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.

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 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 edentulous 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 the palatine part of the bone) and fuses to the palatine part of the bone posteriorly. The maxillary part bears a labial tooth row (with 13 pedicellate teeth on the left and 15 teeth on the right bone).

The palatine part of each maxillopalatine has the anterior and medial processes that form the anterior, lateral, and posterior borders of the choana and the posterior process for the articulation with the pterygoid posteromedially (Fig. S2C, D). The end of the anterior process of the palatine part is dorsally curved and contacts with a similarly curved crest on the dorsal surface of the vomer; these two structures border a large concavity dorsally. According to Sarasin and Sarasin (1887–1890) and Palakkool et al. (2022), this concavity contains the ‘Choanenschleimbeutal’ (= ‘choanal mucous sac’). Each palatine part bears a lingual teeth row. The neurovascular system of the bone is complex and includes a network of canals that access the external surface of the bone through several openings. These canals are presumably for the passage of the branches of the maxillary and ophthalmic divisions of the trigeminal nerve and the associated blood vessels.

Septomaxillae. The paired septomaxillae are small bony elements that form the lateral borders of the nostrils (Fig. S3). Each septomaxilla contacts the nasal dorsally and posterodorsally, the premaxilla anteroventrally, the maxillopalatine posteroventrally, and has a narrow contact with the prefrontal posteriorly. The anterior portion of each bone is rounded in the anterior view and has shaped like an open tube, with dorsal and ventral processes (Fig. S3A). The anterior dorsal process has a ledge-like medial portion. The posterior part of each bone is triangular in lateral view. The ventral edge of each bone is slightly inflected medially. The lateral surface bears numerous neurovascular foramina that are connected with a longitudinal canal on the medial surface of the posterior part of the bone; this canal is open medially and looks like a deep longitudinal groove (Fig. S3B). The neurovascular foramina and the corresponding canal/groove are presumably for the passing of the branch of the ophthalmic division of the trigeminal nerve and the associated blood vessels.

Nasals. Each nasal is a flat and anteroposteriorly elongated bone (the ratio of its maximum width to the midline length is about 0.44) (Fig. S4). Each nasal is a large bone nearly the same size as the parietal. Each bone is trapezoidal, with a tapering posterior process. Each nasal contacts the premaxilla anteriorly, the septomaxilla anterolaterally, the prefrontal laterally, and the frontal posterolaterally. The paired nasals articulate with each other medially along their entire length. Anteriorly, the nasals contribute to the dorsal margin of the nostril. The dorsal surface of each bone bears several neurovascular foramina in the anterior and central portions, presumed to be for branches of the ophthalmic division of the trigeminal nerve and the associated blood vessels (Fig. S4B).

Frontals. Each frontal is a flat bone, longer than wide (the ratio of its maximum width to the maximum length is about 0.46), with its maximum width approximately at the level of mid-length (Fig. S5). Each frontal is somewhat smaller than the nasal and the parietal. The anterior portion is triangular in dorsal and ventral views, with the top oriented anterolaterally. The posterior portion is blunt and rounded in dorsal and ventral views. On the dorsal surface, the anteromedial edge of the anterior portion bears a wide facet for contact with the nasal (Fig. S5B, nf), and the anterolaterally edge of the anterior portion bears a facet for contact with the prefrontal (Fig. S5B, C). On the ventral surface, the anteromedial edge bears a deep groove-like indentation, forming the dorsal part of the canal presumably for the dorsal olfactory nerve (Fig. S5A). Each frontal contacts the nasal anteromedially, prefrontal anterolaterally, and the parietal posteriorly. The paired frontals contact each other along about one-third of their lengths; they are widely separated by the posterior process of the nasal anteriorly and by the anteromedial portions of the parietals posteriorly. The lateral edge of each bone forms the medial border of the temporal fossa. The ridge (Fig. S5C), formed by the lateral edge of each bone, presumably acts as an attachment point for the long adductor of the lower jaw (m. adductor mandibulae longus) (see Lowie et al. 2023). The ventral surface of the lateral edge bears a ventrolaterally oriented, pronounced ridge for contact with the sphenethmoid (Fig. S5C). The dorsal surface of each bone bears several neurovascular foramina, presumed to be for branches of the ophthalmic division of the trigeminal nerve and the associated blood vessels (Fig. S5B).

