The monotypic genus Atopomesus, a poorly known member of the family Characidae, is re-described. Atopomesus pachyodus is endemic to the Rio Negro basin, where it feeds on aquatic insect larvae, and seems to occur mostly in riverine sand banks and beaches. A. pachyodus was originally described as a member of the Characidae subfamily Cheirodontinae, but has been considered as an Characidae incertae sedis based on relatively superficial studies of its dentition and external morphology. However, according to the morphological analyses presented herein, a close relationship between A. pachyodus and the members the subfamily Aphyoditeinae is hypothesized. The inclusion of A. pachyodus in the Aphyoditeinae is based on the shared presence of a long posteriorly directed spiniform process on the posteroventral margin of the orbitosphenoid, the elongation of the transverse process of the neural arch of the third vertebra, and the elongation of the posterior process of the parasphenoid.

Amazon basin, blackwater rivers, freshwater fish, Neotropical ichthyofauna, Characidae systematics