. Integrating morphological and molecular data, we describe the Xuan Lien and Pu Hoat specimens of ichthyophiid caecilians from northern Vietnam as a new species. Furthermore, here we present the first detailed description of the cranial anatomy of Ichthyophis sp. from northern Vietnam based on newly generated micro-CT data allowing comparisons with available data for other members of Ichthyophiidae. Material and Methods Sampling Fieldwork in Pu Hoat Nature Reserve in Nghe An Province, Vietnam, was conducted by N. A. Poyarkov and D. X. Le in May 2019, and in Xuan Lien National Park in Thanh Hoa Province, Vietnam, by A. M. Bragin, A. P. Yuzefovich, and D. X. Le in October 2023 (see Fig. 1). Permissions to conduct fieldwork and collect specimens were granted by the Department of Forestry, Ministry of Agriculture and Rural Development of Vietnam, and by the Forest Protection Departments of the People’s Committees of Nghe An Province (permit number #2089/UBND.VX of 03.04.2019) and Thanh Hoa Province (permit numbers #562/GP of 01.06.2022 and #179/SNN&PTNT-CCKL of 05.10.2023) of Vietnam. Geographic coordinates and altitude were obtained with a Garmin GPSMAP 60CSx GPS receiver (Garmin Ltd, USA) and recorded in datum WGS 84. A living specimen was photographed and subsequently euthanized by a 20% benzocaine solution; tissue samples (liver) were taken for genetic analyses and stored in 96% ethanol prior to specimen fixation in a 4% formaldehyde solution for 24 h with subsequent preservation in 70% ethanol. Collected specimens were deposited in the herpetological collections of the Zoological Museum of Moscow State University (ZMMU), Moscow, Russia, and of the Joint Vietnam-Russia Tropical Science and Technology Research Center (VRTC), Hanoi, Vietnam. Additionally, we employed published morphological and molecular data from the material deposited in the following herpetological collections: AMNH: American Museum of Natural History, New York, New York, USA; CAS: California Academy of Sciences Museum, San Francisco, California, USA; CBC: Centre for Biodiversity Conservation at the Royal University of Phnom Penh, Phnom Penh, Cambodia; FMNH: Field Museum, Chicago, Illinois, USA; IEBR: Institute of Ecology and Biological Resources, Vietnam Academy of Sciences and Technology, Hanoi, Vietnam; KIZ: Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, Yunnan, China; KUHE: Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, Japan; LSUHC: La Sierra University Herpetological Collection, Riverside, California, USA; MNHN: Museum National d’Histoire Naturelle, Paris, France; MZB: Museum Zoologicum Bogoriense, Java, Indonesia; MZMU: Museum of the Zoology Department, Mizoram University, Mizoram, India; NCSM: North Carolina Museum of Natural Sciences, North Carolina, USA; NHMUK: Natural History Museum, United Kingdom (formerly BMNH), London, UK; ZFMK: Zoologisches Forschungsinstitut und Museum Alexander Koenig, Bonn, Germany; ZISP: Zoological Institute, Russian Academy of Sciences, St. Petersburg, Russia. Micro-computed tomography To study the skeletal morphology of Ichthyophis sp., we examined X-ray projections for two collected specimens Poyarkov N et al.: New unstriped Ichthyophis and osteological traits for Asian tailed caecilians 408 (ZMMU A-8208 and VRTC NAP08953) and performed detailed micro-CT scanning of the cranium and atlas for the better-preserved specimen (ZMMU A-8208). The scans were obtained with a SkyScan 1272 microtomograph (Bruker, Billerica, USA) equipped with a Hamamatsu L10101-67 source (Hamamatsu Photonics, Hamamatsu, Japan) and a Ximea xiRAY16 camera (Ximea GmbH, Münster, Germany) at the Biological Faculty of Moscow University. The specimens were scanned at a source voltage of 70 kV and a source current of 135 μA, without an X-ray filter. The samples were rotated 360° with a rotation step of 0.1, with 3 frames averaged per step. A stack of virtual cross sections through the specimens’ skeletal structures (3170 images of 2106×2887 resolution and a pixel size of 5.25 μm) was reconstructed with the software NRecon (Bruker micro-CT, Kontich, Belgium) and imported into the three-dimensional visualization software package Avizo 8.1 for subsequent processing and examination of osteological traits. The obtained scans were deposited in MorphoSource. Additionally, we prepared microCT scans of skulls for two other species of Ichthyophis for comparison purposes: I. nguyenorum (ZMMU NAP-03122) and I. supachaii Taylor, 1960 (ZMMU NAP11349); these scans were also deposited in MorphoSource (see Data availability statement below). Terminology and identification of the osteological features. We followed Wake (2003) and Palakkool et al. (2022) for the terminology of skull elements and their main parts, and we followed Norris and Hughes (1918), Maddin (2011), Maddin et al. (2012), and Palakkool et al. (2022) for identifying cranial foramina and describing details of the braincase. Additionally, for the description of morphological details of some skull bones (namely, the premaxilla and the maxillary part of the maxillopalatine) and the atlas, we followed terminology broadly used for the description of the corresponding structures in some Figure 1. Distribution of the unstriped species of the genus Ichthyophis from Indochina and southern China. Symbols indicating Ichthyophis species are identical to those used in Figure 5; a dot in the center of a symbol indicates the type locality of a species. Localities: 1 Xuan Lien NP, Thanh Hoa Province, Vietnam; 2 Pu Hoat NR, Nghe An Province, Vietnam; 3 Maandi, Jinping County, Yunnan Province, China; 4 Cha Lo, Quang Binh Province, Vietnam; 5 Cat Loc, Cat Tien NP, Lam Dong Province, Vietnam; 6 Phnum Dalai, Phnom Samkos WS, Pursat Province, Cambodia; 7 Luang Prabang, the former French administrative centre of “Haute Laos” (following Geissler et al. 2015; but see the Discussion for the problem of the I. laosensis type locality); 8 Mae Wang, Chiang Mai Province, Thailand; 9 Muang Liep, Sayaboury Province, Laos; 10 Bhuping summer palace, Chiang Mai Province, Thailand; 11 Doi Ang Khang, Chiang Mai Province, Thailand. Vertebrate Zoology 75, 2025, 405–440 409 salamanders (e.g., Skutschas 2009; Jia et al. 2019, 2020). For the full list of abbreviations of the osteological features, see Appendix 1. External morphology Measurements and counts generally followed Geissler et al. (2015). It should be noted that some meristic characters were taken different ways by previous researchers (e.g., Taylor, 1960, 1968, 1969), which complicates direct comparisons of our measurements with the literature data (see Kotharambath et al. 2024 for discussion). We used a Mitutoyo (Kanagawa, Japan) digital caliper to take the following measurements to the nearest 0.1 mm: total length, from snout tip to tail tip (TL); tail length, from the posterior end of vent slit to tip (TAL); body width at first nuchal groove (BW1); midbody width (BW2); body width at the anterior edge of cloacal disc (BW3); head length, from snout tip to first nuchal groove, measured ventrally (HL); head width, measured at mouth corners (HW); upper jaw length, from snout tip to corner of mouth (UJL); lower jaw length, from tip of lower jaw to corner of mouth (LJL); snout projection, from snout tip to anteriormost point of lower jaw (SP); distance between eye and lip, measured in lateral view (EL); snout length, from anterior border of the eye to snout tip (ES); interorbital distance (EE); internarial distance (NN), inter-tentacle distance, measured as the distance between the tentacle apertures (TT); eye-naris distance (EN); eye-tentacle aperture distance (ET); tentacle aperture-naris distance (TN); eye diameter, the widest diameter of the visible part of the eye (ED); and distance from eyes to top of head (ETH). Additionally, we took the following measurements for the description of the type series of the new species (modified from Kamei et al. 2009): length of the first collar directly behind the corner of the mouth (C1); length of the second collar directly behind the corner of the mouth (C2); distance between the tentacle aperture and the snout tip (STTA); distance between the tentacle aperture and the lip (LTA); and length of the cloacal disc (CD). We calculated the following ratios: (1) tentacle aperture-naris distance/eye-tentacle aperture distance (TN/ ET); (2) head length/eye diameter (HL/ED); (3) snout projection/head length (SP/HL); (4) total length/tail length (TL/TAL); tail length/tentacle-snout distance (TL/ STTA); and (5) total length/midbody width (TL/BW2). In addition to measurements, counts were made of the following meristic characters: total number of annuli in dorsal count, from posterior margin of second collar (third nuchal groove) to tail cap (except the latter) (TAD); total number of annuli (except the tail cap) in ventral count (TAV); total number of annuli interrupted (at least one annular groove) by the cloacal disc (AV); total number of annuli posterior to the cloacal disc (TAT); the number of transverse grooves on dorsal surface of collar (TG); the number of premaxillary / maxillary teeth (PMM); the number of vomero-palatine teeth (VP); the number of outer mandibular teeth (DE, also sometimes referred to as dentary teeth); and the number of inner mandibular teeth (IM, also sometimes referred to as splenial teeth). All tooth counts included ankylosed teeth and empty sockets (counted from micro-CT scans). Comparative data for other species of Ichthyophis were obtained from the literature (e.g., Taylor 1960, 1968; Nishikawa et al. 2012a; Geissler et al. 2015; Rao et al. 2024). Morphological data from larval specimens were not used in comparisons with the new species type series. Other abbreviations. Dist. – District; Is. – Island; NP: – National Park; NR – Nature Reserve; Prov.: – Province; WS – Wildlife Sanctuary. Molecular laboratory methods For the molecular phylogenetic analyses, total genomic DNA was isolated using the standard phenol chloroform-proteinase K extraction procedures with consequent isopropanol precipitation for a final concentration of about 1 mg/mL (protocols followed Hillis et al. 1996). We visualized the isolated total genomic DNA using agarose electrophoresis in the presence of ethidium bromide. We measured the concentration of total DNA in 1 μL using NanoDrop 2000 (Thermo Scientific) and consequently adjusted it to ca. 100 ng DNA/μL. We used polymerase chain reaction (PCR) to individually amplify three mtDNA fragments: complete sequences of cytochrome b (cyt b) as well as fragments of 12S rRNA and 16S rRNA genes. Table S1 summarizes the primers used for both PCR and sequencing. For both cyt b and 12S–16S rRNA fragments we used the modified PCR protocol of Nishikawa et al. (2012a): (1) an initial denaturation step at 94°C for 4 min; (2) 33 cycles of denaturation at 94°C for 30 s, annealing at 53°C for 30 s and extension at 72°C for 2 min; (3) a final extension at 72°C for 7 min; and (4) a cooling step at 4°C for storage. We ran all amplifications using an iCycler Thermal Cycler (Bio-Rad). We loaded the PCR products onto 1% agarose gels in the presence of ethidium bromide and visualized them by electrophoresis. The successful targeted PCR products were purified by the Diatom DNA PCR Clean-Up Kit and outsourced to Evrogen (Moscow, Russia) for sequencing; sequence data collection and visualization were performed on an ABI 3730xl Automated Sequencer (Applied Biosystems). The obtained sequences were deposited in GenBank under the accession numbers PV088114–PV088123 (see Table S2). Molecular phylogeny To estimate the phylogenetic relationships of the genus Ichthyophis, we used the newly obtained cyt b, 12S