Tylototriton koliaensis, Poyarkov & Nguyen & Le & Le, 2024

Tylototriton koliaensis sp. nov.

urn:lsid:zoobank.org:act:DB001E6F-71F6-4964-929E-7E2308EF0FB5 ( Fig. 2 View Figure 2 , Fig. 5–8 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ; Tables 4–5 View Table 4 View Table 5 )

Chresonymy

Tylototriton vietnamensis (non Tylototriton vietnamensis Böhme, Schöttler, Nguyen & Köhler, 2005 ) — Böhme et al. (2005, in part); Weisrock et al. (2006); Nguyen et al. (2009, in part); Shen et al. (2012). Tylototriton cf. vietnamensis — Stuart et al. (2010); Gu et al. (2012). Tylototriton asperrimus (non Tylototriton asperrimus Unterstein, 1930 ) — Yuan et al. (2011, in part). Tylototriton ziegleri (non Tylototriton ziegleri Nishikawa, Matsui & Nguyen, 2013 ) — Nishikawa et al. (2013 a, in part); Phimmachak et al. (2015, in part); Hernandez (2016, in part); Jiang et al. (2017, in part); Wang et al. (2018, in part); Than Zaw et al. (2019, in part); Lyu et al. (2021, in part); Liu et al. (2022, in part); Poyarkov et al. (2021 a, in part); Poyarkov et al. (2021 b, in part); Dufresnes & Hernandez (2022, in part); Nguyen et al. (2022, in part).

Holotype

Adult female ZMMU A-8157 (field number NAP-07305), collected from Kolia Organic Farm within the Phia Oac-Phia Den National Park , Thanh Cong Commune, Nguyen Binh District, Cao Bang Province, Vietnam (22.562423°N, 105.862566°E; 1025 m a.s.l.) by Nikolay A. Poyarkov and Son Xuan Le on June 9, 2017. GoogleMaps

Paratypes (n=8)

Eight adult males: ZMMU A-8158–8165 (field numbers NAP-07306–07313), all collected from Kolia Mountain Pass within the Phia Oac-Phia Den National Park , Thanh Cong Commune, Nguyen Binh District, Cao Bang Province, Vietnam (22.59938°N, 105.88328°E; 1341 m a.s.l.) by Nikolay A. Poyarkov on June 9, 2017 GoogleMaps .

Diagnosis

The new species is assigned to the genus Tylototriton and to the subgenus Yaotriton based on its phylogenetic placement and the following combination of morphological attributes: granules on dorsal skin present, dorsolateral bony ridges on head present, dorsolateral series of knob-like warts (rib nodules) present, and quadrate spine absent ( Fig. 5 View Figure 5 ). Tylototriton koliaensis sp. nov. is differentiated from all other members of the subgenus Yaotriton by the following combination of morphological characters: medium body size (SVL: 56.0– 61.4 mm in males; 70.8–73.0 in females); snout bluntly truncate in dorsal view and rounded in lateral profile; head large, as wide as long; gular fold weak; skin rough with fine granules laterally, granules getting smaller at mid-dorsum; supratemporal bony ridges on head wide, protruding, beginning at the snout; sagittal ridge on head indistinct; limbs very long and thin; tips of forelimbs and hindlimbs greatly overlapping when adpressed along body; vertebral ridge prominent, glandular and not segmented or slightly segmented; tail base widened; rib nodules 10–12, small, knob-like, rounded, protruding, arranged in two irregular dorsolateral series intermittent with smaller warts; dorsal background coloration black, ventral coloration dark-gray to black, limbs black; finger and toe tips, parts of soles and palms, and vent bright orange, orange stripe continuing along the ventral edge of tail to the tail tip; bright orange markings on the parotoids or rib nodules absent; vomerine teeth arranged in two gently curving diverging ˄-shaped series, converging anteriorly with their anterior corner forming a sharp angle. At the molecular level, it differs by private nucleotides at the mitochondrial genes ND2 and 16S, as well as in multiple loci across the nuclear genome.

