A striking new genus and species of cave-dwelling frog (Amphibia: Anura: Microhylidae: Asterophryinae) from Thailand

Diagnosis

A medium-sized (19 mm < SVL < 30 mm) member of the Australasian subfamily Asterophryinae (family Microhylidae), with the following combination of morphological attributes: (1) both maxillae and dentaries eleutherognathine, no maxillary teeth; (2) vertebral column procoelous with eight presacral vertebrae (PSV) lacking neural crests; (3) no sagittal crest on cranium; (4) frontoparietals conjoined, connected by long suture; (5) nasals wide, calcified, but not contacting each other medially; (6) vomeropalatines small, not expanded, vomerine spikes absent; (7) cultriform process of parasphenoid comparatively narrow; (8) clavicles present as slender tiny bones, lying on the procoracoid cartilage not reaching scapula or the midline; (9) omosternum absent; (10) sternum large, anterior portion consists of calcified cartilage, xiphisternum cartilaginous; (11) weak dorsal crest present on urostyle, absent on ilium; (12) terminal phalanges large T-shaped; (13) all fingers and toe discs with terminal grooves; (14) subarticular tubercles weak, discernible only at digit basis; (15) toe webbing absent; (16) tympanum distinct; (17) two transverse smooth palatal folds; (18) pupil round; (19) snout rounded, equal to EL; (20) development with a larval stage, tadpole with peculiar dorso-ventrally compressed morphology.

• Type species. Siamophryne troglodytes sp. nov.

• Other included species. None are known at present.

Distribution

To date, S. troglodytes sp. nov. is only known from a small cave system in a karst region of Sai Yok District, Kanchanaburi Province, northern Tenasserim Region, western Thailand (see below the description of the species) (see Fig. 1).

