Taxonomic status of the enigmatic Natrix sexcarinata Wagler, 1824 (Serpentes: Colubridae: Colubrinae)

Abstract

Wagler described Natrix sexcarinata through a specimen collected on the banks of the Amazon River in Brazil, without citing a specific location. The species was later transferred to different Neotropical genera (Chironius, Herpetodryas, Phrynonax, and Pseustes) currently allocated in the Family Colubridae. The taxonomic instability of Natrix sexcarinata is probably due to its brief original description in combination with its supposedly inaccurate illustration. Despite the holotype being lost during World War II, some authors have pointed out that its description is somewhat similar to the widespread concept of Pseustes poecilonotus. More recently, a molecular study proposed the synonymy of the genera Pseustes with Spilotes, resurrecting the Phrynonax genus to allocate Ph. poecilonotus and Ph. polylepis. However, the taxonomic positioning of Natrix sexcarinata remains unsatisfactory with respect to the cis-Andean taxa placed in Phrynonax or even Spilotes throughout the Amazon. Considering the taxonomic and nomenclatural problems mentioned above, we investigated how many Phrynonax species occur along the Amazon Basin and what would be their applicable name. We examined the external morphology (meristic, morphometric, and color pattern data) of 118 Phrynonax specimens and prepared the hemipenes of seven specimens to understand geographic, sexual, and ontogenetic variability parameters. Our result suggests the synonymy of Ph. polylepis and N. sexcarinata under the combination Phrynonax sexcarinatus. Based on available evidence, we reinforce that Phrynonax sexcarinatus is the only congener that occurs along the Amazon Basin. We have also designated a Natrix sexcarinata neotype to stabilize its complex nomenclature.