Parietals. Each parietal is a large trapezoidal and flat bone (Fig. S6); its lateral edge is slightly curved ventrally; parietals are the largest bones of the cranial roof. Each parietal is anteroposteriorly elongated (the ratio of its maximum width to the midline length is about 0.48), with the maximum width approximately at the level of the posterior third of its length. Each parietal is slightly larger than the nasal. Each parietal contacts the frontal anteriorly, the os basale laterally, and overlaps the tip of the dorsomedial process of the sphenethmoid anteromedially and the os basale posteriorly. The paired parietals contact with each other medially along their entire lengths.

The anterior part of the dorsal surface of each bone has a wide, semicircular facet for the overlying frontal (Fig. S6B). Posteriorly to the frontal facet, the dorsal surface divides into the flat medial area, tapering posteriorly, and the slightly curved lateral area. The anterolateral edge of the flat medial area bears a rounded ridge. This ridge is a direct continuation of the similar ridge on the frontal, extending the presumed attachment point for the long adductor of the lower jaw. The lateral curved area is presumed to be for the attachment of the m. depressor mandibulae (Lowie et al. 2023). The lateral edge of each bone is flattened; it faces ventrally and bears a facet for contact with the os basale. The anteromedial part of the ventral surface of each bone bears a small facet for contact with the sphenethmoid. The posterior part of the ventral surface of each bone has a wide facet for the os basale.

Squamosals. Each squamosal is a large (about the same size as the parietal), trapezoidal, and slightly convex bone that forms most of the lateral surface of the temporal region of the cranial roof (Fig. S7). Each squamosal contacts the circumorbital anterodorsally, the maxillopalatine anteroventrally, and overlaps the quadrate posteroventrally. The dorsal edge of each squamosal forms the lateral border of the temporal fossa. The anteroventral part of the lateral surface of each bone has a triangular socket-like facet for contact with the posterior process of the maxillopalatine. The posterior part of the lateral surface has a wide depression, presumably for the attachment of the m. depressor mandibulae (Lowie et al. 2023). On the lateral surface, there are several foramina in the central part and along the anterior border of each bone, dorsally to the maxillopalatine facet (Fig. S7A). These foramina are presumed to be for the branches of the mandibular division of the trigeminal nerve and the associated blood vessels. The posterior part of the medial surface of the bone has a wide facet for presumed contact with the processus ascendens of the quadrate that is bordered by a ridge anteriorly (Fig. S7B).

Prefrontals. Each prefrontal is a flat and relatively small bone (slightly larger than the septomaxilla), elongated anteroposteriorly (the ratio of its maximum width to the midline length is about 0.3) (Fig. S8). Each prefrontal contacts the septomaxilla anteriorly, the nasal anterodorsally, the frontal dorsally and posterodorsally, and the maxillopalatine anteroventrally, ventrally, and posteroventrally. The ventral edge of each bone is thickened and has a facet for contact with the pars facialis (namely, an elongated sharp process) of the maxillopalatine. On the left side of the cranium, each prefrontal appears to be fused with this process.

Circumorbitals. Each circumorbital is a small, crescent-shaped, and curved bone, widely open ventrally, that forms the dorsal and posterodorsal border of the orbital aperture (Fig. S9); it was referred to as postfrontal in some earlier works (Taylor 1969a); these crescentic bones are unique to Ichthyophiidae among caecilians (Nussbaum 1977, 1979; Wilkinson and Nussbaum 1999, 2006). Each circumorbital loosely contacts the squamosal posterodorsally. The anterior and posterior ends of each bone are uneven and bear small notches; circumorbitals are symmetric in shape.

Vomers. Each vomer is a large (about the same size as the nasal) and flat dentigerous bone that forms most of the anterior part of the palate (Fig. S10). Each vomer is subtriangular in the ventral view, with the maximum width at the level just behind the vomerine foramen. Each vomer contacts the premaxilla anteriorly, the maxillopalatine anterolaterally and laterally, the sphenethmoid dorsally, and overlaps the os basale posteriorly. The paired vomers articulate with each other medially along their entire length. Each vomer forms the medial border of the choana. The posterior edge of each vomer is situated slightly anteriorly to the center of the palate (about 44–45% of the cranium length).

The ventral surface of each vomer bears an anterior part of the lingual (= vomero-palatine) tooth row (with 10 functional teeth on the left and 11 on the right bone) that is curved along the anterolateral edge of the bone and is parallel to the labial tooth row. There is a large vomerine foramen in the central part of the ventral surface (Fig. S10A).