Description of holotype

A medium-sized female specimen (SVL: 73.0 mm) in a good state of preservation ( Fig. 6 View Figure 6 ). Head as wide as long (HW/HL: 0.99), large (HL/SVL: 0.20), notably wider than body; rounded hexagonal in shape in dorsal view ( Fig. 6C View Figure 6 ), slightly depressed, gently sloping in profile ( Fig. 6E View Figure 6 ); snout comparatively long and wide, three times longer than eye (UEW/SL: 0.38), bluntly truncated in dorsal and ventral views ( Fig. 6C View Figure 6 ), obtusely rounded in lateral profile ( Fig. 6E View Figure 6 ), almost not projecting beyond lower jaw ( Fig. 6E View Figure 6 ); nostrils located on lateral surfaces of the anterior margin of snout, located notably closer to snout tip than to eye (NSD/ON: 0.68), with antero-lateral orientation, not visible from dorsal view ( Fig. 6C View Figure 6 ); labial fold weakly developed in the posterior one third of jaw length ( Fig. 6C View Figure 6 ); tongue oval, attached to anterior floor of mouth, free posteriorly and laterally; vomerine teeth arranged in two gently curving diverging ˄-shaped series, converging anteriorly but not reaching choanae, with their anterior corner forming a sharp angle, each vomer bearing 34 vomerine teeth ( Fig. 7 View Figure 7 ); parotoids distinct, large, beanshaped, slightly projecting posteriorly ( Fig. 6C View Figure 6 ); dorsolateral supratemporal bony ridges on head wide, slightly protruding, beginning at the snout, continuing to the anterior corner of orbit and further posteriorly to the anterior end of parotoid, posterior ends of supratemporal ridges bulge-shaped, slightly curved inside ( Fig. 6C View Figure 6 ); sagittal bony ridge on head indistinct ( Fig. 6C View Figure 6 ); gular fold weak and almost indistinct ( Fig. 6D View Figure 6 ).

Body habitus moderately stout ( Fig. 6A–B View Figure 6 ); costal folds absent; vertebral middorsal ridge distinct, wide, rough, and glandular, not segmented, running from occiput region to sacrum and further continuing on the anterior part of tail ( Fig. 6A View Figure 6 ). Rib nodules distinct, small, rounded, forming knoblike glandular warts, arranged in two regular longitudinal series on dorsolateral surfaces of dorsum, with 12 nodules placed on each side of body from the area posterior to axilla to the level of the posterior vent margin (tail base) ( Fig. 6A View Figure 6 ). Rib nodules not in contact with each other, their size varies from rounded, distinctly knob-like anteriorly to smaller and of more irregular shape posteriorly; rib nodules intermittent with smaller irregular granules, which makes the dorsolateral series slightly ragged in shape ( Fig. 6A View Figure 6 ; Fig. 8A View Figure 8 ).

Limbs comparatively long and thin ( Fig. 6A–B View Figure 6 ); forelimbs slightly shorter than hindlimbs; relative lengths of limbs: FLL/SVL: 0.31, HLL/SVL: 0.34; fore- and hindlimbs overlapping when adpressed towards each other along body; fingers and toes well developed ( Fig. 6F–G View Figure 6 ), free of webbing or folds; fingers four, comparative finger lengths: 4FL<1FL=2FL<3FL; toes five, comparative toe lengths: 5TL<1TL<2TL<4TL<3TL. Tail comparatively short, subequal to body length (TAL/SVL: 0.97); laterally compressed along entire length, sharply tapering posteriorly; lateral grooves on tail indistinct; dorsal tail fin comparatively low, starting at anterior one fourth of tail length, becoming higher and more distinct posteriorly, with maximal tail height at the middle of tail length, dorsal tail fin smooth; ventral tail ridge smooth; tail tip pointed.

Dorsally skin rough, granulated, with numerous small granules and larger warts present on lateral surfaces of head, body, and tail ( Fig. 6E View Figure 6 , 8A View Figure 8 ); granules getting less dense and notably smaller on dorsum between the dorsolateral rows of rib nodules and at the head basis ( Fig. 6A View Figure 6 , Fig. 8A View Figure 8 ); ventral surfaces smoother with smaller granules arranged in transverse striations ( Fig. 6B View Figure 6 ); throat with numerous tiny flat tubercles ( Fig. 6D View Figure 6 ); surfaces of head ridges and middorsal vertebral ridge rough; limbs dorsally with numerous tiny tubercles, volar and plantar surfaces of hands and feet with tiny grooves forming reticulated pattern ( Fig. 6F–G View Figure 6 ). Flattened outer metacarpal and inner metacarpal tubercles on palms ( Fig. 6F View Figure 6 ); flattened inner metatarsal tubercle distinct on soles ( Fig. 6G View Figure 6 ). Cloacal region slightly swollen, vent as a shirt longitudinal slit, vent edges with numerous small transverse folds ( Fig. 6H View Figure 6 ).