Discrimination from other Asterophryinae genera

Information on character states for other Asterophryinae genera is based on Parker (1934), Zweifel (1972, 2000), Menzies & Tyler (1977), Burton (1986), Zweifel, Menzies & Price (2003), Günther, Stelbrink & von Rintelen (2010), Kraus (2010, 2017), and references therein. The new genus has eleutherognathine maxillae and dentaries and thereby is distinguished from those Asterophryinae genera which have symphignathine state of this trait in both jaws: Asterophrys (including the recently synonymized Pseudocallulops and Metamagnusia; see Rivera et al., 2017) (New Guinea), Callulops (from Sulawesi to New Guinea region), Mantophryne (New Guinea and Louisiade Archipelago), Oninia (New Guinea), and Xenorhina (including the recently synonymized Xenobatrachus Peters & Doria) (New Guinea region). The genus Barygenys (Papua New Guinea region) can be distinguished from the new genus by the presence of symphignathine dentaries and eleutherognathine maxillae (vs. both jaws being eleutherognathine in Siamophryne Gen. nov.). The new genus lacks distinct neural crests on PSV, and therefore can be differentiated from the genera Aphantophryne (from Philippines to New Guinea) and Cophixalus (from Moluccas to New Guinea and northern Australia) (both have well-developed neural crests on PSV); the new genus can be further distinguished from Aphantophryne as it has eight PSV (vs. seven PSV in Aphantophryne). The genus Sphenophryne sensu lato (New Guinea) has well-developed long and slender clavicles (vs. tiny clavicles that do not reach scapula and the midline in the new genus), and broad vomeropalatines that contact each other medially, with a postchoanal portion overlying the palatine region (vs. vomeropalatines not expanded in the new genus); Sphenophryne sensu stricto (S. cornuta Peters & Doria) can be further distinguished by a characteristic spine-like projection on the upper eyelid (vs. smooth upper eyelid in the new genus), it also has arboreal life style (vs. troglophilous life style of the new genus). The genus Genyophryne (recently considered as a synonym of Sphenophryne; see Rivera et al., 2017) can be distinguished from the new genus by absence of clavicles (vs. clavicles present), stout body habitus (vs. slender body habitus) and absence of large finger discs (vs. very large finger discs in the new genus). The genus Liophryne (which is also regarded as a member of Sphenophryne sensu lato based on phylogenetic data of Rivera et al., 2017) can be differentiated from the new genus by the presence of long and slender clavicles (vs. tiny clavicles in the new genus), and by the presence of comparatively small finger discs (vs. large broad fingers discs in the new genus). The genus Oxydactyla (now regarded as a part of Sphenophryne sensu lato; see Rivera et al., 2017) can be distinguished from the new genus by the absence of finger discs (vs. large finger discs present in the new genus). The genus Paedophryne (New Guinea region) can be distinguished from the new genus by a much smaller body size (SVL < 20 mm vs. SVL ≥ 20 mm in the new genus), by cartilaginous phalanges in the first digit (vs. ossified phalanges in the new genus), by FI reduced to a nub (vs. well-developed FI in the new genus), by the absence of clavicles and procoracoids (vs. presence in the new genus) and by having seven PSV (vs. eight PSV in the new genus). By the presence of clavicles and procoracoid cartilage, the new genus can be differentiated from the genus Choerophryne (New Guinea), which lacks these structures; the latter also has palatine portions of vomeropalatines fused with broad sphenethmoids (vs. not fused in the new genus). The genus Copiula (New Guinea) can be distinguished from the new genus by the lack of clavicles (vs. presence in the new genus), by small discs on fingers, which are smaller than those on toes (vs. large finger discs that are larger than toe discs in the new genus), by cartilaginous sternum (vs. ossified anterior portion of sternum in the new genus), and by the presence of a conspicuous rostral dermal gland (vs. rostral gland absent in the new genus). The new genus can be distinguished from Austrochaperina (Australia, New Guinea, and New Britain) by fingers with very wide discs, much wider than penultimate phalanges, by vomeropalatines not expanded and by narrow cultriform process of parasphenoid (vs. discs on fingers absent or small, slightly different in width from penultimate phalanges, vomeropalatines expanded and broad cultriform process of parasphenoid in Austrochaperina). The new genus can be distinguished from Hylophorbus (New Guinea) by comparatively better developed nasals (vs. poorly developed nasals in Hylophorbus), by the presence of large finger discs (vs. discs on fingers usually absent, if present, they are much smaller than toe discs) and by completely smooth skin on dorsum (vs. shagreened to tubercular skin on dorsum in Hylophorbus). The genus Oreophryne (from Philippines and Lesser Sundas to New Guinea and New Britain, see Kraus, 2017) can be differentiated from the new genus by distinct toe webbing (vs. no toe webbing in the new genus), by arboreal or terrestrial life style (vs. troglophilous life style in the new genus), and by expanded vomeropalatines (vs. not expanded in the new genus).

From its sister genus Gastrophrynoides (Peninsular Malaysia and Borneo) the new genus can be easily distinguished by the presence of large and wide finger discs (vs. small finger discs, slightly wider than the penultimate phalanges in Gastrophrynoides), by a comparatively much shorter snout and larger eye (SL equal to EL in the new genus vs. snout 2.5 times longer than eye in Gastrophrynoides) and by a distinct tympanum (vs. tympanum obscured by skin in Gastrophrynoides).

Finally, the sequences of the 12S rRNA–16S rRNA mtDNA fragment for the new genus are markedly distinct from the sequences for all those Microhylidae members, for which homologous sequences are available (see Fig. 2; Table 3).

Etymology

The generic nomen Siamophryne is derived from “Siam”—the old name of present-day Thailand; referring to the range of the new genus, which to date is only known from western Thailand; and the Greek noun “phryne” (φρÚνη; feminine gender), meaning “toad” in English; this root is often used in the generic names in Asterophryinae microhylid frogs. Gender of the new genus is feminine.

• Siamophryne troglodytes sp. nov.

• Figs. 3–10; Table 4.

Holotype

AUP-00500, adult male in a good state of preservation, collected in a limestone cave in Sai Yok District, Kanchanaburi Province, western Thailand, elevation 440 m a.s.l. (approximately in the vicinity of 14°28′N, 98°51′E; exact geographic coordinates not provided for conservation purposes) (see Fig. 10); collected on October 27, 2016, by Montri Sumontha, Jitthep Tunprasert, Nirut Chomngam, and Chatmongkon Suwannapoom.