rRNA, and 16S rRNA sequences of two unstriped specimens from Xuan Lien NP and Pu Hoat NR, together with previously published sequences of these genes for the family Ichthyophiidae, including 25 nominal species and six unnamed species of the genus Ichthyophis (of them 23 striped and eight unstriped species) and four species of Poyarkov N et al.: New unstriped Ichthyophis and osteological traits for Asian tailed caecilians 410 the genus Uraeotyphlus Peters, 1880. Altogether, the concatenated alignment of cyt b, 12S rRNA, and 16S rRNA fragments included sequences from 46 representatives of Ichthyophiidae; the sequences of Epicrionops marmoratus Taylor, 1968 and Rhinatrema bivittatum (GuérinMéneville, 1838) (family Rhinatrematidae) were used to root the tree (data summarized in Table S2). We aligned the nucleotide sequences using the default parameters in MAFFT online (Katoh et al. 2019), visually checked them in BioEdit 7.0.5.2 (Hall 1999) and adjusted them when required. The mean uncorrected genetic p distances between species of the genus Ichthyophis were calculated with MEGA 6.0 with the pairwise deletion option (Tamura et al. 2013) based on cyt b and 16S rRNA sequences. The best-fit substitution models for the data set were selected for genes and codon positions using PartitionFinder 2.1.1 (Lanfear et al. 2012) with corrected Akaike information criterion (AICc); substitution models were estimated for the 12S–16S rRNA region as one partition and the three codon positions (1st, 2nd, and 3rd positions) of the cyt b gene; gaps were treated as missing data. We estimated phylogenetic trees for the concatenated mtDNA fragments data set. We inferred the phylogenetic relationships of Ichthyophis using Bayesian inference (BI) and maximum likelihood (ML) approaches. We used the IQ-TREE online server (Nguyen et al. 2015) to generate the ML tree and assessed the confidence in tree topology by 10000 ultrafast bootstrap replications (UFBS). We conducted BI in the terminal version of MrBayes 3.1.2 (Huelsenbeck and Ronquist 2001). Metropolis-coupled Markov chain Monte Carlo (MCMCMC) analyses were run with one cold chain and three heated chains for 20 million generations and sampled every 2000 generations. The run was checked to ensure the effective sample sizes (ESS) were all above 200 by exploring the likelihood plots using TRACER v. 1.7 (Rambaut et al. 2018). We discarded the initial 1000 trees as burn-in. For BI, we assessed the confidence in tree topology using the posterior probability (PP) of the nodes (Huelsenbeck and Ronquist 2001). We a priori regard the nodes with UFBS values of 95% or higher and PP values over 0.95 as strongly supported; UFBS values between 95% and 90% and PP values between 0.95 and 0.90 were regarded as well-supported, and lower values were regarded as a lack of node support (Minh et al. 2013). Results Skull atlas of an Ichthyophis sp. from northern Vietnam Osteology description The following description is based on micro-computed tomographic reconstruction of the skull and the atlas of the female specimen ZMMU A-8208 from Xuan Lien NP in Thanh Hoa Province of Vietnam. The general morphology of the skull and the braincase are shown in Figures 2 and 3, respectively; morphology of the atlas is presented in Figure 4, morphology of individual skull bones is shown in Figures S1–S14. General features of the skull. The cranium is heavily ossified, dorsoventrally flattened, and more like V-shaped in the dorsal view (with the maximum width at the level where the squamosal overlies the contact between the pterygoid and the quadrate) (Fig. 2A). The medial part of the cranial roof is composed of paired premaxillae, nasals, frontals, and parietals; laterally, the cranial roof includes paired septomaxillae, prefrontals, maxillopalatines, circumorbitals, and squamosals. These dermal bones constitute the major portion of the dorsal and lateral surfaces of the cranium (Fig. 2A, E). The palate is formed by the posteroventral parts of the premaxillae, paired wide vomers, the ventral parts of the maxillopalatines, and narrow pterygoids (Fig. 2B). The braincase consists of the two well ossified compound bones: the sphenethmoid anteriorly and the os basale posteriorly (Fig. 3). The posteriormost part of the dorsal aspect of the cranium is constituted by the dorsal surface of the os basale, which is the only contribution of the braincase to the dorsal and lateral surfaces of the cranium (Fig. 2A, E). The posterior (occipital) and the posterolateral (otic) regions of the cranium are formed entirely by the os basale (Fig. 2D, E). The sphenethmoid is obscured by the dermal cranial roof bones (the nasals and frontals) dorsally, by the maxillopalatines laterally, and is partially obscured by the vomers and the palatal portions of maxillopalatines ventrally, so the posteriormost portion of this bone is still visible in the ventral aspect (Fig. 2B). The posterolateral aspect of the cranium contains the lateral portions of the os basale and pterygoids, the ventral portions of the quadrates and the stapes (Fig. 2E). The cranium bears the following ten major external openings: the foramen magnum and the paired external nostrils, orbital apertures, upper and lower temporal fossae, and choanae. Additionally, the palatal aspect of the cranium contains the openings of the large mediopalatinal cavities (= interpterygoid vacuities) and the lower temporal fossae (= ventral openings of the adductor chambers) (both openings are much larger than the orbital apertures). The nostrils are relatively large, about the same size as the orbital apertures, rounded with terminal and anterior orientation bordered by the premaxillae (medially and ventrally), nasals (dorsally), and septomaxillae (laterally) (Fig. 2C). The orbital apertures are approximately the same size as the nostrils, oval, slightly dorsoventrally compressed, with an anterolateral orientation, and bordered by the circumorbitals (the dorsal and posterodorsal part of the orbital aperture) and the maxillopalatines (the rest of the orbital aperture) (Fig. 2E). The presence of the elongated semicrescent-shaped circumorbitals excludes the prefrontals, frontals, and squamosals from contacting the orbital apertures. The tentacular canal is open laterally, fused with the orbital aperture, and lies entirely in the maxillopalatine. Vertebrate Zoology 75, 2025, 405–440 411 The dorsally oriented upper temporal fossae are elongated and slit-like, widening posteriorly (Fig. 2A). Each temporal fossa is bordered by the prefrontal anteriorly, by the frontal and the parietal dorsally and medially, and by the squamosal laterally and ventrally. The posterior end of the upper temporal fossa is open forming a space between the squamosal, the parietal and the os basale (Fig. 2A). Nussbaum (1977: 13; 1983: 547) noted that some Ichthy Figure 2. General skull morphology of the holotype of Ichthyophis griseivermis sp. nov. (ZMMU A-8208, adult female, 206 mm total length). 3D reconstruction of the skull is shown with colored rendering in dorsal (A), ventral (B), anterior (C), posterior (D) and left lateral (E) views, and of the left lower jaw in labial (F) and lingual (G) views. Scale bar equals 3 mm. Abbreviations: bas – os basale; car – foramen for the carotid artery; ch – choana; corb – circumorbital; fm – foramen magnum; front – frontal; jf – jugular foramen; ltf – lower temporal fossa; mpc - mediopalatinal cavity; mxpal – maxillopalatine; nas – nasal; nos – nostrils; oc – occipital condyles; orb+tc – orbit and tentacular canal; par – parietal; pmx – premaxilla; psa – pseudoangular; prfr – prefrontal; psd – pseudodentary; pt – pterygoid; q – quadrate; smx – septomaxilla; sphen – sphenethmoid; st – stapes; sq – squamosal; utf – upper temporal fossa; vom – vomer; vf – vomeral foramen. Poyarkov N et al.: New unstriped Ichthyophis and osteological traits for Asian tailed caecilians 412 have ‘a zone of weakness or a very narrow gap between the squamosal and parietal that permits lateromedial movement of the cheek region,’ what he identified as as ‘weakly stegokrotaphic skull,’ without providing the details in which Ichthyophis species such condition is observed. In our specimen, the slit-like temporal fossae are wider, open posteriorly, and the area for the attachment of the adductor muscles on the dorsal surfaces of the cranial roof bones (namely, frontals and parietals) resembles the condition described for larval Ichthyophis specimens (see Kleinteich and Haas 2007). Therefore, we refer to this condition of the skull as weakly zygokrotaphic, though further studies are required to confirm if the lateral layer of m. adductor mandibulae externus sensu Wilkinson and Nussbaum (1997) (or m. levator mandibulae longus sensu Kleinteich and Haas 2007) is indeed passing through the upper temporal fossa or not. The large foramen magnum (notably larger than the orbital aperture) faces posteriorly, is rounded and is entirely bordered by the os basale (Fig. 2D). The ventrally oriented choanae are rounded and slightly smaller than the orbital aperture (Fig. 2B). Each choana is bordered by the maxillopalatine and by the vomer anteromedially. The elongate mediopalatinal cavities are posterior to the choanae, approximately of the same width as the choanae and bordered by the maxillopalatines anteriorly, the pterygoids laterally, and the sphenethmoid and the os basale medially. The posterior ends of the mediopalatinal cavities are open; the pterygoids are located somewhat asymmetrically, with the right pterygoid located closer to the basipterygoid process of the os basale than the left one, suggesting some kinesis in this articulation. The lower temporal fossae are situated laterally to the mediopalatinal cavities, are teardrop-shaped (with a pointed anterior part) and bordered by the squamosals laterally, the quadrates posterolaterally and posteriorly, the os basale medially, and the maxillopalatines anteromedially (Fig. 2B). As in all caecilians (Nussbaum 1977, 1983), the lower jaws consist of the two compound and extensively overlapping bones: the dentigerous pseudodentaries and edenFigure 3. Braincase morphology of the holotype of Ichthyophis griseivermis sp. nov. (ZMMU A-8208, adult female). 