Coloration of holotype

In life, dorsum uniformly black ( Fig. 8A View Figure 8 ); venter slate-black to dark gray, slightly lighter than dorsum; limbs black; rib nodules and vertebral ridge dark brownishblack; fingers and toes brownish-orange (ventrally on all of their lengths, dorsally only tips of the digits); parts of soles and palms corresponding to outer and inner metacarpal and inner metatarsal tubercles reddish-orange, vent and ventral tail ridge bright orange, separated by a dark patch at the beginning of the ventral tail fin. After seven years in preservative, dorsal coloration slightly faded, becoming dark brown, while orange markings on limbs and tail faded to cream ( Fig. 6 View Figure 6 ).

Measurements of the holotype

Measurements of the holotype are given in Table 4 View Table 4 . Additional morphometric characters (all in mm) as follows. ICD: 9.4; CW: 15.7; NSD: 2.8; 1FL: 4.6; 2FL: 4.6; 3FL: 5.1; 4FL: 2.8; 1TL: 3.0; 2TL: 4.4; 3TL: 5.6; 4TL: 5.4; 5TL: 2.9.

Variation

All males are generally similar in morphology and coloration and agree with the holotype description in coloration; variation of morphometric characters among the type series is given in Table 4 View Table 4 . Variation of the dorsal coloration in the type series in life is presented in Fig. 8 View Figure 8 : all specimens show dark black to slate-black dorsal coloration without any light markings. The number of rib nodules in males varies from 10 to 12 (mean: 11.1 ± 1.1; n = 8). All paratype males have a notably smaller body size than the single female holotype (SVL: 56.0– 61.4 mm vs. 73.0 mm); their body habitus is more slender, tails are notably longer than body (TAL/SVL: 1.22 ± 0.07 vs. 0.97) and the head is relatively bigger and wider than in the female specimen (MXHW/SVL: 0.29 ± 0.01 vs. 0.24). Male ZMMU A-8158 has a more segmented middorsal ridge ( Fig. 8B View Figure 8 ) than other specimens, in which the middorsal ridge was continuous and almost not segmented. Some noticeable variation was also observed in the shape of the vomerine tooth series: while most paratypes show gently curving, diverging ˄-shaped vomerine tooth series, the paratype male ZMMU A-8158 has a ∩-shaped anterior part of the series with a blunt angle in the middle of the series length.

Osteological description

The following description of skull morphology is based on the tomographic data obtained for the holotype ZMMU A-8157, female ( Fig. 5 View Figure 5 ); the associated metadata are provided in the MorphoSource project 4.

Cranium

Overall, the cranium of Tylototriton koliaensis sp. nov. is heavy and strongly ossified; some elements on the dorsal surfaces of the cranium show traces of hyperossification; the number of cartilaginous elements is comparatively low; the skull shape is acute trapezoidal in dorsal ( Fig. 5A View Figure 5 ) and ventral views ( Fig. 5B View Figure 5 ) and close to right-angled trapezoidal in lateral view ( Fig. 5C View Figure 5 ); snout distinctly truncate in dorsal and ventral views.

Premaxilla

The premaxilla is a paired bone, slightly arched dorsally, bearing teeth; the right premaxilla bears 18, and the left maxilla bears 16 teeth ( Fig. 5B View Figure 5 ). The premaxilla dorsally contributes to the anterodorsal internasal fontanelle (cavum internasale), ventrally contributes to the anteromedial fenestra, and anteriorly forms a ventromedial part of the external nares. Anterior and dorsal surfaces of premaxillary bones are granular with numerous small ossifications ( Fig. 5A View Figure 5 ). The premaxilla contacts the prefrontal, nasal, and maxillary bones posteriorly and contributes to the anterior part of the nasal cavity.

Maxilla

The paired maxilla runs laterally from the nasal capsule to the level of the otic region, bearing teeth on its anterior half; the right maxilla bears 21, and the left maxilla bears 19 teeth ( Fig. 5B View Figure 5 ). Dorsal and lateral surfaces of the maxillary bone are rough and tuberous. The maxilla contacts premaxillar, prefrontal, and vomerine bones anteriorly, and pterygoid and quadrate posteriorly. Anteriorly maxilla forms an anterolateral part of the external nares and dorsomedially contributes to the orbit cavity.