Paratypes

In total, 10 specimens: AUP-00501-00504, four adult males, and AUP-00505-00508, three adult females, collected on October 27, 2016, from the same locality and with the same data as the holotype; ZMMU A-5818 (field ID NAP-06651), adult male, and ZMMU A-5819 (field ID NAP-06652), adult female, collected by T. Ruangsuwan from the same locality as the holotype on August 1, 2016.

Referred specimens

AUP-00509, a larval specimen, Gosner stage 36, collected by T. Ruangsuwan from the same locality as the holotype on August 1, 2016.

Diagnosis

The only known member of the genus Siamophryne Gen. nov. (see Diagnosis of the genus). S. troglodytes sp. nov. is characterized by a combination of the following traits: (1) SVL of six adult males 19.1–24.9 mm, and of five adult females 25.0–27.8 mm; (2) body habitus slender, limbs very long (FLL/SVL ratio 0.69 (0.65–0.74); HLL/SVL 1.50 ratio (1.39–1.67) for both sexes); (3) snout short, rounded, subequal to EL (0.8–1.0 times the length of the eye); (4) eye medium-sized, EL/SVL ratio (0.12–0.14); (5) tips of fingers II–IV expand to broad discs 1.5–2.5 times wider than the penultimate phalanges; toes II–IV with smaller discs slightly wider than the penultimate phalanges, tips of finger I, toe I, and toe V rounded, same width as the penultimate phalanges; (6) finger discs distinctly wider than toe discs; (7) terminal phalanges distinctly T-shaped in F2–F4 and T2–T5; bobbin-shaped in finger I and toe I; (8) subarticular tubercles on fingers weak, indistinct; finger subarticular tubercle formula: 1:1:1:1; better pronounced on toes, toe subarticular tubercle formula: 1:1:1:1:1; (9) outer metatarsal tubercle absent, inner metatarsal tubercle small, rounded; (10) skin of the ventral surface completely smooth, skin of the dorsal and lateral surfaces smooth with rare flat tubercles or pustules; (11) osteological features are the same as for the genus. Other diagnostic features are given in the diagnosis of the genus.

Description of the holotype

Holotype in preservative is shown in Figs. 3 and 4. Medium-sized specimen, with SVL 22.1 (hereafter all measurements in mm), in a good state of preservation, however distal parts of toes II—V slightly dehydrated (Fig. 3E); ventral surface of left thigh dissected for 5 mm and some of femoral muscles removed. Body habitus slender (Fig. 3A); HL slightly shorter than HW (0.95); snout rounded both in profile (Figs. 3C and 4A) and in dorsal view (Fig. 3A), shorter than the diameter of eye (SL/EL 0.91); eyes large, notably protuberant in dorsal and lateral views, pupil oval, horizontal (Fig. 3C); dorsal surface of head flat, canthus rostralis indistinct, gently rounded; loreal region weakly concave; nostril rounded, lateral, located much closer to tip of snout than to eye; tympanum well discernable, circular, tympanic rim not elevated above the skin of temporal area, supratympanic fold absent; vomerine teeth absent, two transverse palatal folds present across the palate anteriorly to the pharynx, both of them with smooth edges, tongue spatulate and free behind, lacking papillae, vocal sac opening not discernable.

Forelimbs comparatively long, less than half length of hindlimbs (FLL/HLL 0.42); hand slightly longer than lower arm and less than half length of forelimb (HAL/FLL 0.36); fingers slender, flattened in cross section, first finger well developed, one-half length of the second finger (1FLO/2FLO 0.50); relative finger lengths: I < II < IV < III (see Figs. 3D and 4B). Finger webbing and dermal fringes absent. First fingertip rounded and slightly dilated, almost the same width as the basal phalanx. Tips of three outer fingers II–IV greatly dilated forming large triangular disks with distinct narrow terminal grooves; relative finger disk widths: II < IV < III; longitudinal furrow on the dorsal surface of fingers absent; flexor tendons visible through translucent skin on ventral surface of fingers; subarticular tubercles on fingers barely distinct at basis of proximal phalanges and almost indistinct under penultimate phalanges of fingers III and IV, subarticular tubercles flat, oval-shaped with unclear borders, finger subarticular tubercle formula: 1:1:1:1 (for fingers I:II:III:IV, respectively); nuptial pad absent; two palmar (metacarpal) tubercles: inner palmar tubercle small, rounded, same size as subarticular tubercles; outer palmar tubercle oval-shaped with indistinct borders, almost the same length as inner palmar tubercle (IPTL/OPTL 0.97); palmar surface smooth, supernumerary palmar tubercles absent.