3D reconstruction of the braincase is shown with colored rendering in dorsal (A), left lateral (B), and right medial (C) views, and of the isolated sphenethmoid in posterior (D) and anterior (E) views. Scale bar equals 1 mm. Abbreviations: Id – incisure for the dorsal branch of the olfactory nerve; Iv – foramen for the ventral branch of the olfactory nerve; II – incisure for the optic nerve; Vop – foramen for the deep ophthalmic branch of the trigeminal nerve; VII – foramen for the trunk of the facial nerve; VIIIa – foramen for the anterior branch of the auditory nerve; VIImd – foramina for the medial branches of the auditory nerve; VIIIp – foramen for the posterior branch of the auditory nerve; bca – basicranial articulation; car+VIIpal – foramen for the carotid artery and the palatal branch of the facial nerve; dmp – dorsomedial process of the sphenethmoid; dv? – incisure for the dorsal vein; fe – endolymphatic foramen; fper – perilymphatic foramen; fv – fenestra vestibuli; jf – jugular foramen; ns – nasal septum; sn – sola nasi (processus conchoides). Vertebrate Zoology 75, 2025, 405–440 413 tulous pseudoangulars; the latter articulate with the quadrates and have an elongated retroarticular process (Fig. 2F, G). The anterior end of the lower jaw is subterminal (Fig. 2E, F). Dentition. The dentigerous elements of the cranium (upper jaw) are the premaxillae, the maxillopalatine, and the vomers. The only dentigerous element of the lower jaw is the pseudodentary. The dentition is organized as two (labial and lingual) continuous sub-parallel rows of teeth on both the cranium and the lower jaw. The upper jaw dentition includes a labial ‘premaxillary-maxillary’ tooth row, which is present on the premaxillae and the maxillary portion of the maxillopalatine, and a lingual ‘vomero-palatine’ row (sensu Wake 1976), which is situated on the vomers and the palatinal portion of the maxillopalatine (Fig. 2B). There are 22/22 teeth on each side of the cranium in the premaxillary-maxillary row (8/8 on the premaxillae and 13/15 on the maxillary portions of the maxillopalatine) and 21/22 teeth in the vomero-palatine row (10/11 on the vomers and 11/11 on the palatinal portion of the maxillopalatine). The premaxillary-maxillary and vomero-palatine tooth rows are approximately equal in length. The vomero-palatine row extends posteriorly to the anterior border of the lower temporal fossa. The lower jaw dentition includes a labial ‘dentary’ (or outer mandibular) tooth row that is located on the dentary portion of the pseudodentary and a lingual ‘splenial’ (or inner mandibular) tooth row (Fig. 2G). The inner mandibular row (also referred to as ‘adsymphyseal tooth row’ sensu Kligman et al. 2023) is placed on the medial portion of the pseudodentary (the origin of the medial part of the pseudodentary in caecilians is unclear, and its homologization with the lower jaw bones of other tetrapods is difficult; see discussion in Kligman et al. 2023). There are 18/20 teeth in the outer mandibular tooth row and 12/13 teeth in the inner mandibular row. The outer mandibular tooth row is notably longer than the inner mandibular tooth row. All functional teeth are bicuspid and pedicellate. The crowns of functional teeth are moderately sized, with gently posteriorly curved tips. The teeth of labial rows (premaxillary-maxillary row on the cranium and outer mandibular tooth row on the lower jaw) are approximately the same size as the teeth of lingual rows (vomero-palatine row on the cranium and the inner mandibular row on the lower jaw). The replacement teeth are present lingually to the base of the corresponding functional teeth. Premaxillae. The paired dentigerous premaxillae articulate with each other medially to form the anterior margin of the snout (Fig. S1). Each premaxilla contacts with the corresponding nasal dorsally, septomaxilla posterodorsally, vomer posteromedially, and maxillopalatine posteriorly, and consists of three main parts: the pars dorsalis (= dorsal or alary process), the pars dentalis, and the pars palatina. Pars dorsalis is relatively long and narrow with a dorsal apex that is sharply pointed and set within a deep notch of the nasal (Fig. S1A). There are two large neurovascular canals presumably for the ophthalmic division of the trigeminal nerve and the associated blood vessels, which nurture the skin, passing through the base and middle parts of the dorsal process. In the posterior view, these canals are visible as large foramina and in the medial view as short deep grooves (Fig. S1B). These canals branch within the bone and emerge on the external surface through several openings. The pars dentalis of each premaxilla is short and slightly curved along the contour of the snout (Fig. S1A). Its lingual surface bears closely spaced pedicellate teeth that form the anterior part of the labial tooth row. The posterior end of each pars dentalis is penetrated by one oblique neurovascular canal that enters the bone posteriorly and almost immediately exits onto the dorsal surface, opening into a deep short groove. This canal is presumably for the passage of the medial branch of the maxillary division of the trigeminal nerve and the associated blood vessels. The pars palatina is a wide ledge along the lingual aspect of the pars dentalis (Fig. S1C, E). In ventral view, the inner margin of this bony ledge runs parallel to the curvature of the outer edge of the premaxilla, except the median part, where it has a pronounced projection (Fig. S1C). The pars palatina articulates along its entire length with the vomer to contribute to the anterior part of the palate. Maxillopalatines. The paired maxillopalatines are compound dentigerous bones with a complex shape (Fig. S2). Each bone consists of a lateral maxillary part and a medial palatine part. Each maxillopalatine articulates with the premaxilla anteriorly, the septomaxilla anterodorsally, the prefrontal dorsally, the squamosal posteriorly, and the vomer and the pterygoid medially to contribute to the lateral part of the palate. The maxillary part of each maxillopalatine has a short anterior process with a blunt anterior end (for the contact with the premaxilla anteriorly and the septomaxilla dorsally), the pars facialis (= facial, dorsal process) in the anterior half of the maxilla (for the contact with the prefrontal) (Fig. S2A), and the posterior process for the articulation with the squamosal posteriorly (Fig. S2D). The central part of each pars facialis is penetrated by a large foramen (Fig. S2A, B). The medial surface of the pars facialis around this large foramen bears a deep oval depression that is presumably holding the vomeronasal organ (Fig. S2A). The posterior part of each pars facialis has a narrow, sharp, and posterodorsally oriented process that borders an anterodorsal edge of the orbital aperture (Fig. S2A). The posterior process of the maxillary part forms the posteroventral border of the orbital aperture. There is a wide ventral gutter-like anterolaterally oriented groove between the sharp process of pars facialis and the posterior process that is confluent with the orbital aperture and corresponds to the tentacular canal (therefore, the tentacular canal is an open groove, but not a closed canal separated from the orbital aperture) (Fig. S2B). The pars palatina of each maxillary part is a wide ledge that contacts the vomer (at the level of the anterior process and the pars facialis of the maxillary part, anterior to Poyarkov N et al.: New unstriped Ichthyophis and osteological traits for Asian tailed caecilians 420 lationships among these three species remained essentially unresolved. Subclade D2 (95/0.98) included two unstriped species: I. lakimi Nishikawa, Matsui & Yambun, 2012 from Sabah, Borneo, and I. mindanaoensis Taylor, 1960 from the Philippines, along with a number of striped species from Borneo, namely I. biangularis Taylor, 1965, I. pauli Nishikawa et al., 2013, I. cf. asplenius Taylor, 1965, and I. nigroflavus Taylor, 1960 (the two latter species were also reported from the Malay Peninsula and Sumatra), as well as two unnamed species, Ichthyophis sp. A and Ichthyophis sp. B from Borneo. Subclade D3 (100/1.0) included an array of species from mainland Southeast Asia (including northeastern India) and Sundaland. The striped species from Myanmar and northeastern India (I. benjii Lalremsanga et al., 2021; I. khumhzi Kamei et al., 2009; and I. multicolor Wilkinson et al., 2014) formed a group (95/1.0) that is sister to all the remaining species from Indochina and Sundaland (98/0.99). Within this former group, I. benjii from Mizoram, India, was a sister species of I. multicolor from Myanmar and I. khumhzi from Mizoram, with the latter making I. multicolor paraphyletic. Within the Indochinese + Sundaland species, I. catlocensis, an unstriped species from southern Vietnam, was suggested as a sister species to all the remaining taxa (98/1.0) (Fig. 5). These latter included a group consisting of two taxa from Peninsular Malaysia: an unstriped species I. larutensis Taylor, 1960, and its undescribed sister species Ichthyophis sp. C (100/1.0). Two striped species, I. hypocyaneus (van Hasselt, 1827) from Java and I. elongatus Taylor, 1965 from Sumatra, formed a well-supported clade with three striped species from the Thai-Malay Peninsula: I. supachaii and two unnamed species, Ichthyophis sp. D from Tanintharyi in Myanmar and Ichthyophis sp. E from southern Thailand (100/1.0). Ichthyophis nguyenorum, a striped species from central Vietnam, formed a well-supported clade with I. cardamomensis, an unstriped species from southern Cambodia (100/0.99). Finally, the I. kohtaoensis complex grouped two striped species from mainland Indochina: I. kohtaoensis (including its junior synonym I. bannanicus) and an unnamed species, Ichthyophis sp. E from Vietnam (98/1.0). Pairwise distances. The uncorrected p distances for the cyt b gene and the 16S rRNA mtDNA fragment among the members of the genus Ichthyophis are summarized in Table S4. The interspecific distances among Ichthyophis species in the cyt b sequences varied from p = 5.3% (between I. supachaii and I. hypocyaneus) to p = 19.8% (between I. elongatus and I. cf. asplenius, and I. elongatus and I. pauli), while in the 16S rRNA sequences, p distances varied from p = 1.5% (between I. supachaii and I. hypocyaneus) to p = 9.9% (between I. orthoplicatus and Ichthyophis sp. A). The genetic divergence of the newly discovered population of Ichthyophis sp. from northern Vietnam in cyt b gene sequences varied from p = 6.5% (with I. yangi) to p = 14.6% (with Ichthyo phis sp. E), while in the 16S rRNA gene sequences p distances varied from p = 4.5% (with I. chaloensis) to p = 7.6% (with I. khumhzi, Ichthyophis sp. D, Ichthyophis sp. E). Inter-group genetic differentiation between the two samples of Ichthyophis sp. from northern Vietnam was small and comprised p = 0.9% (for the 16S rRNA gene fragment); the two samples of Ichthyophis sp. shared the same cyt b haplotype. Systematics Our molecular phylogenetic analyses strongly suggested that the newly discovered caecilian populations from Xuan Lien NP and Pu Hoat NR in northern Vietnam belong to the genus Ichthyophis, within which they formed, among the sampled species, a distinct species-level lineage, which was closely related to two recently discovered unstriped species: I. yangi from Yunnan Province, China, and I. chaloensis from central Vietnam (Fig. 5). Genetic divergence of Ichthyophis sp. from northern Vietnam from its putative sister species was not especially high (6.5–6.9% in the cyt b gene and 4.5% in the 16S rRNA gene) but generally corresponded to the species-level divergence in these genes reported for many recognized species of this genus (for example, I. supachaii and I. hypocyaneus, or I. mindanaoensis and I. cf. asplenius; see Table S4). Overall, based on the external morphology and geographic proximity of their distributions, the unstriped species I. chaloensis, I. yangi, and Ichthyophis sp. from northern Vietnam appeared to be most closely related to the enigmatic species I. laosensis, described from central Laos and to date reliably known only from the holotype MNHN 1928.95. Geissler et al. (2015) revised the unstriped species of Ichthyophis from Indochina and, along with descriptions of three new species, provided a redescription of available type materials for I. laosensis, I. acuminatus, and I. youngorum. Though the absence of the recently collected materials and molecular data for I. laosensis hampers the interpretation of our phylogenetic results, in the case of significant morphological differences, the taxonomic recognition of morphologically distinct populations as species seems to be reasonable. Morphological examination of the two specimens of Ichthyophis sp. from northern Vietnam indicated the presence of several diagnostically important traits related both to body proportions and to meristic characters, including the number of annuli, teeth, and vertebrae, which allowed distinguishing this population from all other unstriped species of Ichthyophis in mainland Southeast Asia, as well as from all