Prefrontal

The prefrontal (sensu Huxley 1875) is a paired bone located between nasal anteriorly, maxillary laterally, prefrontal posteriorly, and frontal medially. The dorsal surface of the prefrontal is densely tuberous, forming the medial part of the supratemporal cranial crests ( Fig. 5A View Figure 5 ). The prefrontal contributes posteriorly to the orbit cavity and ventrally to the nasal cavity.

Nasal

The paired nasals are located anteriorly; they contact premaxillary anteromedially, maxillary anterolaterally, prefrontal posterolaterally, and frontal posteromedially ( Fig. 5A View Figure 5 ). The nasal forms the dorsal part of external nares and contributes ventrally to the nasal cavity. Dorsal surfaces of nasal bones granular, forming the anterior part of the supratemporal cranial crests. A pair of nasals meet anteriorly and form internasal fontanelle medially.

Frontal

The frontal is a paired bone, forming the large portion of the cranial roof. The frontal is located between the nasal anteriorly, prefrontal anterolaterally, orbitosphenoid ventrally, squamosal posterolaterally, and parietal posteromedially. A pair of frontals meet along the midline forming a suture ( Fig. 5A View Figure 5 ), anteromedially contribute to the internasal fontanelle, and ventromedially to the orbit cavity. A broad and massive posterolateral process of each frontal contacts squamosal forming fronto-squamosal arc ( Fig. 5A View Figure 5 ). The frontal bones' dorsal surface is tubercular, with ossified granules becoming denser posteriorly. The sagittal cranial crest not pronounced; posterolaterally frontal highly tuberous, contributing to the medial part of supratemporal cranial crest; anteromedially frontal is almost smooth.

Parietal

The paired parietals meet each other along their length, forming a suture ( Fig. 5A View Figure 5 ); they contact frontal anteriorly, orbitosphenoid anteroventrally, occipito-otic complex laterally and posteriorly, and touch squamosal dorsolaterally. The dorsal surface of the parietal is depressed and smooth posterolaterally, elevated, and granular anteromedially; however, no sagittal crest is formed.

Vomer

The vomer is a paired bone, covering the cranial base between the internal nares and bearing vomerine teeth forming two gently curving, diverging ˄-shaped series ( Fig. 5B View Figure 5 ); each vomer bears 34 teeth. The vomer is located between prefrontal anteriorly, maxillary anterolaterally, orbitosphenoid dorsomedially, and parasphenoid posteriorly, covering the latter with its posterior process ventrally. A pair of vomers forms an anteromedial fenestra, meets medially behind this fenestra, and contributes to the nasal cavity anterodorsally and orbit cavity laterally.

Parasphenoid

The single parasphenoid is a large sword-shaped bone plate, covered with the posterior portion of the vomers ( Fig. 5B View Figure 5 ). The parasphenoid contacts vomers anterolaterally, orbitosphenoid laterally, occipito-otic complex laterally and posteriorly, and the posteromedial portion of the parasphenoid contributes to the ventral edge of the foramen magnum.

Squamosal

The paired squamosals exhibit a T-shaped structure with massive anterior process and stem portion, which is located at the right angle to the jaw arc ( Fig. 5A, C View Figure 5 ). The anterior process of the squamosal has a rough and tuberous dorsal surface; it contacts the posterolateral process of the frontal anteriorly, forming the fronto-squamosal arc. Squamosal is barely attached to parietal with a small dorsomedial portion, while mostly being in contact medially with occipito-otic complex. The stem portion of the squamosal contacts quadrate ventrally and pterygoid ventromedially. The squamosal is densely tuberous dorsally, with a well-developed posterior process, forming the posterior part of the supratemporal cranial crest ( Fig. 5C View Figure 5 ).

Quadrate

The quadrate is a small paired bone contacting squamosal dorsally, pterygoid medially, maxillary anteriorly, and forming the jaw joint with the mandible ventrally ( Fig. 5B View Figure 5 ). The lateral surface of the quadrate is granular.

