|Amphiesma stolatum from Thailand. JCM|
Asia is the geographic center for the origin of most modern snake lineages. The Asian natricines known as keelbacks (Amphiesma) are widely distributed and inhabit a variety of niches and exhibit significant morphological variation. The genus keelbacks in the genus Amphiesma comprise at least 42 species of small to medium-sized (not exceeding one meter) of snakes. They are terrestrial to semiaquatic, oviparous, and generally considered harmless or mildly venomous.
Species in this group range throughout southern, eastern, and southeastern Asia, from Pakistan and India to eastern China, north into southernmost Russia and Japan, and southwards to Sumatra. In the mid-20th century Edmund Malnate used morphological characters including hemipenial morphology, dentition, and external scalation to divided the genus Natrix sensu lato into several genera, revalidating the genus Amphiesma Duméril, Bibron, and Duméril with the type species Amphiesma stolatum. The diagnostic characters of Amphiesma are defined as: hemipenes and sulci spermaticus simple; maxillary teeth in continuous series, gradually becoming larger posteriorly in the series or the last two teeth abruptly enlarged; terrestrial; internasals broad anteriorly, nostrils lateral; apical pits present or absent. Recent molecular phylogenies suggest that this genus is not monophyletic, and that additional cryptic diversity is present. All analyses consistently show Amphiesma consists of three distinct, monophyletic lineages.
In a recently published paper Guo et al (2014) divide Amphiesma into three genera, Amphiesma, Hebius, and Herpetoreas. The genus Amphiesma is monotypic, Herpetoreas contains three species, and Hebius comprises the remaining 39 species. On the basis of a combination of molecular analyses and external morphological comparisons, they describe a new species in the Herpetoreas group from China as H. burbrinki sp. nov. Several other species are shown to be non-monophyletic or contain significant levels of intraspecific genetic diversity. They also found another Old World natricine genus, Xenochrophis non-monophyletic and suggest further taxonomic revisions are needed in the Natricinae, at multiple levels.
Guo P, Liu Q, Zhang L, Ll J X, Huang Y, Pyron RA. 2014. A taxonomic revision of the Asian keelback snakes, genus Amphiesma (Serpentes: Colubridae: Natricinae), with description of a new species. Zootaxa,3873: 425-440.