other congeners (summarized in Table 1). As a basis for further taxonomic research on Southeast Asian Ichthyophiidae, we also provided a detailed description of osteological traits of this population, including a detailed morphological description of skull bones and the atlas. Overall, our results supported the hypothesis that the recently discovered population of Ichthyophis sp. from northern Vietnam represents an undescribed species, which we formally describe below. Vertebrate Zoology 75, 2025, 405–440 421 Table 1 – part 1. Comparison of morphological characters of Ichthyophis griseivermis sp. nov. with other unstriped species of the genus Ichthyophis from Indochina and China. Diagnostic differences from the new species are marked in bold. Morphological data from larval specimens were not used in comparisons with the new species type series. For character abbreviations see Materials and methods. Type status abbreviations: H – holotype; P – paratype. Morphological data are taken from Taylor (1960), Geissler et al. (2015), Rao et al. (2024), and the present study. Species Ichthyophis griseivermis sp. nov. I. yangi I. catlocensis I. chaloensis I. laosensis Museum IDs ZMMU A-8208 VRTC NAP-08953 KIZ 2024R1448 KIZ 2023048 KIZ 2024R1445 ZFMK 88976 IEBR A.2011.16 MNHN 1928.95 Type status H P H P P H P H Sex F M F F F F F F Morphometry (in mm) TL 206.0 242.0 321.4 307.4 329.5 183.5 215.7 318.0 TAL 2.4 3.6 2.9 3.0 3.9 2.1 3.7 3.7 BW1 6.6 6.9 10.6 10.5 8.0 5.3 6.9 11.4 BW2 9.8 11.2 13.9 13.8 10.5 7.1 7.6 16.1 BW3 4.9 5.0 5.8 5.7 4.2 3.7 3.2 6.3 HL 8.8 9.8 14.6 15.4 17.0 7.3 9.4 10.9 HW 6.1 6.9 8.7 8.1 9.5 4.8 6.3 9.7 UJL 6.5 8.0 10.8 9.6 10.5 5.4 7.2 10.8 LJL 6.2 7.5 10.3 9.2 10.1 5.2 6.3 10.3 SP 0.6 0.8 0.8 0.6 0.7 0.5 1.3 0.6 EL 0.7 0.8 0.8 0.7 1.0 0.6 0.8 1.7 ES 4.0 4.7 5.7 5.5 5.6 3.6 4.8 6.3 EE 4.6 5.8 5.9 5.1 6.6 4.3 4.8 7.3 NN 1.8 2.2 2.0 1.7 2.6 2.0 1.8 3.2 TT 4.2 4.9 6.1 5.5 5.4 4.2 4.7 7.5 EN 2.8 3.7 4.2 4.3 5.4 2.9 3.6 5.0 ET 1.1 1.5 1.2 1.1 1.2 0.6 1.3 1.6 TN 2.4 3.1 1.3 1.3 1.3 2.7 2.8 4.3 ED 0.6 0.7 1.0 1.1 1.2 0.5 0.3 1.0 ETH 0.7 0.9 –––––– C1 2.2 2.8–––––– C2 2.4 3.0 –––––– STTA 3.6 4.5 –––––– LTA 0.6 0.7 –––––– CD 1.5 3.0 –––––– Measurement ratios TN/ET 2.2 2.1 1.1 1.1 1.1 4.5 2.2 2.7 HL/ED 15.5 14.5 15.2 13.8 14.2 14.6 31.3 10.9 SP/HL 0.06 0.08 0.05 0.04 0.04 0.07 0.14 0.06 TL/TAL 85.5 66.7 110.8 103.9 83.6 87.4 58.3 85.9 TL/BW2 21.1 21.6 23.2 22.4 31.4 25.8 28.4 19.8 ES/HL 0.46 0.48 0.39 0.36 0.33 0.49 0.51 0.58 EL/HL 0.08 0.08 0.05 0.04 0.06 0.08 0.09 0.16 Meristic characters TAD 306 301 369 372 372 342 344 346 TAV 298 292 367 369 368 340 342 345 AV 4 4 666533 TAT 1 1 222552 PMM 44 48 – 51 53 44 37 33 VP 43 48 – 44 44 51 54 36 DE 38 38 – 48 49 27 26 35 IM 25 33 –36 35 16 11 30 VERT 112 111 114 114 –110 110 112 TG 0 0 –––––– Table 1 – part 2. Species I. youngorum I. acuminatus I. cardamomensis Museum IDs FMNH 189250 FMNH 189251 FMNH 189252 FMNH 189253 AMNH A20875 NHMUK 1921.4.1.338 NHMUK 1961.2055 NHMUK 1961.2056 NHMUK 1961.2057 LSUHC 9335 CBC 01185 LSUHC 10106 Type status H P P P H P P P P H P P Sex M M – (larva) F (larva) M F F F – (larva) FFF Poyarkov N et al.: New unstriped Ichthyophis and osteological traits for Asian tailed caecilians 422 Family Ichthyophiidae Taylor, 1968 Genus Ichthyophis Fitzinger, 1826 Ichthyophis griseivermis sp. nov. https://zoobank.org/ACB099C7-7C20-4F23-93F4-ED9C536C2FAF https://www.morphosource.org/concern/media/000708101 Table 1; Figures 2–8, S1–S16 Holotype. ZMMU A-8208 (field tag NAP-15224), an adult female, from Xuan Lien National Park, Bat Mot Commune, Thanh Hoa Province, Vietnam (elevation 800 m a.s.l.; geographic coordinates: 19.985°N, 104.974°E), collected by A. P. Yuzefovich and A. M. Bragin from a bank of mountain stream on October 28, 2023. Paratype. VRTC NAP08953 (field tag NAP-08953), adult male, from Pu Hoat Nature Reserve, Tien Phong Commune, Nghe An Province, Vietnam (elevation 815 m a.s.l.; geographic coordinates: 19.755°N, 104.796°E), Species I. youngorum I. acuminatus I. cardamomensis Museum IDs FMNH 189250 FMNH 189251 FMNH 189252 FMNH 189253 AMNH A20875 NHMUK 1921.4.1.338 NHMUK 1961.2055 NHMUK 1961.2056 NHMUK 1961.2057 LSUHC 9335 CBC 01185 LSUHC 10106 Morphometry (in mm) TL 208.0 217.0 82.0 189.0 294.0 199.0 203.0 215.0 172.0 183.0 321.7 289.1 TAL 2.9 3.1 1.8 3.5 3.9 2.8 2.4 3.6 2.0 3.1 3.8 6.0 BW1 8.1 8.0 – 7.8 10.6 7.8 6.4 8.1 6.7 7.1 9.2 8.9 BW2 11.2 10.5 4.8 10.6 13.7 10.9 8.1 10.6 8.0 7.9 8.6 8.0 BW3 5.5 5.0 2.5 5.3 5.4 4.3 3.6 5.9 3.9 4.1 5.4 4.9 HL 10.4 10.4 – 9.3 14.6 8.8 9.1 10.1 8.8 8.1 11.7 9.2 HW 7.6 7.3 3.9 7.0 9.8 6.7 6.2 7.0 6.3 5.8 7.9 7.1 UJL 7.6 7.3 2.8 4.5 10.6 6.4 6.2 6.2 5.9 6.9 10.2 9.5 LJL 6.7 6.9 2.2 4.5 9.3 6.0 5.6 5.6 5.9 6.0 9.7 9.0 SP 0.1 0.1 0.3 0.7 0.8 0.4 0.1 0.3 0.4 0.7 0.5 0.6 EL – – – – – – – – – 1.0 1.0 – ES – – – – 5.8*– – – – 4.1 6.0 5.5 EE 4.5 4.8 2.4 4.2 6 4.3 4 4.6 4.3 4.3 5.6 5.2 NN 2.4 2.2 1.4 1.7 2.6 1.3 1.1 1.4 1.3 2.2 2.8 2.6 TT 5.9 4.9 – – 7.2 5.0 4.6 5.5 5.0 4.7 6.2 5.8 EN 3.8 3.6 1.7 2.8 4.8 3.1 3.2 3.2 3.1 3.5 4.7 4.2 ET 1.1 1.0 – – 1.3 0.7 0.8 0.8 0.8 0.9 1.2 1.1 TN 2.7 2.5 – – 3.1 2.0 2.0 2.1 2.5 2.9 3.5 3.1 ED – – – – 0.7* – – – – 0.7 1.0 0.8 ETH – – – – – – – – – – – – C1 – – – – – – – – – – – – C2 – – – – – – – – – – – – STTA – – – – – – – – – – – – LTA – – – – – – – – – – – – CD – – – – – – – – – – – – Measurement ratios TN/ET 2.5 2.5 – – 2.4 2.9 2.5 2.6 3.1 3.2 2.9 2.8 HL/ED – – – – 20.9 – – – – 11.6 13.2 11.5 SP/HL 0.01 0.01 – 0.08 0.05 0.05 0.01 0.03 0.05 0.09 0.04 0.06 TL/TAL 71.7 70 45.6 54.0 43.0 71.1 84.6 59.7 86 59 84.7 48.1 TL/BW2 18.6 20.7 17.1 17.8 21.5 18.3 25.1 25.1 21.5 23.2 37.4 36.1 ES/HL – – – – 0.40 – – – – 0.51 0.51 0.60 EL/HL – – – – – – – – – 0.12 0.09 – Meristic characters TAD 318 328 – 317 302 296 – 301 327 322 364 340 TAV – – – – – – – – – 320 359 338 AV 5 6 – 6 6 7 5 6 3 5 4 5 TAT 4 4 –43 2 3 2 36 4 6 PMM 28 22** –23** 40 43 37 41 35 23 38 38 VP 33** 40 –33** 46 39 38 43 37 29 28 28 DE 28** 29** – 28** 42 43 38 42 31 20 34 34 IM 19** 18** –15** 32 26 26 26 19 19 21 22 VERT 108 108 104 108 109 108 – 111 111 120 – 120 TG – – – – – – – – – – – – * Based on Taylor (1960, 1968); ** Damaged, no exact count possible. Vertebrate Zoology 75, 2025, 405–440 423 collected by N. A. Poyarkov from the bank of a river on May 15, 2019. Etymology. The specific name “griseivermis” is a Latin noun in the nominative singular, given in apposition, derived from the Latin adjective “griseus” for “grey” and the Latin noun “vermis” for “worm.” The new species is named in reference to its characteristic uniform grey body coloration. The specific epithet also alludes to Grey Worm, the commander of the Unsullied, the warrior-eunuchs of Astapor with an unparalleled reputation for combat in George R. R. Martin’s fictional work “A Song of Ice and Fire” (also known as “Game of Thrones”). We suggest the following common names for the new species: “Grey Worm Caecilian” (in English), “Ếch giun xám Bắc Trung Bộ” (in Vietnamese), and “Seryi rybozmey” (“Серый рыбозмей,” in Russian). Diagnosis. The new species Ichthyophis griseivermis sp. nov. differs from other members of the genus Ichthyophis by the following combination of the morphological characters: unstriped body lacking lateral yellow stripe; adult total length 206–242 mm (based on two available specimens); snout blunt and rounded (snout length/head length ratio 0.06–0.08); tentacle aperture located closer to eye than to naris (tentacle aperture-naris distance/tentacle aperture-eye distance ratio 2.1–2.2); premaxillary and maxillary teeth 44–48, vomero-palatine teeth 43–48, dentary teeth 38, inner mandibular teeth 25–33; tail very short, acuminate, ending in a nipple-like cap; annuli angulate, total 301–306 (dorsal count), four interupted by cloacal disc, one posterior to cloacal disc, the degree of annuli angulation decreasing from head to cloaca with grooves appearing almost orthoplicate at mid-body and posteriorly; vertebrae 111–112; scales in one series per annulus (dosolaterally), present only in the posterior half of body. Description of the holotype. Adult female (Figs 6–8), specimen in a good state of preservation (Fig. S15); a small oblique scar on the dorsal surface of body above the cloaca (Fig. 7E, G), small (< 15 mm) midventral longitudinal incision at midbody with some viscera protruding, including yellowish round mature ova (ca. 6 mm in diameter). Body subcylindrical; head, nuchal region, and trunk slightly dorsoventrally compressed. Body tapering posteriorly, more abruptly at about one-fifth of body length (Fig. 6C–D), ending in blunt tail tip, with a small nipple-like terminal cap (Fig. 7E–H). Tail downturned towards tip, very short (TL/TAL ratio 85.5), slightly longer than tentacle-snout distance (TAL/STTA ratio 0.68). Head longer than wide (HW/HL ratio 0.70); head dorsal surface slightly flattened (Fig. 7A–B). Head somewhat more like Vthan U-shaped in dorsal view (Fig. 7C). In dorsal view, head width at the level of mouth corner notably smaller than the width of the first collar (HW/ BW1 ratio 0.92); head narrowing towards the tentacles and gradually tapering from the tentacles to the snout (Fig. 7C). In lateral view, head conically tapering on the distance between the first collar and nares (Fig. 7A–B). Nares located much closer to the tip of the snout than to eye (ES/EN ratio 1.4; Fig. 4A–B). Lip margin flat and straight; corners of mouth notably closer to the throat than to top of head (Fig. 7A–B); mouth subterminal, with the upper and lower lips nearly identical in length (SP/ UJL ratio 0.10; Fig. 7D); in ventral view, gular region flat (Fig. 7D). Eyes very small (ED/HL ratio 0.07), eye diameter slightly larger than that of naris and subequal to tentacle aperture; covered by grayish-white semitranslucent skin; eyes round, surrounded by narrow whitish ring, forming a dark-gray central disc, with a very small round pupil visible through the skin (Fig. 7A–B). In lateral view, eyes located almost equidistant from lip and top of head (EL/ETH 0.91) (Fig. 7A–B). Tentacle apertures located over two times closer to eye than to naris (EN/ET ratio 2.6; TN/ET ratio 2.2; Fig. 7A–B), almost reaching the edge of the upper lip; subequal in size to the eye (Fig. 7A–B). Tentacular papilli elevated above the adjacent skin and visible in dorsal, lateral, and ventral views (Fig. 7A–D). Naris small, oval with anterolateral orientation (Fig. 7A–B). Teeth small, notably recurved, almost hookshaped, located in two rows on upper and lower jaws (PMM 44, VP 43, DE 38, IM 25); outer mandibular tooth series approximately the same length as vomero-palatine tooth series. Tongue triangular with an acuminate tip, plicate posteriorly, lacking a distinct longitudinal medial groove; choanae narrow. The first collar slightly wider than the head at the mouth corner level (HW/BW1 ratio 0.92); the second collar gradually widens posteriorly; collar grooves widely incomplete dorsally, more distinct on the ventral surface (Fig. 7D) and on the sides (Fig. 7A–B); medially, anterior and posterior borders of collar region not well discernable, edged by the anteriormost body annuli (Fig. 7C–D); transverse nuchal grooves on dorsal surface of collar absent; in ventral view, second collar slightly longer than first (C1/C2 ratio 0.91); the anteriormost annuli complete in both ventral and dorsal aspects (Fig. 6C–D); body grooves encircle venter by forming an angle pointing towards tail, with the degree of angulation decreasing from head to cloaca: grooves distinctly angulated in the anterior one-third of body length (Fig. 6D) and appear almost orthoplicate at mid-body and posteriorly, with only a small medial portion of groove (ca. 0.5 mm in length) forming a shallow angle (Fig. 