Pterygoid

The paired pterygoids are triangular in shape. The processus oticum is directed dorsomedially, with a prominent excavation in lateral view ( Fig. 5B View Figure 5 ). The processus palatinum is directed anterolaterally with its ventral margin rounded, bearing a bulb-shaped pterygoid groove in lateral view ( Fig. 5C View Figure 5 ). The processus quadratum of the pterygoid is directed posteriorly.

Orbitosphenoid

The orbitosphenoid is a single bone that forms the anterior portion of the neurocranium, bearing on each side a large foramen opticum that opens in its posterior part ( Fig. 5C View Figure 5 ). The orbitosphenoid contacts prefrontal dorsally, parietal posterodorsally, occipito-otic complex posteriorly, parasphenoid posteroventrally, and vomer ventrally and contributes to the nasal cavity anteriorly and orbit cavity laterally.

Occipito-otic complex

The occipito-otic complex is a paired structure comprised of fused prootic and exoccipital bones. This complex contacts squamosal dorsolaterally, pterygoid laterally, parasphenoid ventrally, parietal anteromedially, and narrowly touches orbitosphenoid anterolaterally. A pair of these structures meet dorsally, behind the parietals ( Fig. 5A View Figure 5 ). These bones form the lateral part of the otic capsule, fenestra ovalis, foramen magnum, and bear paired occipital condyles.

Stapes

The paired stapes encompass a disc-like footplate and stylus. The footplate was barely seen in our stacks, which indicates its poor or partial ossification; the stylus was completely invisible, likely due to it being cartilaginous. Examination of cleared and stained specimens is needed to clarify the morphology of this structure.

Operculum

The paired oval operculum covers the posterior part of the fenestra ovalis and is attached to the lateral side of the occipito-otic complex ( Fig. 5B View Figure 5 ). It was likely cartilaginous in all the specimens examined and, thus, is invisible in our reconstructions. Examination of cleared and stained specimens is needed to clarify the morphology of this structure.

Saccular otoconia

The otic capsule in Tylototriton is filled with calcium carbonate, which was visible in our reconstructions. This structure was notably less dense than the other bones, which indicates poor or partial mineralization. The saccule was close to a hemisphere or ellipsoid in shape.

Mandible

The lower jaw has a shape of solid bony arch and comprises three bone elements, namely, paired dentaries, prearticulars, and the Meckel’s cartilage ( Fig. 5C View Figure 5 ), which is poorly seen in our reconstructions. Only a trace of Meckel’s cartilage on the proximal surface, anterior of the prearticular and in between the two portions of the dentary is visible; the poorly-ossified posterior edge of the Meckel’s cartilage corresponds to the location of the articular, which forms the jaw joint with the quadrate.

Dentary

The paired dentaries comprise most of the mandible and bear 38–41 teeth across the two-thirds of their length ( Fig. 5C View Figure 5 ). The dentary contacts articular posterolaterally and prearticular posteromedially, contributing to the coronoid process. A pair of dentaries meet anteriorly with no trace of mentomeckelian between them.

Prearticular

The paired prearticulars cover the posterior part of the medial surface of the mandible ( Fig. 5C View Figure 5 ). The prearticular, together with the dentary, forms the coronoid process.

Hyobranchial apparatus

The hyobranchial apparatus consists of a few predominantly cartilaginous elements. In our scans, we were able to observe only three bony elements: ceratobranchial, ceratohyal, and hypobranchial I.

Etymology

The specific epithet ‘ koliaensis ’ is a Latinized toponymic adjective in nominative singular and is given in reference to the Kolia Mountain Pass and Kolia Farm, the type locality of the new species. The exploration of the Ngan Son Mountains in Cao Bang Province dates back to the early 20th century, when, starting in 1901, French colonialists built a road and camps for gold and tin mining in the area of Phia Oac-Phia Den NP. The local legend says that a female French engineer named “Kolia” (also spelled as “Cô Lê A”) participated in construction of the road to the mountain pass and of a small French settlement on the slopes of the Phia Oac Mountain, built as a mountain retreat during hot summer months (presently known as “Nhà Pháp,” or “French houses”). Local people believe that this French lady was eaten by a tiger during her work in Phia Oac, and the place was named “Kolia” for her memory.

Suggested common names

We suggest the following common names: Cao Bang Crocodile Newt (English), Cá cóc Cao Bằng (Vietnamese), Kaobangskiy krokodilovyi triton (Russian), Tylototriton de Cao Bang (French).

