Vietnamophryne occidentalis is a small, miniaturized Microhylidae described from a single male specimen with a snout-vent length of 20.5 mm. It has a stout body with a head that is as long as it is wide and rather flat on the dorsal surface. The snout is short, shorter than the eye diameter, obtuse in dorsal view, and round in lateral view. The rounded, laterally positioned, nostrils are closer to the tip of the snout than the eye. The distinct canthus rostralis is rounded. The loreal region is vertical. The distance from the eye to the nostril is 6.7% of the snout-vent length and about than half the eye length. The eyes are of medium size and protrude slightly in the dorsal and lateral views. The pupil is round and horizontal. The distinct tympanum is relatively small, rounded, and positioned near the eye. There is a tympanic rim above the skin of the temporal area that is not elevated. The distinct supratympanic fold is thick, rounded, and glandular (Poyarkov et al. 2018).
The forelimbs are long, nearly half the length of the hind limbs. The hand is short, shorter than half of the forelimb length and much shorter than the lower arm. The palmar tubercles are round and small with the outer being shorter than the inner. The palm is otherwise smooth and without supernumerary tubercles. The round fingers are long and slender. The first finger is well developed, and is nearly half the length of the second finger. The relative finger lengths are I < II < IV < III and the fingers lack webbing, fringes, and longitudinal furrows. The subarticular tubercles are weak and indistinct. The fingertips are rounded, do not expand into discs, and do not have terminal grooves. Nuptial pads were not observed in the holotype (Poyarkov et al. 2018).
The hind limbs are relatively short and thick. The tibia is nearly half the snout-vent length. When adpressed along the body, the tibiotarsal articulation reaches the eye. The tarsus is smooth and without a tarsal fold. The foot is short. The outer metatarsal tubercle is absent, and the inner metatarsal tubercle is small (4.2% of snout-vent length), rounded, and flattened. The relative toe lengths are I < II < V < III < IV. The toes are rounded, with weak and indistinct subarticular tubercles, without webbing or dermal fringes, and do not have terminal grooves. The tips of toes I, II, and V do not expand, but toe III is weakly expanded and toe IV is notable dilated (Poyarkov et al. 2018).
The skin on the dorsal, lateral, ventral surfaces are generally smooth including the forelimbs and upper eyelids. The supratympanic fold has thick, low, glandular ridges. The posterior dorsal surfaces of trunk, tail, and hind limbs have loosely scattered, small, flat tubercles. There is a distinct dorsomedial vertebral skin ridge that extends from the tip of the snout to the level of the cloaca, which is particularly discernible on the dorsal midline and head (Poyarkov et al. 2018).
Vietnamophryne is considered a part of the Asterophryinae subfamily of Microhylids. Vietnamophryne can be distinguished from other Asterophryniae genera via morphological traits. Asterophrys, Pseudocallulops, Metamagnusia, Callulops, Mantophryne, Oninia, Xenorhina, and Xenobatrachus have symphignathine dentaries, whereas Vietnamophryne has eleutherognathine dentaries, and Barygenys has both. Aphantophryne and Cophixalus can be distinguished from Vietnamophryne by lack of neural crests on the presacral vertebrae. Vietnamophryne can be distinguished from its sister genus, Siamophryne, by the formers’ stout body hiatus and generally smaller body size, as well as lack of clavicles, bobbin-shaped terminal phalanges, a single smooth palatal fold, lack of vomeropalatines, a sternum that is fully cartilaginous, and lack of digit discs. Members of Vietnamophryne have an externally distinct tympanum which distinguish it from the genera Glyphoglossus, Microhyla, Micryletta, Kaloula, Phrynella, Metaphrynella, and Gastrophrynoides, which have hidden tympanum. Vietnamophryne can be further distinguished from Gastrophrynoides by the former having a short, rounded, or obtuse snout. Vietnamophryne can be distinguished from Phrynella and Metaphrynella by the former’s lack of subarticular tubercles. The absence of a tibiotarsal projection differentiates Vietnamophryne from Chaperina and a bony ridge along the posterior border of the choana differentiates Vietnamophryne from Kaloula (Poyarkov et al. 2018).
Vietnamophryne occidentalis can be distinguished from Vietnamophryne inexpectata and Vietnamophryne orlovi by the former having a larger body size, bright orange-red ventral coloration with brown patterning, and relatively longer forelimbs. Vietnamophryne inexpectata can be further differentiated from V. occidentalis by the former having warty skin, a reduced first finger, and larger tympanum. Vietnamophryne orlovi can be further distinguished from V. occidentalis by the former having a head that is longer than wide with a notably longer snout, and a eye-nostril distance that is nearly equal to the eye length (Poyarkov et al. 2018).