6B). Total number of annuli TAD 306, TAV 298 (dorsal and ventral counts, respectively); vertebrae 112. Cloacal slit longitudinal, located in an oval cloacal disc, interrupting four annuli on both sides (Fig. 7H). Tail bearing two annular grooves delimiting a single annulus and terminating in a distinct nipple-like terminal cap (Fig. 7G–H); scales in one series per annulus (in dorsolateral view), present only in the posterior half of body; scales oval in shape. Coloration. In life (Figs 6–8), body uniformly greybrown; somewhat lighter on venter; with a slight pinkish-purple tint on lower flanks and belly (Fig. 8); annular grooves dark-grey, annuli greyish; margins of nares, lips, nares and tentacles whitish-beige; eyes dark-blue with a narrow whitish circle around. After 15 months in preservative (Fig. S15), dark grey on dorsum and somewhat Poyarkov N et al.: New unstriped Ichthyophis and osteological traits for Asian tailed caecilians 424 lighter ventrally; cloacal disc white; tail cap white; eyes, tentancles, and nares encircled by a narrow white margin. Variation. Variation in measurements and meristic characters of the type series is presented in Table 1. The paratype (VRTC NAP08953, adult male) was found as a desiccated specimen and is in a moderate condition of preservation (Fig. S16). Paratype body significantly dorsoventrally flattened; a large transverse incision present in the posterior one-third of the specimen length on the Figure 6. The holotype of Ichthyophis griseivermis sp. nov. in life (ZMMU A-8208, adult female). A Dorsal view of annuli in the middle of the body; B ventral view of annuli in the middle of the body; C general dorsolateral view of the body, right side; D general ventrolateral view of the body, right side. Scale bar equals 5 mm. Photographs by A. M. Bragin. Vertebrate Zoology 75, 2025, 405–440 425 Figure 7. Details of external morphology of the holotype of Ichthyophis griseivermis sp. nov. in life (ZMMU A-8208, adult female). A Lateral view of the head, right side; B lateral view of the head, right side; C dorsal view of the head; D ventral view of the head; E lateral view of the tail, left side; F lateral view of the tail, right side; G dorsal view of the tail; H ventral view of the tail. Scale bar equals 5 mm (all photographs shown in one scale). Photographs by A. M. Bragin. Poyarkov N et al.: New unstriped Ichthyophis and osteological traits for Asian tailed caecilians 426 ventral side. The copulatory organ was extruded from the cloaca and damaged prior to specimen collection; the remains of the everted organ are visible on the ventral and lateral aspects of the specimen (Fig. S16A, B), but do not allow us to provide a description of its morphology. Overall, the male paratype VRTC NAP08953 is generally very similar in most morphological traits to the holotype; however, it is c. 17% longer than the holotypeis (TL 242 mm), and has a slightly lower number of annuli (TAD 301, TAV 292), one less vertebra (VERT 111), and a greater number of teeth in all tooth series (PMM 48; VP 48; DE 38; IM 33) all within the range of intraspecific variation expected for an Ichthyophis. It is identical to the holotype in in the number of annuli interrupted by the cloacal disc (AV 4) and the number of annuli posterior to the cloacal disc (TAT 1); and similar in body proportions. Due to desiccation and storage in ethanol for over five years, the original dark coloration of the paratype has significantly faded; the specimen is uniformly brown; the head and ventral surfaces are light-brown (Fig. S16). Comparisons (external morphology). The new species lacks light lateral stripes, so it can be easily distinguished from all striped members of the genus Ichthyophis, and its comparisons with the unstriped congeners are the most pertinent. We will first compare Ichthyophis griseivermis sp. nov. with seven currently recognized unstriped species of the genus Ichthyophis from the Indochinese region (including Vietnam, Cambodia, Laos, and Thailand) and China; the main diagnostic characters separating the new species from these species are summarized in Table 1. Ichthyophis griseivermis sp. nov. is a comparatively small-sized species (TL 206.0–242.0 mm): though only two specimens of the new species are known to date and it is impossible to be confident about its maximal body size, the presence of mature ova in the holotype at least indicates that this specimen is adult at the TL of 206.0 mm. This can arguably distinguish the new species from its sister species I. yangi (endemic to Yunnan Province, China) (TL 307.4–329.5 mm) and, with less confidence, from I. laosensis (known only from northern Laos) (TL of the only known holotype 318.0 mm). In body proportions, by the relatively longer eye-snout distance (ES/HL ratio 0.46–0.48), the new species can be further distinguished from I. yangi, in which the snout is much shorter (ES/HL ratio 0.33–0.39), and, though with less confidence, from I. acuminatus (distributed in northwestern Thailand and northwestern Laos) (ES/ HL ratio 0.40; however, this comparison should be taken with caution as its calculation is based on the ES measurement by Taylor 1960). At the same time, I. laosensis and, arguably, I. cardamomensis (endemic to the Cardamom Mountains, southwestern Cambodia) have a comparatively longer snout than the new species (ES/HL ratios 0.58 and 0.51–0.60, respectively). Ichthyophis griseivermis sp. nov. has the tentacle aperture being situated comparatively farther from the eye (TN/ET ratio 2.1–2.2) than in most other unstriped Indochinese Ichthyophis, including I. acuminatus (TN/ET ratio 2.4–2.9), I. carFigure 8. The holotype of Ichthyophis griseivermis sp. nov. in life in situ (ZMMU A-8208, adult female). Photograph by A. M. Bragin. Vertebrate Zoology 75, 2025, 405–440 427 da momensis (TN/ET ratio 2.8–3.2), I. catlocensis (endemic to Lam Dong Province of southern Vietnam) (TN/ET ratio 4.5), I. laosensis (TN/ET ratio 2.7), and I. youngorum (endemic to northwestern Thailand) (TN/ ET ratio 2.5). At the same time, in the new species, the tentacle aperture is situated comparatively closer to the eye than in I. yangi, where it is located almost in between the nostril and the eye (TN/ET ratio: 1.1). Relative eye size is larger in the new species (HL/ED ratio 14.5–15.5) than in I. chaloensis (endemic to Quang Binh Province, central Vietnam) (HL/ED ratio 31.3) and, arguably, in I. acuminatus (HL/ED ratio 20.9; however, this comparison should be taken with caution as its calculation is based on the ED measurement by Taylor 1960), but is smaller than in I. laosensis (HL/ED ratio 10.9) and generally smaller than in I. cardamomensis (HL/ED ratio 11.5–13.2). Ichthyophis griseivermis sp. nov. has a relatively shorter distance between eye and lip (EL/HL ratio 0.08) than in I. laosensis (EL/HL ratio 0.16), but this distance is greater than in I. yangi (EL/HL ratio 0.04–0.06). The new species has a relatively more projecting snout (SP/HL ratio 0.06–0.08) than I. acuminatus (SP/HL ratio 0.01–0.05) and I. youngorum (SP/HL ratio 0.01), and a slightly more projecting snout than I. yangi (SP/HL ratio 0.04–0.05), but a shorter snout projection than I. chaloensis (SP/HL ratio 0.14). The new species has a comparatively shorter tail (TL/TAL ratio 66.7–85.5) than I. chaloensis (TL/TAL ratio 58.3). Ichthyophis griseivermis sp. nov. has a comparatively wider body (TL/BW2 ratio 21.1–21.6) than I. cardamomensis (TL/BW2 ratio 23.2–37.4), I. catlocensis (TL/BW2 ratio 25.8), and I. chaloensis (TL/BW2 ratio 28.4); though this character should be taken cautiously, as a significant variation in body width has been reported earlier for a larger sample size of I. glutinosus (Nussbaum and Gans 1980). Differences between the new species and its congeners observed in meristic characters should be taken with caution due to a small sample size available for our examination. Nevertheless, I. griseivermis sp. nov. has notably fewer total annuli both in dorsal count (TAD 301–306) and in ventral count (TAV 292–298) than I. catlocensis (TAD 342; TAV 340), I. chaloensis (TAD 344; TAV 342), I. laosensis (TAD 346; TAV 345), I. cardamomensis (TAD 322–364; TAV 320–359), and I. yangi (TAD 369–372; TAV 367–369). The new species has generally fewer annuli interrupted by the cloacal disc (AV 4) than in I. acuminatus (AV 5–7), I. youngorum (AV 5–7), and I. yangi (AV 6), slightly fewer annuli interrupted by the cloacal disc than I. catlocensis (AV 5), and slightly more than I. chaloensis (AV 3) and I. laosensis (AV 3). By having only a single annulus posterior to the cloacal disc (TAT 1), the new species is distinguished from I. acuminatus (TAT 2–3), I. cardamomensis (TAT 2–6), I. catlocensis (TAT 5), I. chaloensis (TAT 5), and I. youngorum (TAT 4). The new species has a slightly higher number of labial premaxillary-maxillary teeth (PMM 44–48) than I. acuminatus (PMM 37–43), I. cardamomensis (PMM 23–38), I. chaloensis (PMM 37), I. laosensis (PMM 33), and I. youngorum (PMM 22–28), but fewer premaxillary-maxillary teeth than in I. yangi (PMM 51–53). Ichthyophis griseivermis sp. nov. has slightly fewer vomero-palatine teeth (VP 43–48) than I. catlocensis (VP 51) and I. chaloensis (VP 54), but more than in I. laosensis (VP 36), I. cardamomensis (VP 28– 29), and I. youngorum (VP 33–40; note that the lower value was obtained from a partially damaged specimen and may be erroneous). The new species has more dentary (labial) teeth (DE 38) than I. catlocensis (DE 27), I. chaloensis (DE 26), and I. youngorum (DE 28–29; note that these values were obtained from partially damaged specimens and may be erroneous), but fewer than I. yangi (DE 48–49). At the same time, the new species has more inner mandibular teeth (IM 25–33) than I. catlocensis (16), I. chaloensis (IM 11), I. youngorum (IM 18–19; note that these values were obtained from partially damaged specimens and may be erroneous), and I. cardamomensis (IM 19–22). Ichthyophis griseivermis sp. nov. further differs from I. cardamomensis by fewer vertebrae (VERT 111–112 vs. 120). Furthermore, I. griseivermis sp. nov. can be readily diagnosed from the following unstriped species of Ichthyophis which occur outside the Indochinese Region and southern China. In particular, the new species differs from I. lakimi (Sabah, Borneo) by having more premaxillary-maxillary (labial) teeth (PMM 44–48 vs. 16–25) and by having more inner mandibular teeth (IM 25–33 vs. 14) (though the tooth counts for I. lakimi reported by Nishikawa et al. 2012 appear to be much lower than are typically known for Ichthyophis and require a re-examination); from I. billitonensis Taylor, 1965 (Belitung Is., Indonesia) by having more inner mandibular teeth (IM 25–33 vs. 2); and by having more annuli in dorsal count (TAD 301–306 vs. 251–254); from I. dulitensis Taylor, 1960 (Sarawak, Borneo) by the absence of scales in the anterior half of body (vs. present); by having more inner mandibular teeth (IM 25–33 vs. 8); and by the absence of a light marking on the throat (vs. present); from I. glandulosus Taylor, 1923 (Basilan Is., Philippines) by having more annuli in dorsal count (TAD 301–306 vs. 273–286); and by a higher number of vertebrae (VERT 111–112 vs. 102); from I. javanicus Taylor, 1960 (Java Is., Indonesia) by having less annuli both in dorsal count (TAD 301–306 vs. 351), and in ventral count (TAV 292– 298 vs. 348); and fewer annuli posterior to the cloacal disc (TAT 1 vs. 10); from I. larutensis (Peninsular Malaysia) by the presence of inner mandibular teeth (vs. absent); and by a slightly higher number of vertebrae (VERT 111–112 vs. 107); from I. mindanaoensis (Mindanao Is., Philippines) by having more inner