Abstract
Prorivulus auriferus, new genus and species, from the coastal plains of northeastern Brazil, is described. It is a member of a monophyletic unit, including Rivulus and the clade comprising all annual fish rivulid genera, diagnosed by two unambiguous synapomorphies: unossified interhyal and distal process of the second epibranchial absent. Prorivulus is hypothesized to be the sister group of the monophyletic assemblage including Rivulus and the clade comprising all annual fish rivulid genera, which is defined by four unambiguous synapomorphies: shortened uncinate process of the third epibranchial, articular face of the distal edge of first hypobranchial anteriorly expanded, ventral process of angulo-articular narrowed, and posterior flange of cleithrum absent. Prorivulus is distinguished from other basal rivulid taxa by having five branchiostegal rays.

Key words: Killifish, Cyprinodontiformes, Rivulidae, Neotropica, systematics, taxonomy, new genus, new species

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