Hindlimbs long and slender, TL is half of SVL (0.49); tibiotarsal articulation of adpressed limb reaching the eye level; foot shorter than tibia (FL/TL 0.89); relative toe lengths I < II < V < III < IV; tarsus smooth, tarsal fold absent; tips of all toes slightly dilated forming small spatulate disks on all toes except toe I (Figs. 3E and 4C), each disk with weak terminal groove similar to that on fingers, relative toe disk widths: I < II < V < III < IV; toes slightly flattened in cross section, dermal fringes absent; toe webbing absent between all toes; subarticular tubercles on toes more distinct than on fingers, oval-shaped, elevated, toe subarticular tubercle formula: 1:1:1:1:1 (for toes I:II:III:IV:V, respectively); single metatarsal tubercle: inner metatarsal tubercle rounded, flattened.

Skin on dorsal and dorsolateral surfaces smooth with rarely scattered flat tubercles (Fig. 3A), tubercles getting larger and more prominent on dorsal surfaces of hindlimbs; dorsal surface of forelimbs smooth with few small tubercles on forearm; upper eyelids smooth; ventral sides of trunk, head and limbs completely smooth (Fig. 3B); dermal ridges or skin macroglands absent.

Measurements of holotype (in mm)

SVL 22.1; HL 6.7; SL 2.7; EL 3.0; N–EL 1.9; HW 7.1; IND 2.3; IOD 2.2; UEW 1.8; FLL 15.4; LAL 10.4; HAL 5.6; IPTL 1.0; OPTL 1.1; HLL 37.1; TL 10.8; FL 9.6; IMTL 0.9; 1TOEL 2.0; 2TOEL 4.7; 3TOEL 7.4; 4TOEL 9.6; 5TOEL 7.3; 1FW 1.0; 2FDW 1.2; 3FDW 1.3; 4FDW 1.3; 1TDW 0.2; 2TDW 0.2; 3TDW 0.3; 4TDW 0.5; 5TDW 0.3; 1FLO 1.9; 2FLO 3.7; 3FLO 5.6; 4FLI 4.5; TMP 1.2; TEY 0.8.

Coloration of holotype in life

Coloration rather uniform: dorsal surface of head and trunk chocolate brown; dorsal surface of limbs ochre-brown; ventral surface of limbs and throat light orange-pink, lateral sides of body gray to gray-brown; ventral surfaces of body pinkish-gray. Fingers and toes gray with dark brown mottling. Tympanum grayish with slight brown mottling. Tubercles on dorsal surfaces of body, limbs and head copper-brown. Iris uniform dark brown; pupil black; sclera bluish-gray.

Coloration of holotype in preservative

Coloration in preservative is shown on Fig. 3. After preservation in ethanol, dorsal coloration changed to grayish-brown, upper eyelids dark-gray (Fig. 3A), ventral surface of chest, belly, limbs turn yellowish-gray (Fig. 3B); iris coloration faded and turned black.

Morphological variation of the type series

Measurements of the type series that show variation in morphometric characteristics are given in Table 4. Coloration of paratype in life is shown in Fig. 5. Specimens show no significant variation in coloration. Specimens vary in the body size: females (SVL 25.0–27.8; mean 26.3 ± 0.7; n = 5) are larger than males (SVL 19.1–24.9; mean 22.4 ± 2.1; n = 6). Paratypes in situ had generally darker coloration with dark gray-brown coloration of dorsum and gray coloration of ventral surfaces and body flanks (Figs. 10C and 10D).

Osteological characteristics

Based on microtomographic data from ZMMU A-5818, adult male. The main skeletal features are shown in Fig. 6. Details of skull morphology are presented in Fig. 7.