mandibular teeth (IM 25–33 vs. 16–22); from I. monochrous (Bleeker, 1858) (Borneo Is., Indonesia) by having more annuli in dorsal count (TAD 301–306 vs. 247); by having more inner mandibular teeth (IM 25–33 vs. 8); and by a greater number of vertebrae (VERT 111–112 vs. 103); from I. orthoplicatus (Sri Lanka) by having fewer annuli posterior to the cloacal disc (TAT 1 vs. 7); and by having slightly more inner mandibular teeth (IM 25–33 vs. 18–20); from I. sikkimensis Taylor, 1960 (northeastern India) by having more annuli in dorsal count (TAD 301–306 vs. 276–292); by a greater number of vertebrae (VERT 111–112 vs. Poyarkov N et al.: New unstriped Ichthyophis and osteological traits for Asian tailed caecilians 428 106–108); and by having slightly more inner mandibular teeth (IM 25–33 vs. 18–20); from I. singaporensis Taylor, 1960 (Singapore) by the absence of scales on the anterior half of body (vs. present); by having more annuli in dorsal count (TAD 301–306 vs. 260–273); by having fewer annuli posterior to the cloacal disc (TAT 1 vs. 7); and by having more inner mandibular teeth (IM 25–33 vs. 6–10); from I. sumatranus Taylor, 1960 (Sumatra Is., Indonesia) by the absence of scales on the anterior half of body (vs. present); and by having fewer annuli posterior to the cloacal disc (TAT 1 vs. 7); from I. weberi Taylor, 1920 (Palawan Is., Philippines) by the presence of inner mandibular teeth (vs. absent); and by having fewer both in dorsal count (TAD 301–306 vs. 313–329), and in ventral count (TAV 292–298 vs. 304–322). Comparisons (cranial features). Overall, the holotype specimen of I. griseivermis sp. nov. (ZMMU A-8208; for detailed osteological description see above) is characterized by the following combination of cranial and dental characteristics: cranium more like V-shaped in the dorsal view; circumorbital present as a small bone, crescent-shaped and widely open ventrally; circumorbital and frontal not in contact; tentacular canal is open laterally and confluent with the orbital aperture; frontals in contact one another along about one-third of their lengths (i.e., short midline contact of frontals following Wilkinson et al. 2014); the posterior edge of vomer is situated slightly anteriorly to the center of the palate (at about 44–45% of the cranium length); ventral edge of the posterior process of pterygoid is situated barely below the level of premaxillary-maxillary teeth; the anterior part of the parasphenoid portion of os basale is wide; occipital condyles are widely spaced in ventral view; retroarticular process of pseudoangular elongated, with its rounded posterior end oriented nearly dorsally; and teeth are moderately sized, with gently posteriorly curved tips. Below, we compare the cranial features of I. griseivermis sp. nov. with the few congeners for which skull morphology reconstructions are available via MorphoSource or from previous publications: I. asplenius, I. kohtaoensis, I. tricolor, I. multicolor, I. nguyenorum, and I. supachaii (based on Wilkinson et al. 2014; McGrath-Blaser et al. 2025 and our data, see the Data Availability Statement below). Comparisons with I. sikkimensis are limited because only the dorsal aspect of the skull has been shown for this species (Gower et al. 2017: fig. 2A). Additionally, we examined skull descriptions and illustrations of I. beddomei, I. glutinosus, I. larutensis, I. mindanaoensis, I. nigroflavus, I. singaporensis, and I. weberi presented in Taylor (1969: figs 2–11). Table S5 summarizes the comparative morphological data on skull morphology for the new species and 14 other members of the genus Ichthyophis. The new species differs from all other Ichthyophis species examined in having a small, crescent-shaped circumorbital, lacking the ventral portion, and bordering only the upper posterior corner of the orbital aperture (vs. a larger circumorbital with a posterior ventral process). The smooth, gradually tapering retroarticular process with its posterior end oriented dorsally is also a characteristic feature of the new species. Additionally, the new species differs from I. asplenius, I. beddomei, I. glutinosus, I. kohtaoensis, I. larutensis, I. mindanaoensis, I. multicolor, I. nguyenorum, I. nigroflavus, I. singaporensis, I. supachaii, I. tricolor, and I. weberi, but resembles I. sikkimensis in having a weakly zygokrotaphic skull (vs. the stegokrotaphic or weakly stegokrotaphic condition of the skull). Furthermore, the new species differs from I. asplenius, I. kohtaoensis, I. tricolor, I. multicolor, I. nguyenorum, and I. supachaii in lacking the stapedial foramen (vs. present in all these species; for other species the structure of the stapes has not been described or illustrated). Additionally, I. griseivermis sp. nov. further differs from I. kohtaoensis in having a more like V-shaped cranium in the dorsal view (vs. more like U-shaped), in having the posterior edge of the vomer situated slightly anteriorly to the center of the palate, about 44–45% of the cranium length (vs. the vomer is situated around the center of the palate, about 50% of the cranium length), and by the laterally expanded foramen magnum with widely spaced occipital condyles (vs. dorsoventrally expanded foramen magnum with closely positioned condyles). The new species further differs from I. tricolor in having a laterally open tentacular groove confluent with the orbital aperture (vs. a laterally closed tentacular groove not confluent with the orbital aperture), by having moderately sized teeth with gently posteriorly curved tips (vs. enlarged teeth with strongly curved tips), by the ventral edge of the posterior process of the pterygoid situated barely below the level of premaxillary-maxillary teeth (vs. far below the level of premaxillary-maxillary teeth), and by the absence of the contact between circumorbital and frontal (vs. present). The new species further differs from I. multicolor in having the posterior edge of the vomer situated slightly anteriorly to the center of the palate, about 44– 45% of the cranium length (vs. vomer situated around the center of palate, about 50% of the cranium length), and in the absence of the contact between circumorbital and frontal (vs. present). The new species further differs from I. asplenius in having a more rounded tip of the snout (vs. more blunt), in more ventrally located nostrils in lateral view (vs. more dorsally), in having a laterally open tentacular canal confluent with the orbital aperture (vs. partly laterally closed tentacular canal not confluent with the orbital aperture), in rounded choanae (vs. subtriangular), and in the absence of the lateroventral process of the squamosal (vs. present). The new species further differs from I. nguyenorum in having widely spaced occipital condyles in ventral view (vs. almost confluent) and in a comparatively wider anterior part of the parasphenoid portion of os basale (vs. narrow). The new species further differs from I. supachaii in having widely spaced occipital condyles in ventral view (vs. almost confluent) and in having a laterally open tentacular groove confluent with the orbital aperture (vs. partly laterally closed tentacular groove not confluent with the orbital aperture). The new species differs from I. sikkimensis in having a longer contact between the frontal and prefrontal (vs. shorter, Vertebrate Zoology 75, 2025, 405–440 429 L-shaped contact). The new species further differs from I. beddomei in having a more like V-shaped cranium in the dorsal view (vs. more like U-shaped), in lacking the contact between frontal and circumrobital (vs. wide contact), in having oblique and narrow posterior edge of prefrontal in dorsal view (vs. transverse), and in having long anterior process of pterygoid (vs. short). Ichthyophis griseivermis sp. nov. further differs from I. glutinosus in having a more like V-shaped cranium in the dorsal view (vs. more like U-shaped), in having rounded snout tip in dorsal aspect (vs. blunt), in having a laterally open tentacular groove confluent with the orbital aperture (vs. a laterally closed tentacular canal), in having palatine tooth series terminating at the level of the anterior border of the adductor chamber (vs. extends behind the level of the anterior border of the adductor chamber), in having widely spaced occipital condyles in ventral view (vs. closely positioned), in having shorter midline contact between frontals (vs. longer), and in having oblique and narrow posterior edge of prefrontal in dorsal view (vs. rounded). The new species further differs from I. larutensis in having rounded snout tip in dorsal aspect (vs. blunt), in having a laterally open tentacular groove confluent with the orbital aperture (vs. a laterally closed tentacular canal), in having palatine tooth series terminating at the level of the anterior border of the adductor chamber (vs. terminates anterior to the adductor chamber), in having widely spaced occipital condyles in ventral view (vs. closely positioned), in having shorter midline contact between frontals (vs. longer), and in having long anterior process of pterygoid (vs. short), and in a comparatively wider anterior part of the parasphenoid portion of os basale (vs. narrow tapering). The new species further differs from I. mindanaoensis in having a more like V-shaped cranium in the dorsal view (vs. more like U-shaped), in having rounded choanae in ventral view (vs. distinctly triangular), in having a laterally open tentacular groove confluent with the orbital aperture (vs. a laterally closed tentacular canal), in having shorter midline contact between frontals (vs. longer), and in a comparatively wider anterior part of the parasphenoid portion of os basale (vs. narrow tapering). The new species further differs from I. nigroflavus in having a more like V-shaped cranium in the dorsal view (vs. more like U-shaped), in having rounded choanae in ventral view (vs. anteroposteriorly elongated, oval), in having widely spaced occipital condyles in ventral view (vs. in contact), in having long anterior process of pterygoid (vs. short), and in a comparatively wider anterior part of the parasphenoid portion of os basale (vs. narrow tapering). Ichthyophis griseivermis sp. nov. further differs from I. singaporensis in having a more like V-shaped cranium in the dorsal view (vs. more like U-shaped), in having rounded snout tip in dorsal aspect (vs. blunt), in having rounded choanae in ventral view (vs. subtriangular), in having a laterally open tentacular groove confluent with the orbital aperture (vs. a laterally closed tentacular canal), in having palatine tooth series terminating at the level of the anterior border of the adductor chamber (vs. extends behind the level of the anterior border of the adductor chamber), in having widely spaced occipital condyles in ventral view (vs. in contact), and in a comparatively wider anterior part of the parasphenoid portion of os basale (vs. narrow tapering). The new species further differs from I. weberi in having a more like V-shaped cranium in the dorsal view (vs. more like U-shaped), and in having long anterior process of pterygoid (vs. short). Finally, a scan of the skull of the I. laosensis specimen from Laos (NCSM 86611) is currently available on MorphoSource (https://www.morphosource.org/concern/ media/000059682); however, unfortunately, the comparison of the new species with this cranial reconstruction appears impossible, since it is likely that the specimen NCSM 86611 is a sub-adult individual with some of the skull elements unossified or not in the definitive condition. Distribution and natural history notes. Currently, I. griseivermis sp. nov. is known from two protected areas in northern