Comparisons

According to mitochondrial phylogeny, Tylototriton koliaensis sp. nov. falls into Clade I, which corresponds to the T. asperrimus species group of the subgenus Yaotriton . Differences between the new species and all other members of the subgenus Yaotriton are summarized in Table 5 View Table 5 . The new species can be easily distinguished from all members of the subgenus Tylototriton (clades 4–5 in Fig. 2 View Figure 2 ), as well as from T. panhai and T. ngoclinhensis , by a dark body coloration, except for bright orange palms and soles, vent region, and ventral ridge of tail (vs. light color markings present on parotoids, lips, vertebral ridge, rib nodules, limbs, and ventral tail ridge).

Tylototriton koliaensis sp. nov. can be distinguished from T. anhuiensis , T. asperrimus , T. broadoridgus , T. dabienicus , T. daloushanensis , T. hainanensis , T. liuyangensis , T. lizhenchangi , T. tongziensis , T. vietnamensis , and T. wenxianensis by having prominent bony ridges on the head, denser granules on the body, large and small warts surrounding the rib nodules, and distinct interspaces between the rib nodules (vs. comparatively less developed ridges on head, smoother skin, slightly flattened to nearly indistinct rib nodules with no distinct interspaces between them). The new species differs from T. maolanensis , T. notialis , and T. thaiorum by a blackish coloration of dorsum (vs. dark brown); from T. asperrimus by comparatively larger eyes and a thinner and lower tail, as well as a weak, almost indistinct gular fold (vs. gular fold distinct); from T. pasmansi by a smaller male body size (SVL: 56.0– 61.4 mm vs. 61.6– 70.7 mm), the head wider than long (vs. slightly longer than wide); from T. sini by a smaller body size in both sexes (SVL: 56.0– 61.4 mm in males, 70.8–73.0 mm in females vs. 62.0– 66.5 mm in males, 79.3 mm in a single female), the snout truncated in dorsal view (vs. rounded), and the head wider than long (vs. slightly longer than wide); from T. sparreboomi by a smaller male body size (SVL: 56.0– 61.4 mm vs. 63.2–68.7 mm) and the head wider than long (vs. longer than wide).

Although Tylototriton koliaensis sp. nov. was previously confounded with its sister species T. ziegleri , it can be readily distinguished by several coloration, morphological and cranial specificities.

First, the dorsal coloration varies from black to slate-black in Tylototriton koliaensis sp. nov. ( Fig. 2 View Figure 2 , 8 View Figure 8 ), while it is generally lighter, varying from blackish-brown ( Fig. 9A, C View Figure 9 ) to dark-brown ( Fig. 2 View Figure 2 , 9B View Figure 9 ) or orange-brown in T. ziegleri ; the tail fin is dorsally completely black in Tylototriton koliaensis sp. nov., while it presents an orange-brown dorsal margin in T. ziegleri ( Fig. 2 View Figure 2 , 10 View Figure 10 ); the dorsum is homogeneous in Tylototriton koliaensis sp. nov, while it shows weak orange markings on rib nodules or head in some specimens of T. ziegleri ( Fig. 2 View Figure 2 , 9C View Figure 9 ).

Second, Tylototriton koliaensis sp. nov, differs from T. ziegleri in body size and for several body ratios (see Table 4 View Table 4 ). Specifically, the males of Tylototriton koliaensis sp. nov. are generally smaller than the males of T. ziegleri , although with wide overlap (SVL: 58.5 ± 2.0 mm, range 56.0– 61.4 mm vs. 62.4 ± 4.5 mm, range 52.7–67.4 mm); the males of Tylototriton koliaensis sp. nov. have wider heads (MXHW/SVL: 0.29 ± 0.01 vs. 0.27 ± 0.01), greater distance between nostrils (IND/SVL: 0.10 ± 0.006 vs. 0.08 ± 0.004), shorter axilla-groin distance (AGD/SVL: 0.50 ± 0.01 vs. 0.52 ± 0.02), longer forelimbs (FLL/SVL: 0.36 ± 0.03 vs. 0.34 ± 0.02), longer hindlimbs (HLL/SVL: 0.38 ± 0.03 vs. 0.35 ± 0.01), and shorter upper eyelids (UEL/SVL: 0.068 ± 0.003 vs. 0.072 ± 0.003) than the males of T. ziegleri .