Skull clearly longer than wide (Fig. 6). Frontoparietals separate along their entire length, longer than broad, narrower anteriorly than posteriorly, connected with each other medially with long suture, lack a sagittal crest, partially fused to exoccipital posteriorly (Fig. 7A). Exoccipitals separate, contact each other medially. Nasals large but not meeting each other at midline, lacking posterior ramus, chondrified peripherally, overlying the dorsal portion of sphenethmoid (Fig. 7A). Sphenethmoid ossified laterally and dorsally, but chondrified anteriorly and ventrally (Fig. 7B). Prootics partially chondrified, lacking dorsal crest (Fig. 7C). Squamosal L-shaped, well-ossified, distally chondrified, articulating on anterolateral surface of prootic (Fig. 7C). Columella comparatively small, centrally ossified (Fig. 7C), distally chondrified; tympanic annulus completely chondrified. Premaxilla with slender, weakly mineralized dorsal process; labial process of premaxilla well-ossified (Fig. 7D). Maxilla largely chondrified, ossified in central part. Anterior ends of maxillaries contact labial portions of well-developed premaxillaries (eleutherognathine condition) (Fig. 7D). Quadratojugal ossified in posterior portion. Vomers mostly chondrified plates meeting at midline, without teeth or lateral processes; parts edging choanae weakly ossified (Fig. 7C). Mentomeckelians ossified, connected to dentaries and to each other by strips of cartilage (Fig. 7B); dentaries not fused (eleutherognathine condition). Parasphenoid smooth; cultriform process of parasphenoid rather narrow, terminates at the level of sphenethmoid with an anterior notch (Fig. 7B). Hyoid plate completely cartilaginous, anterolateral (alary) processes of hyoid plate present, recurved, posterolateral processes thinner than alary processes; posteromedial processes strongly ossified, elongated, straight, wider at proximal ends, chondrified at distal ends (Fig. 6B).

Eight nonimbricate procoelous PSV, stout, length approximately from one-third to one-half of width; PSV longer anteriorly, narrowing progressively to the posterior; all except the first with wide diapophyses, which are also longer anteriorly (3d PSV has the longest transverse processes), decreasing in length progressively to the posterior, with chondrified tips (Figs. 6A and 6B). Diapophyses of vertebrae 2, 7, and 8 oriented anteriad, those of third and sixth—straight, and those of fourth and fifth oriented posteriad. Neural crests on PSV absent. Sacrum with slightly expanded diapophyses. Urostyle with weak dorsal crest running along about 75% of its shaft; ilia smooth, lacking dorsal crest (Fig. 6A).

Coracoids, scapulae, and suprascapulae present; first two fully ossified; suprascapula largely chondrified. Coracoids robust with narrow distal ends oriented anteriad; proximal ends greatly expanded (Fig. 6B). Omosternum absent. Procoracoid cartilage well-developed, extends from the scapula to the midline of the girdle. Clavicles present as slender, tiny, slightly recurved bones, lying on the procoracoid cartilage, not reaching scapula or the midline of the girdle (Fig. 6B). Sternum large, anterior portion consists of calcified cartilage (Fig. 6B), xiphisternum completely cartilaginous.

Bones of hands (Figs. 6C and 6D) with six largely calcified carpal elements: carpale distale I small, carpale distale II–IV fused into a single large element, prepollex and a large radiale lie between a small Y-element and an elongated ulnare. Metacarpals long and fully ossified; phalangeal formula: 2-2-3-3; all phalanges ossified; distal phalanx of finger I bobbin-shaped; terminal phalanges of fingers II–IV with greatly expanded, T-shaped tips, approximately three to four times wider than penultimate phalanges (Figs. 6C and 6D). Foot (Figs. 6E and 6F) with four tarsal elements, including ossified tarsale distale II–III, central and a prehallux; prehallux ossified. Metatarsals fully ossified, long and relatively more massive than metacarpals; phalangeal formula: 2-2-3-4-3; all phalanges ossified (Figs. 6E and 6F). Terminal phalanges of all toes with slightly expanded, T-shaped tips; equal in width to penultimate phalanges on toes I and V, slightly exceeding the width of penultimate phalanges on toes II–IV (Figs. 6E and 6F).