The dorsal surface of each vomer is complex. The anterior part bears an anterodorsally oriented premaxillary process that is in articulation with the pars dorsalis of the premaxilla (Fig. S10B). The base of the process is perforated by a longitudinal canal presumably for the ophthalmic division of the trigeminal nerve and the associated blood vessels that continue into the corresponding canal in the premaxilla. Posteriorly, there is a longitudinal groove that runs parallel to the medial line and almost reaches the level of the middle of each bone. Laterally and posterolaterally to this groove, the dorsal surface has a bulge. This bulge is limited posterolaterally by the curved crest. The bulge contains the inner vomerine cavity that opens ventrally through the vomerine foramen and posteriorly and posterolaterally through the neurovascular canals. The curved crest separates a large concavity from the bulge (Fig. S10B). As mentioned above, this concavity is dorsally bordered by the anterior process of the palatine part of the maxillopalatine and the curved crest on the dorsal surface of the vomer and presumably contains the ‘Choanenschleimbeutal’ sensu Sarasin and Sarasin (1887–1890) and Palakkool et al. (2022).

Pterygoids. Each pterygoid is a small and anteroposteriorly elongated bone (Fig. S11). Each pterygoid contacts with the maxillopalatine anterolaterally and with the quadrate posterodorsally. Each pterygoid forms the lateral border of the mediopalatinal cavities (= interpterygoid vacuities) and the medial border of the lower temporal fossae. Each pterygoid consists of the anteromedially oriented anterior process, the dorsomedially oriented basal process, and the posteroventrally oriented posterior process (Fig. S11A). In lateral view, the ventral edge of the posterior process is situated barely below the level of the tips of the teeth of the labial (premaxillary-maxillary) row. The basal process and the posterior process comprise the expanded and subvertical portion of each bone. The anterior process is tapering anteriorly, and it contacts the posterior portion of the palatal part of the maxillopalatine anterolaterally. The basal process is low, and its posterodorsal part contacts the quadrate. There is no pronounced contact between the basal process of each pterygoid and the os basale.

Quadrates. Each quadrate is a relatively small bone that forms a joint with the lower jaw (= the articular facet for the pseudoangular) (Fig. S12). Each quadrate contacts with the pterygoid anteroventrally and is largely overlapped by the squamosal dorsolaterally. Each quadrate consists of two distinct parts: the condylar portion and the moderately elongated processus ascendens (sensu Palakkool et al. 2022) (Fig. S12A, E). The condyle of each quadrate is lateromedially narrowed, with a depressed middle part. The condyle faces ventrolaterally. The long axis of the condyle is situated at an angle of 50° to the median axis of the skull. The processus ascendens has a relatively wide base and an ascending part narrowing dorsomedially (Fig. S12B). The dorsolateral surface of the processus ascendens has a wide facet for contact with the medial surface of the squamosal. There are three processes associated with the base of the processus ascendens: the elongated anteromedial process for the contact with the pterygoid, the posterolaterally oriented processus oticus (sensu Palakkool et al. 2022), and the short anterolateral process (= processus jugalis sensu Ramaswami 1941) (Fig. S12B, E). The anteromedial process is shelf-like; its ventral surface is coarse and forms a shallow wide groove for a syndesmotic connection with the lateral flanges of the os basale and the pterygoid. The processus oticus has a very short base, which is semicircular in the outline, and a thin posterior flange on its dorsal surface, which covers the cartilaginous connection between the processus oticus and the columellar process of the stapes. The anterolateral process has a wave-like outline (Fig. S12A, B) forming two distinct notches for the syndesmotic connection with the lateral edge of the squamosal.