Vietnam: from Xuan Lien NP in Thanh Hoa Province and Pu Hoat NR in Nghe An Province (Fig. 1). Though at present the new species can be considered as endemic to a narrow montane area in the western parts of Thanh Hoa and Nghe An provinces of Vietnam, its occurrence in the adjacent parts of Houaphanh Province of Laos is anticipated; both known localities are located just 3–5 km from the Vietnam-Laos national border. The possible occurrence of the new species in Ben En NP in Thanh Hoa Province and Pu Huong NR in Nghe An Province also cannot be excluded, and further field survey efforts are needed to clarify the extent of its distribution. The type locality is at an elevation of 800 m asl. The holotype was found on the bank of a small forest stream with a stony bottom and steep clay bank (Fig. S17). The surrounding secondary montane evergreen forest is severely damaged by regular logging, but some old trees remain, including Cunninghamia konishii Hayata (Cupressaceae), forming mixed polydominant forests with Symingtonia populnea (R. Br. ex Griff.) (Hamamelidaceae), Carallia suffruticosa Ridl. (Rhizophoraceae), Engelhardtia roxburghiana Wall (Juglandaceae), Guarea excelsa Kunth (Meliaceae), Castanopsis ferox (Rosb.) (Fagaceae), Michelia mediocris Dandy (Magnoliaceae), Pellionia radicans var. grande (Gagnep.) H. Schroter (Urticaceae), Ardisia quinquegona Blume (Primulaceae), Litsea acutivena Hayata and Litsea yunnanensis Y. C. Yang & P. H. Huang (Lauraceae), and Alniphyllum fortunei (Hemsley) Makino (Styracaceae). The undergrowth is well developed; soil is densely covered with grasses, sedges, and fern thickets. The holotype was found in a small ravine at the foothill of a low mountain range composed of clays and shales with a limestone base. The holotype was found at night (ca. 23:00 h) while slowly crawling among the stones on the banks of a small mountain stream during a rain with an ambient air temperature Poyarkov N et al.: New unstriped Ichthyophis and osteological traits for Asian tailed caecilians 436 the Western Ghats of peninsular India in the collection of London’s Natural History Museum and their implications for taxonomy and distribution. Journal of Vertebrate Biology 73: 1–12. https://doi. org/10.25225/jvb.23073 Kupfer A, Müller H (2004) On the taxonomy of ichthyophiid caecilians from southern Thailand: A reevaluation of the holotype of Ichthyophis supachaii Taylor 1960 (Amphibia: Gymnophiona: Ichthyophiidae). Amphibia-Reptilia 25: 87–97. https://doi.org/10.1163/ 1 56853804322992869 Kupfer A, Nabhitabhata J, Himstedt W (2005) Life history of amphibians in the seasonal tropics: Habitat, community and population ecology of a caecilian (genus Ichthyophis). Journal of Zoology 266: 237–247. Lalremsanga HT, Purkayastha J, Biakzuala L, Mathipi M, Muansanga L, Hmar GZ (2021b) A new striped species of Ichthyophis Fitzinger, 1826 (Amphibia: Gymnophiona: Ichthyophiidae) from Mizoram, northeast India. Amphibian & Reptile Conservation 15: 198–209. Lalremsanga HT, Purkayastha J, Mathipi M, Muansanga L, Decemson H, Biakzuala L (2021a) Range extension of Ichthyophis multicolor Wilkinson et al., 2014 to India and first molecular identification of Ichthyophis moustakius Kamei et al., 2009. Check List 17: 1021– 1029. https://doi.org/10.15560/17.4.1021 Lanfear R, Calcott B, Ho SY, Guindon S (2012) PartitionFinder: Combined selection of partitioning schemes and substitution models for phylogenetic analyses. Molecular Biology and Evolution 29: 1695– 1701. https://doi.org/10.1093/molbev/mss020 Lowie A, De Kegel B, Wilkinson M, Measey J, O’Reilly JC, Kley NJ, Gaucher P, Adriaens D, Herrel A (2023) The anatomy of the head muscles in caecilians (Amphibia: Gymnophiona): Variation in relation to phylogeny and ecology? Journal of Anatomy 242: 312–326. https://doi.org/10.1111/joa.13763 Lowie A, De Kegel B, Wilkinson M, Measey J, O’Reilly JC, Kley NJ, Gaucher P, Brecko J, Kleinteich T, Herrel A, Adriaens D (2022) Regional differences in vertebral shape along the axial skeleton in caecilians (Amphibia: Gymnophiona). Journal of Anatomy 241: 716–728. https://doi.org/10.1111/joa.13682 Maddin HC (2011) Deciphering morphological variation in the braincase of caecilian amphibians (Gymnophiona). Journal of Morphology 272: 850–871. https://doi.org/10.1002/jmor.10953 Maddin HC, Russell AP, Anderson JS (2012) Phylogenetic implications of the morphology of the braincase of caecilian amphibians (Gymnophiona). Zoological Journal of the Linnean Society 166: 160–201. http://dx.doi.org/10.1111/j.1096-3642.2012.00838.x McGrath-Blaser SE, Grafe U, Torres-Sánchez M, McLeod DS, Stuart BL (2025) Cutaneous bacterial microbiome description and new locality record of the poorly known striped caecilian Ichthyophis asplenius Taylor, 1965 in Bruneian Borneo. Journal of Natural History 59: 29–43. https://doi.org/10.1080/00222933.2024.2442749 Minh Q, Nguyen MAT, von Haeseler A (2013) Ultrafast approximation for phylogenetic bootstrap. Molecular Biology and Evolution 30: 1188–1195. https://doi.org/10.1093/molbev/mst024 Mulcahy DG, Lee JL, Miller AH, Chand M, Thura MK, Zug GR (2018) Filling the BINs of life: Report of an amphibian and reptile survey of the Tanintharyi (Tenasserim) Region of Myanmar, with DNA barcode data. ZooKeys 757: 85–152. https://doi.org/10.3897/zookeys.757.24453 Müller H, Oommen OV, Bartsch P (2005) Skeletal development of the direct-developing caecilian Gegeneophis ramaswamii (Amphibia: Gymnophiona: Caeciliidae). Zoomorphology 124: 171–188. https:// doi.org/10.1007/s00435-005-0005-6 Müller J (1835) Ueber die Kiemenlöcher der jungen Coecilia hypocyanea. Archiv für Anatomie, Physiologie und wissenschaftliche Medicin 1835: 391–398. Nguyen LT, Schmidt HA, von Haeseler A, Bui MQ (2015) IQ-TREE: A fast and effective stochastic algorithm for estimating maximum likelihood phylogenies. Molecular Biology and Evolution 32: 268–274. https://doi.org/10.1093/molbev/msu300 Nguyen SV, Ho CT, Nguyen TQ (2009) Herpetofauna of Vietnam. Chimaira, Frankfurt, 768 pp. Nguyen TV, Duong TV, Luu KT, Poyarkov NA (2020) A new species of Kurixalus (Anura: Rhacophoridae) from northern Vietnam with comments on the biogeography of the genus. Journal of Natural History 54: 195–223. https://doi.org/10.1080/00222933.2020.1728411 Nguyen TV, Liu S, Tran VT, Tran TG, Trofimets AV, Dau VQ, Poyarkov NA (2024a) Range extension and expanded description of Micryletta hekouensis Liu, Hou, Mo & Rao, 2021 (Amphibia: Anura: Microhylidae), with comments on Micryletta of northern Vietnam. Herpetozoa 37: 111–121. https://doi.org/10.3897/herpetozoa.37.e120524 Nguyen TV, Liu S, Wilkinson JA, Tran TG, Tran PN, Dau VQ, Poyarkov NA (2024b) Range extension and expanded description of Rhacophorus napoensis Li, Liu, Yu & Sun, 2022 (Amphibia: Anura: Rhacophorinae) with the first country record from Vietnam. Herpetozoa 37: 123–136. https://doi.org/10.3897/herpetozoa.37.e122317 Nguyen TV, Pham CT, Nguyen TQ (2016) New records and an updated list of snakes (Squamata: Serpentes) from Xuan Lien Nature Reserve, Thanh Hoa Province, Vietnam. Journal of Biology 38: 324– 332. https://doi.org/10.15625/0866-7160/v38n3.8448 Nishikawa K, Matsui M, Yambun P (2012b) A new unstriped Ichthyophis (Amphibia: Gymnophiona: Ichthyophiidae) from Mt. Kinabalu, Sabah, Malaysia. Current Herpetology 31: 67–77. https://doi.org/ 10.5358/hsj.31.67 Nishikawa K, Matsui M, Yong HS, Ahmad N, Yambun P, Belabut DM, Sudin A, Orlov NL, Ota H, Yoshikawa N, Tominaga A, Shimada T, Shimada T (2012a) Molecular phylogeny and biogeography of caecilians from Southeast Asia (Amphibia, Gymnophiona, Ichthyophiidae), with special reference to high cryptic species diversity in Sundaland. Molecular Phylogenetics and Evolution 63: 714–723. http://dx.doi.org/10.1016/j.ympev.2012.02.017 Nishikawa K, Matsui M, Yoshikawa N, Khonsue W, Pomchote P, Hibino M, Nguyen T. T, Sanamxay D (2021) Ichthyophis bannanicus Yang, 1984, a junior subjective synonym of I. kohtaoensis Taylor, 1960 (Amphibia, Gymnophiona, Ichthyophiidae). Alytes 38: 1–17. Norris HW, Hughes SP (1918) The cranial and anterior spinal nerves of the caecilian amphibians. Journal of Morphology 31: 489–560. Nussbaum RA (1977) Rhinatrematidae: A new family of caecilians (Amphibia: Gymnophiona). Occasional Papers of the Museum of Zoology, University of Michigan 682: 1–30. Nussbaum RA (1979) The taxonomic status of the caecilian genus Uraeotyphlus Peters. Occasional Papers of the Museum of Zoology, University of Michigan 687: 1–20. Nussbaum RA (1983) The evolution of a unique dual jaw-closing mechanism in caecilians (Amphibia: Gymnophiona) and its bearing on caecilian ancestry. Journal of Zoology 199: 545–554. Nussbaum RA, Gans C (1980) On the Ichthyophis (Amphibia: Gymnophiona) of Sri Lanka. Spolia Zeylanica 35: 137–154. Olson DM, Dinerstein E, Wikramanayake ED, Burgess ND, Powell GV, Underwood EC, D’amico JA, Itoua I, Strand HE, Morrison JC, Loucks CJ (2001) Terrestrial Ecoregions of the World: A new map of life on Earth. A new global map of terrestrial ecoregions provides an innovative tool for conserving biodiversity. BioScience Vertebrate Zoology 75, 2025, 405–440 437 51: 933–938. https://doi.org/10.1641/0006-3568(2001)051[0933:- TEOTWA]2.0.CO;2 Palakkool SR, Gower DJ, Kotharambath R (2022) Osteology of the caecilian Gegeneophis carnosus (Beddome, 1870) (Amphibia: Gymnophiona: Grandisoniidae) from the Western Ghats of peninsular India. Vertebrate Zoology 72: 561–576. https://doi.org/10.3897/ vz.72.e79911 Peter K (1898) Die Entwicklung und funktionelle Gestaltung des Schädels von Ichthyophis glutinosus. Morphologisches Jahrbuch 25: 554–628. Pham CT, Nguyen TQ, Hoang CV, Ziegler T (2016) New records and an updated list of Amphibians from Xuan Lien Nature Reserve, Thanh Hoa Province, Vietnam. Herpetology Notes 9: 31–41. Poyarkov NA, Nguyen TV, Arkhipov DV (2021b) A new species of the genus Tylototriton (Amphibia, Caudata, Salamandridae) from central Vietnam. Taprobanica 10: 4–22. https://doi.org/10.47605/tapro. v10i1.244 Poyarkov NA, Nguyen TV, Le SX, Le DX, Arkhipov DV, Gorin VA, Hernandez A, Dufresnes C (2024) Multiple lines of evidence for a new cryptic species of Tylototriton (Amphibia, Salamandridae) from northern Vietnam. Alytes 41: 56–98. Poyarkov NA, Nguyen TV, Popov ES, Geissler P, Pawangkhanant P, Neang T, Suwannapoom C, Ananjeva NB, Orlov NL (2023) Recent progress in taxonomic studies, biogeographic analysis, and revised checklist of reptiles in Indochina. Russian Journal of Herpetology 30: 255–476. https://doi.org/10.30906/1026-2296-2023-30-5-255476 Poyarkov NA, Nguyen TV, Popov ES, Geissler P, Pawangkhanant P, Neang T, Suwannapoom C, Orlov NL (2021a) Recent progress in taxonomic studies, biogeographic analysis and revised checklist of amphibians in Indochina. Russian Journal of Herpetology 28: 1–110. https://doi.org/10.30906/1026-2296-2021-28-3A-1-110 Poyarkov NA, Nguyen TV, Trofimets AV, Gorin VA (2020) A new cryptic species of the genus Microhyla (Amphibia: Microhylidae) from Langbian Plateau, Vietnam. Taprobanica 9: 136–163. https://doi. org/10.47605/tapro.v9i2.228 Ramaswami LS (1941) Some aspects of the cranial morphology of Uraeotyphlus narayani Seshachar (Apoda). Records of the Indian Museum 43: 143–207. Ramaswami LS (1942) An account of the head morphology of Gegeneophis carnosus (Beddome), Apoda. Journal of the Mysore University 3: 205–220. Rambaut A, Drummond AJ, Xie D, Baele G, Suchard MA (2018) Posterior summarization in Bayesian phylogenetics using Tracer 1.7. Systematic Biology 67: 901–904. https://doi.org/10.1093/sysbio/ syy032 Rao DQ, Zhou HX, Mo MZ, Yu ZY, Li X, Liu S (2024) The second species of Caecilian in China. Asian Herpetological Research 15: 