Third, Tylototriton koliaensis sp. nov. shows unique cranial characters compared to T. ziegleri . Specifically, Tylototriton koliaensis sp. nov. often has wider vomerine tooth series, in the form of two gently curving diverging ˄-shaped lines with the anterior corner of the series forming a sharp angle ( Fig. 5B View Figure 5 ), while in T. ziegleri , the anterior part of the vomerine tooth series is ∩-shaped, their anterior corner is rounded, and the anterior parts of their branches are almost parallel, diverging in the posterior one-third of their length, forming a blunt angle ( Fig. 5E View Figure 5 ); but note that the shape of the vomerine tooth series is subjected to individual variation. Moreover, Tylototriton koliaensis sp. nov. shows an excavation on the processus oticum of pterygoid ( Fig. 5C View Figure 5 ) (vs. absent in T. ziegleri , Fig. 5F View Figure 5 ), the dorsal grove on the processus palatinum of the pterygoid is bulb-shaped ( Fig. 5C View Figure 5 ) (vs. slit-like in T. ziegleri , Fig. 5F View Figure 5 ), the lateral margin of the processus palatinum of pterygoid is rounded ( Fig. 5B View Figure 5 ) (vs. flattened in T. ziegleri , Fig. 5E View Figure 5 ), the saccular otoconia is poorly mineralized (vs. completely mineralized in T. ziegleri ), and the stapedial footplate is partially ossified (vs. completely ossified in T. ziegleri ).

Distribution

To date, Tylototriton koliaensis sp. nov. has been recorded from the Ngan Son Mountain Range in Bao Lac and Nguyen Binh districts of Cao Bang Province, at elevations between 1025 and 1425 m a.s.l. (Bernades et al. 2017; this study), and is reliably known from only three localities (localities 1–3 in Fig. 1 View Figure 1 ). Hernandez (2016) reported this species (as T. ziegleri ) from 920 m. a.s.l. elevation in Jingxi County of Guangxi Province, China, but based on personal field observations and without voucher specimen collection that could have allowed subsequent reassessments. Given the overall morphological similarity within the T. asperrimus species group, species identification based on photographs or personal observations, i.e., without molecular or quantitative morphological data, should be taken with caution. Accordingly, and acknowledging that the Guangxi population is geographically proximate to the confirmed range of the new species, we herein treat this population as Tylototriton cf. koliaensis sp. nov.

Natural history notes

At the type locality in Phia Oac-Phia Den NP, Tylototriton koliaensis sp. nov. is known from primary subtropical broadleaf montane forests on granite substrates. Phia Oac Mt. (1935 m a.s.l.) is the highest peak of the Ngan Son-Yen Lac Mountain Range, the major mountain system of northeastern Vietnam (Cao Bang, Bac Kan, and Thai Nguyen provinces). The occurrence of Tylototriton koliaensis sp. nov. in other montane forest areas of the Ngan Son-Yen Lac Mountain Ridge at similar elevations is thus considered likely. The polydominant evergreen subtropical forests in Phia Oac-Phia Den NP at elevations of 1200–1500 m a.s.l. are characterized by thick bamboo undergrowth and are dominated by trees from the families Fagaceae (genera Lithocarpus Blume and Castanopsis (D. Don) Spach ), Sapindaceae (genus Acer L.), Platanaceae (genus Platanus L.), Elaeocarpaceae (genus Elaeocarpus L.), Ericaceae (genus Rhododendron L.), Lauraceae (genus Cinnamomum Schaeff. ), and Theaceae (genus Schima Reinw. ex Blume ), with thick layers of moss and numerous epiphytic plants ( Orchidaceae , Ericaceae, Pteridophyta ). At higher elevations, they are characterized mainly by bamboo vegetation, patches of grasses, and woody shrubs ( Fig. 11A View Figure 11 ; Hernandez 2016; Bernardes et al. 2017; Poyarkov et al. 2018; this study).