Sphenethmoid. The sphenethmoid consists of the laterally expanded main body, the elongated anteriorly projecting nasal septum, and the posteriorly projecting dorsomedial process (Fig. 3). The main body accounts for slightly less than half of the total length of the sphenethmoid. The lateral wall of the main body has a relatively long and broad anterolateral process, which forms a flat facet for contact with the frontal. Ventrally to the process, the lateral wall forms a short canal with an anterodorsally oblique posterior margin presumably for the deep optic branch of the trigeminal nerve (Fig. 3B). The posterior edge of the lateral wall, behind this canal, bears one to three foramina of different sizes. Two dorsally prominent structures referred to as sola nasi by Maddin et al. (2012) are present but weakly ossified (Fig. 3A, E); these structures were referred to as processus conchoides or eminentia olfactoria by earlier authors (e.g., Sarasin and Sarasin 1887–1890). The ventral surface of the main body bears a deep medial groove for contact with the parasphenoid rostrum of the os basale. The posterior margin of the floor of the sphenethmoid is deeply incised, exposing a large area of the os basale ventral to it. The nasal septum dorsoventrally tapers slightly towards the tip (Fig. 3B). Its dorsal sutural surface for contact with the nasals has a uniform shape along the whole length of the septum. The ventral sutural surface of the septum has a small anterior expansion and forms a wide lateral flange in its posterior half, which fuses with the processus conchoides, forming a deep cavity presumably for the ventral branches of the olfactory nerves. The foramina presumed to be for the ventral branches of the olfactory nerves are located somewhat close to the midline of the sphenethmoid. The dorsal branches of the olfactory nerves are presumably not confined within the anterior wall of the sphenethmoid and pass through a cavity formed by its dorsal edge. The dorsomedial process has the same width as the nasal septum and does not extend behind the level of the medial wall (Fig. 3B).

Os basale. The os basale consists of the main body, formed by the fusion of the occipital and otic bones, the ossified antotic walls, and the ventral part formed by the parasphenoid (Fig. 3). The parasphenoid rostrum ends in a sharp point, not reaching the base of the nasal septum (Fig. 3B). The ventral surface of the rostrum bears a W-shaped facet for contact with the vomers. The body of the parasphenoid bears distinct lateral flanges, which form the surface for the basicranial joint. Posteriorly to the basicranial joint, a large foramen presumably leading to the carotid artery canal system is present on the ventral surface of the parasphenoid. Upon entering the parasphenoid, the carotid canal is immediately divided into two grooves, which run anteriorly along the medial and the lateral sides of the antotic wall. The posterior edge of the parasphenoid is reaching the level of the base of the condyles and bears a pair of robust semicircular ridges running towards the lateral margins of the otic capsules. The dorsal surface of the parasphenoid bears distinct depressions for the cerebral hemispheres and the hypophysis (Fig. 3A).

The antotic wall has a weak incisure presumably for the ophthalmic nerve on its anterior edge and a large subcircular foramen in its posterior portion, through which the branches of the trigeminal nerve presumably exit the braincase (Fig. 3B, C). The dorsoposterior edge of this foramen has a large incisure, presumably for passage of the dorsal vein. The otic region has well-developed dorsal processes, which meet each other on the midline without fusion (Fig. 3A). Anterior to the processes, there is a deep facet for the contact with the parietal. The fenestra vestibuli is large, circular, and slightly incises the lateral margin of the otic capsule when viewed posteriorly (Fig. 2D) (for comparison of this character see Maddin et al. 2012). Anterior to the fenestra vestibuli, the dorsal surface of the os basale bears a small anterolateral flange. The condyles are semicircular and lie far apart from each other (Fig. 2D).

The medial wall (= inner surface) of the otic capsule region bears a number of foramina. The anterior-most foramen is presumed to be for the trunk of the facial nerve (Fig. 3C). Upon entering the foramen, the nerve is presumably almost immediately divided into the palatal branch, which passes anteriorly into the medial carotid canal, and the hyomandibular branch, which passes posteriorly above the stapes. Immediately posterior and dorsal to this foramen, there is a much smaller foramen leading to the ampulla of the anterior semicircular canal—presumably for the anterior auditory nerve (Fig. 3C). The medial auditory nerve presumably enters the ear capsule through the four similarly sized small foramina, which chaotically perforate the medial wall behind the level of the facial foramen (Fig. 3C). Posterior to these small foramina, there is a slightly larger and more dorsally positioned foramen presumably for the posterior auditory nerve (Fig. 3C). Posterior to the auditory nerves foramina, there is the perilymphatic foramen, which is the largest and has an oval outline (Fig. 3C). Dorsally to the perilymphatic foramen, almost at the base of the dorsal process of the os basale, there is an endolymphatic foramen, which has an anterodorsally oblique outline. The jugular foramen opens laterally at the base of the condyle and can be seen in the lateral view (Fig. 3B, C).

