The Anablepidae are a family of mostly livebearing cyprinodontiform fishes with few living representatives, but an increasingly diverse fossil record. According to recent contributions, an important part of their evolution can be traced back to Miocene remains in Central and Northwestern Argentina. In this article we describe the oldest known fossil species of Jenynsia, the most diverse genus of the family, from upper Miocene lacustrine deposits in Catamarca, Argentina. The new taxon is herein described based on many specimens of both females and sexually dimorphic males and a phylogenetic hypothesis is provided. The new species was recovered as the sister group of the remaining taxa of the nominal subgenus Jenynsia in our final hypothesis. However, it was also recovered as forming a clade with the extinct †Sachajenynsia pacha, and this clade as sister group of the subgenus Jenynsia, under one of the explored analytical conditions. The evolutionary history of Jenynsia was related in the literature to marine transgressions and the species radiation was dated through evolutionary model-based methods. Our findings and interpretations challenge the mostly marine-driven diversification of Jenynsia and the new taxon is older than predicted by the published estimations. This study reinforces the hypothesis that sexual reproduction by means of internal fertilization was already well-established in the Miocene for anablepids.Fil: Sferco, Marta Emilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Aguilera, Gaston. Fundación Miguel Lillo; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - Tucumán. Unidad Ejecutora Lillo; ArgentinaFil: Góngora, José Matías. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; ArgentinaFil: Mirande, Juan Marcos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - Tucumán. Unidad Ejecutora Lillo; Argentina. Fundación Miguel Lillo; Argentina