The Phia Oac-Phia Den NP, where Tylototriton koliaensis sp. nov. has also been recorded, experiences a comparatively colder climate. Under the influence of the monsoon tropical climate of northeast Vietnam, which comprises cold winters and summer rains, the area has a mean annual temperature of 20.6°C, a mean annual precipitation of 1718 mm, and an average humidity rate of 83.4 % ( Averyanov et al. 2003; Le 2005; Le et al. 2021). Unusually for northern Vietnam, the temperature can fall below zero, and snow is not rare in December and January. The dry season extends from November to April, with a mean precipitation of 295 mm (17.2 % of total annual rainfall); the rainy season runs from May to November, with the peak of precipitation in July and August, which average 1423 mm (82.8 % of total annual rainfall; Le 2005). These conditions support a variety of forest types, particularly low to high montane broadleaf evergreen forests ( Tran et al. 2014). Though vegetation covers approximately 84 % of the total area of Phia Oac-Phia Den NP, it mostly consists of secondary forests or plantations, while the primary and undisturbed forests are found only above 1,000 m a.s.l. ( Tran et al. 2014).

Tylototriton koliaensis sp. nov. breeds in the summer, when rainfall is abundant. During the breeding season, from April to July, it occurs in slow-flowing streams and temporary pools of 30–50 cm of depth ( Fig. 11B View Figure 11 ). It hides under rocks and cavities during its winter terrestrial phase. In Cao Bang Province, Paramesotriton guangxiensis ( Huang, Tang & Tang, 1983) is sympatric with Tylototriton koliaensis sp. nov., but it presumably occupies cooler streams at higher elevations ( Hernandez 2016). Other species of amphibians recorded at the type locality of Tylototriton koliaensis sp. nov. include Vietnamophryne orlovi Poyarkov, Suwannapoom, Pawangkhanant, Aksornneam, Duong, Korost & Che, 2018 , Raorchestes cf. malipoensis Huang, Liu, Du, Bernstein, Liu, Yang, Yu & Wu, 2023 , Kurixalus cf. hainanus (Zhao, Wang & Shi in Zhao et al. 2005), Gracixalus gracilipes ( Bourret, 1937) , Gracixalus cf. jinxiuensis (Hu in Hu et al. 1978), Polypedates mutus ( Smith, 1940) , and Ophryophryne microstoma Boulenger, 1903 .

Several natural history features that pertain to Tylototriton koliaensis sp. nov. were documented by Bernardes et al. (2017). These authors reported that females are supposadly larger than males and have distended abdomens during the breeding season, whereas males tend to have more robust-looking limbs than females. Females deposit their eggs on the ground (terrestrial oviposition) from April to July, and tadpoles undergo metamorphosis in July. However, in June 2017, we also observed egg deposition directly in the water at the type locality, in the context of large aggregations of males and females in the slow-moving parts of partially flooded mountain streams. In Bao Lac District, Cao Bang Province, Bernardes et al. (2017) observed eggs and larvae in July, and reported egg diameters (from randomly sampled eggs) of 7.2 mm to 8.9 mm (mean: 8.1 ± 0.8 mm, n = 28). On average, the egg clutches of Tylototriton koliaensis sp. nov. appear to be bigger than other Yaotriton members, with a mean size of 67 ± 32 eggs, deposited on rock and soil substrates at a distance of 50 ± 28 cm from the water ( Bernardes et al. 2017). Bernardes et al. (2017) also found clutches of Tylototriton koliaensis sp. nov. on the slopes of a slow-flowing forest stream, suggesting some flexibility in the breeding habitat.

According to Bernardes et al. (2017), larvae of the new species have relatively broad head, the interorbital distance being wider than the internarial distance, a pointed tail tip, the dorsal tail fin being higher than the ventral fin, and the tail being shorter than the body. The early larval stages of Tylototriton koliaensis sp. nov. lack balancers, while the more advanced larval stages have dark ground color with the exception of yellow digits and ventral fin ( Bernardes et al. 2017).

Conservation status

Habitat loss due to destructive anthropogenic activities, such as road construction, agriculture, forest destruction caused by logging, hydroelectric development, and rock excavation, are the main threats faced by the new species ( Hernandez 2016). During the past French colonial era, mining and farming activities resulted in the clearing of large areas of evergreen montane forest in Cao Bang Province, and possibly caused local population declines. At present, Tylototriton koliaensis sp. nov. is only distributed in one protected area, Phia Oac-Phia Den NP, which covers 663,798 ha. Given its presumably small range that potentially continues to be subjected to many threats, we suggest to list Tylototriton koliaensis sp. nov. under the IUCN’s Red List category of Endangered (EN) (Anonymous 2019).