Stapes. Each stapes is a small bone that overlaps the fenestra ovalis (Fig. S13). Each stapes consists of a footplate and a columellar process (= style) (Fig. S13A). The footplate is oval and dorsoventrally compressed. There is a small foramen perforating the footplate dorsally to the base of the columellar process. The columellar process is anterodorsally oriented and extends in the direction of the processus oticus of the quadrate (but does not contact it). The columellar process ends in a depression indicating that in life its distal-most end was finished in cartilage. The base of the columellar process is dorsoventrally expanded, forming a robust circular ridge (Fig. S13C, E). The stapedial foramen is absent; however, there is a deep groove formed by the ventral part of the footplate and the ridge of the base of the columellar process (Fig. S13E); the stapedial artery presumably passes through this groove.

Pseudodentaries. The paired elongated dentigerous pseudodentaries articulate with each other medially in the symphyseal area to form the anterior tip of the mandible (Fig. S14). The posterior half of each bone is overlapped by the pseudoangular medially (lingually) (Fig. S14C). Each bone is the widest in the anterior presymphyseal part (in dorsal and ventral views). Each bone has a relatively straight occlusal margin and a nearly similar depth along its entire length except for the tapering posterior end. Each pseudodentary has a pronounced ‘splenial’ ridge (sensu Palakkool et al. 2022) bearing a lingual inner mandibular tooth row at its anterior and central portions (Fig. S14C). The posterior portion of the ‘splenial’ ridge lacks teeth. There is a distinct medial ridge at the level of the posterior half of the inner mandibular tooth row, which forms a facet for contact with the anterior process of the pseudoangular.

The labial outer mandibular tooth row is longer than the lingual inner mandibular tooth row and terminates at the level of the posterior end of the ‘splenial’ ridge. There are numerous neurovascular foramina (Fig. S14A) that presumably represent the openings of the inner canals for the branches of the mandibular division of the trigeminal nerve (external and alveolar branches) and the alveolar branch of the facial nerve (Norris and Hughes 1918).

The external branch of the mandibular division of the trigeminal nerve (r. mandibularis externus) presumably enters the pseudodentary through two large foramina on the lingual side posterior to the level of the posterior ends of the outer and inner mandibular tooth rows. After passing through the bone, it exits through numerous foramina at the base of the teeth on the lingual side and along the entire length of the pseudodentary on the labial side (Fig. S14A, C). The combined alveolar branches of the mandibular division of the trigeminal nerve and of the facial nerve presumably pass in a groove below the facet for contact with pseudoangular.

Pseudoangulars. Each pseudoangular is an elongate bone that overlaps the pseudodentary medially (lingually) (Fig. S14C). Each pseudoangular consists of the anterior process, the processus condyloideus, the processus internus, and the retroarticular process. The anterior process is tapering anteriorly. The processus condyloideus (= condyle) is situated on the dorsal margin of the bone, anterior to the retroarticular process, and forms a joint with the cranium (= the articular facet for the quadrate) (Fig. S14A). The processus internus is a small lingually oriented projection on the lingual surface of the bone, ventrally to the processus condyloideus (Fig. S14B). Anteriorly to the processus condyloideus, the dorsal margin of each bone has a high curved ridge and a high dorsal projection. The posterior end of the curved ridge terminates between the processus condyloideus and the processus internus, and the anterior end of this ridge is fused with the dorsal projection. The anterior end of the curved ridge and the dorsal projection border a large foramen dorsolaterally (Fig. S14B). This is the foramen of the canalis primordialis presumably for the passing of the mandibular division of the trigeminal nerve and the mandibular artery. Ventrally to the foramen of the canalis primordialis, the labial surface of the bone bears a large foramen presumably for the passage of the intermandibular branch of the mandibular division of the trigeminal nerve (Fig. S14C).

The retroarticular process is elongated, with its rounded posterior end oriented nearly dorsally (Fig. S14F). At the base of the retroarticular process, posterior to the processus internus, there is a large oblique foramen presumably for the passage of the alveolar branch of the facial nerve (Fig. S14F).

Atlas. The atlantal centrum is very short (Fig. 4). The anterior cotyles are large and have a crescent shape in the anterior view (Fig. 4A). The articular surfaces of the anterior cotyles have a complex, saddle-like shape, with a larger lateral portion facing anterodorsally and a smaller medial portion facing ventrolaterally. The lateral portions are inclined dorsally (at about 35° from the vertical plane), and their articular surfaces are nearly flat (Fig. 4C, F). In the anterior view, the medial portions are situated at about 50° from the horizontal plane. The left and right anterior cotyles are widely separated and do not contact along the midline. The intercotylar tubercle (= odontoid process; = tuberculum interglenoideum), the transverse processes (= rib-bearers), the ventrolateral ridges, the lateral alar ridges, the lateral depressions, and the hypapophysis are absent as in other caecilians. The posterior cotyle is nearly circular in posterior outline. A large spinal nerve foramen perforates the base of the neural arch. On the left side of the atlas, this foramen is paired (which is also the case for all of the trunk vertebrae) (Fig. 4C).

