Isanophis boonsongi new comb., preserved
holotype (FMNH 135328). From top to
bottom: Dorsal view – Ventral view –
Lateral view of the head and neck, left side.
Photographs by Patrick David.
There is little doubt that Southeast Asia harbors the most diverse assemblage of living snake species. And, a number of species from the Indochinese region, including Thailand, are still poorly known only, in some cases known only from their holotype or type series, or at best a handful of specimens. Natricid snakes are particularly diverse in Southeast Asia and three genera contain species that seem to be restricted to very small ranges, they are all aquatic and despite being described in the mid-20th century have remained enigmatic.
Angel’s stream snake, Paratapinophis praemaxillaris described by Angel in 1929, has been known from two syntypes from northern Laos, and six other specimens from China and Thailand. Two other natricine species, Pararhabdophis chapaensis and Parahelicops annamensis both described by Bourret in 1934, were previously known from their respective holotypes. However, Stuart (2006) described a second specimen of P. annamensis, from Laos in 2006. Recently, intensive fieldwork in northern Vietnam and Laos, recovered about 10 specimens of Parahelicops annamensis and Pararhabdophis chapaensis each. Another rare species, Parahelicops boonsongi described by Taylor and Elbel in 1958 was described on the basis of a single specimen from Loei Province in northeastern Thailand. Subsequently, two additional specimens, also from Loei Province were found by Cox in 1995.
Taylor and Elbel placed their new species to the genus Parahelicops because of morphological similarities with P. annamensis, such as the single prefrontal. However, the generic status of Parahelicops has been controversial since its description. It was established by Bourret for a new species, Parahelicops annamensis, on the basis of a single specimen with the following characters: 25 subequal maxillary teeth, the last two enlarged; head quite distinct from the neck; eye small with a round pupil; nostrils directed upwards; two internasals, a single prefrontal; elongated body, slightly laterally compressed; dorsal scales keeled, without apical pits, in 15 rows; tail long; subcaudals paired; hypapophyses developed throughout the vertebral column. Bourret (1934b) also noted its similarity to Opisthotropis but differed in dentition, having its head distinct from the neck, and its elongated body.
Parahelicops boonsongiwas described by Taylor and Elbel in 1958 and is known from only three specimens from Thailand. It has been placed either in the genus Parahelicops Bourret, 1934, along with Parahelicops annamensis, as well as the genus Opisthotropis Günther, 1872. In a new paper David et al. (2015) compared its morphological characters with those of P. annamensis and with three other relevant genera, Opisthotropis, Pararhabdophis Bourret, 1934, and Paratapinophis Angel, 1929. Parahelicops boonsongi is phenotypically distinct from Parahelicops annamensis, Opisthotropis, and all other natricine genera. The authors erect a new genus, Isanophis gen. nov., to accommodate Parahelicops boonsongi. How these snakes are related to each other and other natricids remains to be determined.
David, P., Pauwels, O. S., Nguyen, T. Q., & Vogel, G. (2015). On the taxonomic status of the Thai endemic freshwater snake Parahelicops boonsongi, with the erection of a new genus (Squamata: Natricidae). Zootaxa, 3948(2), 203-217.