In life, the dorsal coloration of the body and limbs is dark brick brown with the tubercles and pustuals on the posterior portion of the animal being darker brown and grey, respectively. The lateral sides of the head, including the canthus rostralis and the ventral side of the supratympanic fold are dark brown to almost black. The ventral surfaces are an orange coloration with weak brown marbling or flecks that are denser at the throat and limbs. The fingers and toes are dark brown on the dorsal surface and gray-brown to gray with occasional reddish blotches on the ventral surface. Finally, the pupils are black and the iris is a uniform dark brown. In preservative, the pattern remained unchanged after one year, however, the dorsal coloration changed to dark brown and the reddish tint on dorsum and ventral surfaces faded to a yellowish-grey color (Poyarkov et al. 2018).
Because the description is based off of a single holotype specimen, there is no information on species variation in V. occidentalis (Poyarkov et al. 2018).
The species authority is: Poyarkov Jr., N. A., Chatmongkon, S., Pawangkhanant, P., Akrachai, A., Van Duong, T., Korost, D. V., Che, J. 2018. A new genus and three new species of miniaturized microhylid frogs from Indochina (Amphibia: Anura: Microhylidae: Asterophryinae). Zoological Research 39(3): 130-155.
Based on Maximum Likelihood and Bayesian Inference of a 12S to 16S mtDNA gene fragement, V. occidentalis is sister to the clade composed of the rest of the Vietnamophryne members, V. inexpectata and V. orlovi. The genus Vietnamophryne is sister to the monospecifc genus, Siamophryne (Poyarkov et al. 2018).
The genus name, Vietnamophryne, is a combination of a reference to Vietnam, the country where species of this genus were found, and the Greek noun “phryne”, which means “toad” and is a root commonly used for Asterophryinae frogs (Poyarkov et al. 2018).
The species epithet, “occidentalis”, is a Latin adjective meaning “western”, referring to the type locality of the new species, which is in the westernmost areas where members of subfamily Asterophryinae have been found (Poyarkov et al. 2018).
The suggested common name of the species in English is “Chiang Rai Dwarf Frog” and in Thai is “Eung Tham Khaera Chiang Rai” (Poyarkov et al. 2018).
The holotype of this species is in poor condition as it was partially decayed before preservation and had lost soft tissues from the belly and left hind limb. Muscle from the left thigh and femoral area were also removed for genetic analysis (Poyarkov et al. 2018).
At the time of its description, V. occidentalis was only known from its type locality at an elevation of 1000 - 1050 m a.s.l. on the slope of Doi Tung Mountain, Pong Ngam District, Chaing Rai Province, northern Thailand near the border with Myanmar, where the species’ occurrence is likely. Two individuals were, respectively, found under a log and in leaf litter in a subtropical montane forest on limestone outcrops after heavy rain (Poyarkov et al. 2018).
The climate of the Doi Tung Mountain is monsoonal with three distinct seasons: cool-dry from November to February, hot-dry from March to May, and rainy from May or June to November. Average rainfall in the Chiang Rai Province is 2500 mm at 1200 m elevation. The temperatures range from 13 °C – 21 °C over the course of the year. The typical montane forest canopy trees include Schima wallichii, Sarcosperma arboretum, Cinmnamomim iners, Balakata baccata, and several Fagaceae (Poyarkov et al. 2018).
Based on genus attributes, it is assumed that V. occidentalis has a secretive lifestyle, and spends considerable time underground or in leaf litter (semi-fossorial). Only two specimens were encountered over two intensive surveys. The first occurrence of V. occidentalis was a male specimen found in February, midday after heavy rain under a tree log at elevation 1000 m. However, this specimen was not collected. The second specimen was found in April, also during the day, in leaf litter near a trail after heavy rain (Poyarkov et al. 2018).
No calling activity during the day was recorded, and the reproductive biology and diet is assumed to be similar to its sister genera, Siamophryne, and related members in the Microhylidae family (Poyarkov et al. 2018).
Associated amphibians and reptiles in the areas include Microhyla berdmorei, Microhyla heymonsi, Sylvirana nigrovittata, Rhacophorus rhodopus, Theloderma albopunctatum, Theloderma gordoni, Acanthosaura lepidogaster, Pseudocalotes microlepis, Tropidophorus thai, Oreocryptophis porphyraceus cf. porphyraceus, and Ovophis monticola (Poyarkov et al. 2018).
There is difficulty determining the trends and threats of this species currently because of low sample size and the species’ rarity. While, the montane forests of Indochina where the Vietnamophryne species are found are nearly undisturbed, and contain some of the richest herpetofaunas in the world, deforestation is a growing threat in Indochina and Southeast Asia. Habitat loss and modifications to natural environments are recognized as a major threat to amphibians. Most forest specialists are restricted to primary undisturbed broadleaf evergreen montane forests. More surveys in Thailand and Myanmar are needed to elucidate the range and population status of the species (Poyarkov et al. 2018).