The atlantal neural arch is high. In the lateral view, the anterior edge of the neural arch is oblique; its dorsal part projects anteriorly beyond the level of the anterior edge of the anterior cotyles. The neural spine is not well pronounced, but there is a very low and swollen median ridge. The anterior part of the dorsal surface of the neural arch is smooth. The posterior part of the dorsal surface of the neural arch is lower than the anterior part and bears a low median depression. The postzygapophyseal processes are oriented ventrolaterally (at approximately 20° from the midline) and are elongate and narrow in ventral outline. The neural canal is broad, high, and nearly round in its posterior outline. The inner surface of the base of the neural arch bears a pair of spinal cord supports that extend medially into the neural canal (Fig. 4B). The dorsal surface of the centrum bears a small medial outgrowth at the level of the spinal cord supports.

Sequence variation. A total of 3967 aligned base pairs (1143 bp from cyt b, and 2824 bp from the fragment containing 12S rRNA and 16S rRNA genes) were obtained for our study. Sequence characteristics, including the estimated transition/transversion bias, nucleotide frequencies, and suggested best-fit models of DNA evolution for each genetic marker and partition, are summarized in Table S3. We deposited the newly obtained sequences in GenBank (see Table S2 for the accession numbers and voucher details; the final alignment is available upon request).

Phylogenetic relationships.ML and BI analyses of our mtDNA alignment recovered trees with identical topologies, except for a few relationships that do not affect our interpretations of the results (Fig. 5). Our updated matrilineal genealogy was generally consistent with the earlier works of Nishikawa et al. (2012a), Wilkinson et al. (2014), Geissler et al. (2015), and Rao et al. (2024), and unambiguously confirmed the reciprocal monophyly of two major groups within the Ichthyophiidae, corresponding to the genus Ichthyophis (100/1.0; hereafter, branch support values are given for MLUFBS and BIPP values, respectively) and Uraeotyphlus (100/1.0). The position of ‘Ichthyophis’ bombayensis Taylor, 1960 (including its junior synonym ‘I.’ peninsularis Taylor, 1960) is especially interesting as this species supposedly was treated as a sister group to all other Uraeotyphlus species (100/1.0) in accordance with earlier studies of the group (e.g., Nishikawa et al. 2012a; Wilkinson et al. 2014). This phylogenetic position recently prompted Dubois et al. (2021) to formally transfer this species to Uraeotyphlus, a taxonomy that is now widely accepted (Frost 2025). In agreement with the earlier data (Gower et al. 2002; Nishikawa et al. 2012a), the striped and unstriped species of Ichthyophiidae were intermixed with each other and did not form respective monophyletic groups (Fig. 5).

Intragenerically, our analyses revealed four deeply-diverged clades with unclear phylogenetic relationships (see Fig. 5). Clade A (100/1.0) regrouped two species from Northeastern India: striped I. moustakiusKamei et al., 2009, and I. cf. garoensis Pillai & Ravichandran, 1999, while Clade B (100/1.0) included two species from Sri Lanka: striped I. glutinosus (Linnaeus, 1758) and unstriped I. orthoplicatus Taylor, 1965; the phylogenetic placement of the clades A and B with respect to the remaining Ichthyophis species remained unclear. Clade C (89/0.93) included two striped taxa from southern peninsular India, i.e., I. tricolor Annandale, 1909 and I. cf. beddomei Peters, 1880; this clade received low support in ML analysis and was moderately supported in BI analysis; it was reconstructed as a sister group of Clade D, which joined all the remaining species of Ichthyophis, though only with low branch support values (81/0.90).

Clade D (99/1.0) was well-supported and regrouped Ichthyophis taxa from East and Southeast Asia (including northeastern India). It was further subdivided into three major subclades (Fig. 5). Subclade D1 (100/1.0) included three unstriped species of Ichthyophis from Indochina and southern China: I. chaloensis, I. yangi, and Ichthyophis sp. from northern Vietnam; phylogenetic relationships 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. khumhziKamei et al., 2009; and I. multicolorWilkinson 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 Ichthyophis 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.