Larval morphology

Tadpole morphology description is based on a single larval specimen AUP-00509, Gosner stage 36, collected from the type locality. External appearance and coloration of the tadpole in life is shown in Fig. 8; morphological details in preservative are presented in Fig. 9. Body measurements are given below.

The single encountered tadpole was attributed to S. troglodytes Gen. et sp. nov. based on the following evidence: (1) morphological features characteristic for Microhylidae larvae in general (flattened body, mouthparts lack lateral lobes, and keratinized structures); (2) collected in a crevice in the limestone cave, where adult animals were observed (see Figs. 10E and 10F); (3) species identification confirmed by mtDNA sequence of short 16S rRNA gene fragment (up to 500 bp), identical to that of adult specimens (GenBank accession number: MG682559, see Table 2).

Standard tadpole measurements (AUP-00509, Gosner stage 36) (all in mm, taken by TR): ToL: 35.1; BL: 12.1; TaL: 21.8; BW: 7.9; BH: 5.4; TH: 4.0; SVL: 13.6; SSp: 5.6; UF: 1.0; LF: 1.0; IN: 1.2; IP: 1.7; RN: 1.9; NP: 1.9; ED: 1.0; MW: 2.6.

In dorsal view (Figs. 8A and 9B), body elongated (BW/BL 0.65) guitar-shaped with large bluntly rounded anterior part and a distinct narrowing behind the eyes that forms notably angular body edges; snout blunt with a shallow medial notch. From lateral view (Fig. 9A), both head and body strongly flattened. Tail strong, with well-developed muscles, less than two times longer than the body (Tal/BL 1.80), tail width at tail basis subequal to IOD; tail tip gently rounded. Tail fins very low, with even edges; dorsal fin not extending on the trunk, ventral fin same height as dorsal fin (LF/UF 0.94). A wide gular fold across the anterior half of body on the level of ca. 1/3 of BL (Figs. 8B and 9B). Spiracle medial, narrow, slit-like, transversal; located in the anterior half of body (SSp/SVL 0.41); covering membrane with even free margin. Vent tube medial, oblique.

Eyes dorsal, very small (ED/BL 0.08); with dorsolateral orientation of pupils. Small unpigmented narial protuberances present in nostril area. Mouth terminal, oriented anteroventrally, not visible from dorsal view (Fig. 9B), mouth opening transverse, slit-like from ventral view (Fig. 9C), relatively wide (MW/BW 0.33). From lateral view (Fig. 9A), upper labium slightly projecting and overhanging over the mouth opening; upper labium with even edge; lower labium short, with U-shaped edge. Protuberances in mouth angles, lateral lobes, and keratinized mouthparts absent. No papilla seen on mouth floor or mouth roof. Lingual anlage rounded.

Coloration

In life at stage 36 larva almost unpigmented and in situ appears translucent with dark-black colored eyes (Figs. 10E and 10F). In laboratory conditions, live tadpole (Fig. 8) shows semitransparent body with weak brownish pigmentation on dorsal surfaces of body and tail; a strongly-pigmented dark stripe runs along dorsal surface from interorbital region to tail base (Fig. 8A). Tail fins and ventral sides unpigmented, in life this allows to see heart, liver, and major blood vessels from ventral view (Fig. 8B). Limb buds pigmented with brown. In preservative, brown color fades to brown-gray (Fig. 9).

Natural history notes

Siamophryne troglodytes Gen. et sp. nov. has a troglophilous life style and to date is only known from a small limestone cave system in western Thailand. All specimens were collected within a narrow area inside a limestone cave located on elevation 440 m a.s.l. in a polydominant tropical forest in Sai Yok District, Kanchanaburi Province, western Thailand (Fig. 10A). The cave was examined twice on the 1st of August and the 27th of October, 2016. In both cases, adult specimens of S. troglodytes Gen. et sp. nov. were only recorded inside the cave, at a distance of more than 25 m from the entrance, sitting on walls of the cave (Figs. 10B and 10D) or hiding inside small caverns in limestone (Fig. 10C) or under flat stones. Despite the thorough search, no animals were recorded near the cave entrance or in the forest close to the cave. Animals were active from 23:00 to 24:00, when the air temperature inside the cave was 28 °C in August and 26 °C in October, in both cases with 100% humidity. No calling activity was recorded during both surveys. Diet and enemies of the new frog are unknown.