279–288. https://doi.org/10.3724/ahr.2095-0357.2024.0043 Rowley JJL, Dau VQ, Cao TT (2017) A new species of Leptolalax ( Anura: Megophryidae) from Vietnam. Zootaxa 4273: 61–79. Rowley JJL, Nguyen SN, Dau VQ, Nguyen TT, Cao TT (2011) A new species of Gracixalus (Anura: Rhacophoridae) with a hyperextended vocal repertoire from Vietnam. Zootaxa 3125: 22–38. San Mauro D, Gower DJ, Oommen OV, Wilkinson M, Zardoya R (2004) Phylogeny of caecilian amphibians (Gymnophiona) based on complete mitochondrial genomes and nuclear RAG1. Molecular Phylogenetics and Evolution 33: 413–427. http://dx.doi.org/10.1016/j. ympev.2004.05.014 Santos RO, Wilkinson M, Zaher H (2025) An overview of the postcranial osteology of caecilians (Gymnophiona, Lissamphibia). Anatomical Record 2025: 1–26. http://dx.doi.org/10.1002/ar.70000 Sarasin P, Sarasin F (1887–1890) Ergebnisse naturwissenschaftlicher Forschungen auf Ceylon in den Jahren 1884–1886 Band II: Zur Entwicklungsgeschichte und Anatomie der Ceylonesischen Blindwühle Ichthyophis glutinosus. C. W. Kreidels, Wiesbaden, 264 pp., plate XV. Schmidt A, Wake MH (1990) Olfactory and vomeronasal systems of caecilians (Amphibia: Gymnophiona). Journal of Morphology 205: 255–268. Sherratt E, Gower DJ, Klingenberg CP, Wilkinson M (2014) Evolution of cranial shape in caecilians (Amphibia: Gymnophiona). Evolutionary Biology 41: 528–545. https://doi.org/10.1007/s11692-0149287-2 Skutschas PP (2009) Re-evaluation of Mynbulakia Nesov, 1981 (Lissamphibia: Caudata) and description of a new salamander genus from the Late Cretaceous of Uzbekistan. Journal of Vertebrate Paleontology 29: 659–664. https://doi.org/10.1671/039.029.0326 Stuart BL, Seateun S, Sivongxay N, Phimmachak S (2024) Amphibians (Class Amphibia) In: WCS 2024. Wildlife in Lao PDR, 2024 Status Report. Wildlife Conservation Society Lao PDR Program, Vientiane, 1–459. Tamura K, Stecher G, Peterson D, Filipski A, Kumar S (2013) MEGA6: Molecular Evolutionary Genetics Analysis Version 6.0. Molecular Biology and Evolution 30: 2725–2729. https://doi.org/10.1093/molbev/mst197 Taylor EH (1960) On the caecilian species Ichthyophis monochrous and Ichthyophis glutinosus and related species. University of Kansas Science Bulletin 40: 37–120. Taylor EH (1968) The Caecilians of the World: A Taxonomic Review. University of Kansas (Science Bulletin), Lawrence, KS, vii, 848 pp. Taylor EH (1969a) Skulls of Gymnophiona and their significance in the taxonomy of the group. The University of Kansas Science Bulletin 48: 585–687. Taylor EH (1969b) Miscellaneous notes and descriptions of new forms of caecilians. University of Kansas Science Bulletin 48: 281–296. Trofimets AV, Dufresnes C, Pawangkhanant P, Bragin AM, Gorin VA, Hasan M, Lalremsanga HT, Muin MA, Le DX, Nguyen TV, Suwannapoom C, Poyarkov NA (2024) Four in one: an integrative taxonomic revision of the Microhyla berdmorei complex (Amphibia: Anura: Microhylidae) illustrates the tremendous amphibian diversity of Southeast Asia. Vertebrate Zoology 74: 595–641. https://doi. org/10.3897/vz.74.e127937 Visser MHC (1963) The cranial morphology of Ichthyophis glutinosus (Linne) and Ichthyophis monochrous (Bleeker). Annale Universiteit van Stellenbosch 38A: 67–102. Wake MH (1976) The development and replacement of teeth in viviparous caecilians. Journal of Morphology 148: 33–63. https://doi. org/10.1002/jmor.1051480104 Wake MH (1980) Morphometrics of the skeleton of Dermophis mexicanus (Amphibia Gymnophiona). Part I. The vertebrae, with comparison to other species. Journal of Morphology 165: 17–130. https://doi.org/10.1002/jmor.1051650202 Wake MH (2003) The osteology of caecilians. In: Heatwole H, Davies M (Eds) Amphibian Biology, Vol. 5. Surrey Beatty, Chipping Norton, 1809–1876. Wiedersheim R (1879) Die Anatomie der Gymnophionen. G. Fischer, Jena, 101 pp. Poyarkov N et al.: New unstriped Ichthyophis and osteological traits for Asian tailed caecilians 438 Appendix 1 List of abbreviations of the osteological features. acot – anterior cotyles addr – ridge for the attachment of the long adductor of the lower jaw alp – anterolateral process of the quadrate amp – anteromedial process of the quadrate ap – anterior process apm – anterior process of the maxillary part of the maxillopalatine app – anterior process of the palatal part of the maxillopalatine bas – os basale bca – basicranial articulation bp – basal process of the pterygoid canp – canalis primordialis car – foramen for the carotid artery car+VIIpal – foramen for the carotid artery and the palatal branch of the facial nerve ccr – curved crest of the vomer ch – choana con – condyle corb – circumorbital cp – columellar process of the stapes cpr – ridge of the columellar process of the stapes dm – depression for the attachment of the m. depressor mandibulae dmp – dorsomedial process of the sphenethmoid dp – dorsal process of the septomaxilla dv? – incisure for the dorsal vein fe – endolymphatic foramen ff – facet for contact of the nasal or the parietal with the frontal fm – foramen magnum fp – facial process of the maxillopalatine fper – perilymphatic foramen fpl – footplate of the stapes front – frontal fv – fenestra vestibuli jf – jugular foramen lg – longitudinal grooves of the vomer ltf – lower temporal fossa mo – medial outgrowth of the atlantal centrum mp – medial portion of the dorsal process of the septomaxilla mpc - mediopalatinal cavity mpp – medial process of the palatal part of the maxillopalatine mxf – facet for contact of the prefrontal or the squamosal with the maxillopalatine mxpal – maxillopalatine na – neural arch nas – nasal nf – facet for contact of the frontal with the nasal nos – nostrils ns – nasal septum oc – occipital condyles orb+tc – orbit and tentacular canal p – posterior process of the maxillary part of the maxillopalatine pa – processus ascendens of the quadrate par – parietal pc – processus condyloideus of the pseudoarticular pcot – posterior condyle pdent – pars dentalis of the premaxilla pdor – pars dorsalis of the premaxilla pff – facet for contact of the frontal with the prefrontal pi – processus internus of the pseudoarticular pmp – premaxillary process of the vomer pmx – premaxilla po – processus oticus of the quadrate pp – posterior process of the palatal part of the maxillopalatine ppal – pars palatina ppn – posterior process of the nasal prfr – prefrontal Wilkinson M (2012) Caecilians. Current Biology 22: R668–R669. Wilkinson M, Nussbaum RA (1996) On the phylogenetic position of the Uraeotyphlidae (Amphibia: Gymnophiona). Copeia 1996: 550–562. Wilkinson M, Nussbaum RA (1999) Evolutionary relationships of the lungless caecilian Atretochoana eiselti (Amphibia: Gymnophiona: Typhlonectidae). Zoological Journal of the Linnean Society 126: 191–223. http://dx.doi.org/10.1111/j.1096-3642.1999.tb00153.x Wilkinson M, Nussbaum RA (2006) Caecilian phylogeny and classification. In: Exbrayat J-M (Ed.) Reproductive Biology and Phylogeny of Gymnophiona (caecilians). Science Publishers, Inc., Enfield, NH, 39–78. Wilkinson M, Presswell B, Sherratt E, Papadopoulou A, Gower DJ (2014) A new species of striped Ichthyophis Fitzinger, 1826 (Amphibia: Gymnophiona: Ichthyophiidae) from Myanmar. Zootaxa 3785: 45–58. https://doi.org/10.11646/zootaxa.3785.1.4 Wilkinson M, San Mauro D, Sherratt E, Gower DJ (2011) A nine-family classification of caecilians (Amphibia: Gymnophiona). Zootaxa 2874: 41–64. https://doi.org/10.11646/zootaxa.2874.1.3 Wilkinson M, Sheps J, Oommen OV, Cohen BL (2002) Phylogenetic relationships of Indian caecilians (Amphibia: Gymnophiona) inferred from mitochondrial rRNA gene sequences. Molecular Phylogenetics and Evolution 23: 401–407. http://dx.doi.org/10.1016/s10557903(02)00031-3 Zhang P, Zhou H, Chen YQ, Liu YF, Qu LH (2005) Mitogenomic perspectives on the origin and phylogeny of living amphibians. Systematic Biology 54: 391–400. https://doi.org/10.1080/10635150590945278 Vertebrate Zoology 75, 2025, 405–440 439 psa – pseudoangular psaf – facet for contact of the pseudodentary with the pseudoangular psd – pseudodentary pt – pterygoid pzp – postzygopophyseal processes q – quadrate qf – facet for contact of the squamosal with the quadrate rap – retroarticular process scs – spinal cord support smx – septomaxilla sn – sola nasi (processus conchoides) snf – spinal nerve foramen spf – facet for contact of the parietal with the sphenethmoid sphen – sphenethmoid sq – squamosal sqf – facet for contact of the quadrate with the squamosal st – stapes sta – groove for the stapedial artery tc – tentacular canal utf – upper temporal fossa vcf – foramina leading to the inner vomerine cavity vf – vomeral foramen vlr – ventrolateral ridge of the frontal vo – vomeronasal organ cavity vom – vomer vp – ventral process of the septomaxilla Id – incisure for the dorsal branch of the olfactory nerve Iv – foramen for the ventral branch of the olfactory nerve II – incisure for the optic nerve Vim – foramen for the intermandibular branch of the trigeminal nerve Vmd – foramina for the mandibular branch of the trigeminal nerve Vmde – foramina for the external branch of the mandibular division of the trigeminal nerve Vmx – foramen for the maxillary branch of the trigeminal nerve Vop – foramen for the deep ophthalmic branch of the trigeminal nerve Vop+mx – foramina for the maxillary and the deep ophthalmic branches of the trigeminal nerve V+VII – groove for combined alveolar branches of the trigeminal and facial nerves VII – foramen for the trunk of the facial nerve VIIalv – foramina for the alveolar branch of the facial nerve VIIIa – foramen for the anterior branch of the auditory nerve VIIIp – foramen for the posterior branch of the auditory nerve VIIImd – foramina for the medial branches of the auditory nerve Poyarkov N et al.: New unstriped Ichthyophis and osteological traits for Asian tailed caecilians 440 Supplementary Material 1 Figures S1–S17 Authors: Poyarkov NA, Skorinova DD, Bragin AM, Kolchanov VV, Gorin VA, Trofimets AV, Yuzefovich AP, Le DX, Nguyen TV, Skutschas PP (2025) Data type: .pdf Explanation notes: Figure S1–S14. 3D reconstructions of the holotype of Ichthyophis griseivermis sp. nov. — Figure S15. The holotype of Ichthyophis griseivermis sp. nov. in preservative. — Figure S16. The paratype of Ichthyophis griseivermis sp. nov. in preservative. — Figure S17. Natural habitat of Ichthyophis griseivermis sp. nov. at the type locality in Xuan Lien NR, Thanh Hoa Province, northern Vietnam. Copyright notice: This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/ odbl/1.0). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited. Link: https://doi.org/10.3897/vz.74.e149399.suppl1 Supplementary Material 2 Tables S1–S5 Authors: Poyarkov NA, Skorinova DD, Bragin AM, Kolchanov VV, Gorin VA, Trofimets AV, Yuzefovich AP, Le DX, Nguyen TV, Skutschas PP (2025) Data type: .pdf Explanation notes: Table S1. Primers used in this study. — Table S2. Sequences and voucher specimens of the family Ichthyophiidae and outgroup taxa used in this study. — Table S3. Characteristics of analyzed DNA sequences and the proposed optimal evolutionary models for gene and codon partitions as estimated in PartitionFinder 2.1.1. — Table S4. Uncorrected p–distances (percentage) between the sequences of cyt b mtDNA gene (above the diagonal) and between the sequences of 16S rRNA mtDNA gene (below the diagonal) of species of the genus Ichthyophis included in the phylogenetic analyses. — Table S5. Potentially diagnostic characters of skull morphology that differ among 15 species of the genus Ichthyophis. Copyright notice: This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/ odbl/1.0). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited. Link: https://doi.org/10.3897/vz.74.e149399.suppl2