Three tadpoles (one of which was collected) were observed during the survey on the 1st of August, 2016, in a small water-filled cavity in the limestone on the floor of the cave, ca. 10 m from the cave entrance (Figs. 10E and 10F). The cavity was filled with water, the average depth was 4–5 cm; mosquito larvae (Chironomidae) were also observed in the same water body. Four other tadpoles (not collected) were discovered in another similar water-filled cavity inside the cave (30 m from the cave entrance).

The cave system where S. troglodytes Gen. et sp. nov. was discovered is inhabited by several species of bats which produce significant amount of guano that accumulates on the cave floor. According to a local guide, the locals mine this guano and that affects the ecosystem of the cave.

Comparisons

As for the genus. S. troglodytes Gen. et sp. nov. is a medium-sized frog (19 mm < SVL < 30 mm) and can be easily distinguished from all other microhylids inhabiting the mainland Southeast Asia by long limbs with digits bearing large disks, with those on fingers up to 2.5 times wider than the penultimate phalanges (vs. finger tips not expanded into prominent discs in Kalophrynus, Glyphoglossus, Microhyla, and Micryletta); by the absence of tibiotarsal projection and uniform dull-brownish coloration of ventral surfaces (vs. the presence of bony tibiotarsal projection and belly with bright saffron-yellow spots in Chaperina); by single small subarticular tubercle with indistinct borders at the basis of each finger (vs. subarticular tubercles of the fingers greatly enlarged to form accessory adhesive organs in Phrynella and Metaphrynella); by the lack of a bony ridge along the posterior border of each choana (vs. well-developed bony ridge along the posterior border of each choana in Kaloula), by the presence of a distinct tympanum (vs. hidden tympanum in genera Glyphoglossus, Microhyla, Micryletta, Kaloula, Phrynella, and Metaphrynella).

The new species can be further distinguished from Gastrophrynoides, another Asterophryinae genus inhabiting the mainland Southeast Asia, by slender body habitus (vs. body habitus robust in Gastrophrynoides, see Fig. 2 for details), by a short rounded snout, that is 0.8–1.0 times the ED (vs. long, pointed snout that is 2.6–3.0 times the diameter of the eye in Gastrophrynoides) and by a well-developed tympanum (vs. tympanum hidden in Gastrophrynoides).

Distribution

As for the genus. At present, S. troglodytes Gen. et sp. nov. is known from a single limestone karst cave in Sai Yok District of Kanchanaburi Province in western Thailand. To date, numerous surveys in the nearby karst massifs have not yielded discoveries of additional populations of the new species. However, further fieldwork in Kanchanaburi Province of Thailand and the adjacent parts of Tanintharyi Division of Myanmar are required.

Conservation status

This species was unknown to science and even to local people for a surprisingly long time despite intense human activity in this region; that suggests a possibly very limited range. Despite our numerous attempts to find additional localities of the new species in adjacent limestone areas, we failed to detect other populations of this troglophilous frog. In this regard, we consider a disclosure of the detailed collecting locality information premature. In the case of narrow-ranged species of amphibians, the locality information should be released only after effective conservation measures have been taken and legal protection status have been established (see discussion in Hou et al., 2014). Until then, this information can be requested from the School of Agriculture and Natural Resources, University of Phayao. Currently, the only known habitat of S. troglodytes Gen. et sp. nov. is endangered (EN) due to illegal guano mining with the use of explosives, which is destroying the cave ecosystem. Given the available information, we suggest S. troglodytes Gen. et sp. nov. to be considered as an EN species following IUCN’s Red List categories (IUCN Standards and Petitions Subcommittee, 2016).

Etymology

The specific name “troglodytes” is a Latin adjective in the nominative singular meaning “cave-dweller”, derived from the Greek “τρωγλoδύτης”, with “trogle” meaning “hole, mouse-hole” and “dyein” meaning “go in, dive in”; referring to the troglophilous biology of the new species, which was recorded only in a limestone karst cave system.

Suggested common names

We recommend the following common names for the new species: “Tenasserim Cave Frog” (English); “Eung Tham Tenasserim” (Thai).