LAUSR

Abstract Potamosiren magdalenensis Reinhart, is an extinct species of manatee (Sirenia, Trichechidae, Trichechinae), which has only been recorded for the middle Miocene Honda Group, in the La Venta area (Huila Department, Colombia). A new specimen referable to Potamosiren cf. P. magdalenensis is reported herein, collected from the early Miocene Barzalosa Formation. This unit crops out in the Pubenza locality, at the Tocaima municipality of the Cundinamarca Department, Colombia. The material described here represents the first evidence of a mammal from the Barzalosa Formation, the earliest record of Potamosiren so far reported and one of the only two trichechid records for the early Miocene of South America. The new specimen adds to a small but growing record of extinct trichechids, increasing the fossil record of this group in South America and allowing us to further explore their evolutionary history. The early Miocene appearance of trichechines coincides geographically and temporally with the onset of the Pebas Mega-Wetland System, which likely provided favourable conditions for the invasion of freshwater ecosystems of this group of fully aquatic mammals. Finally, the depositional environments represented by the Barzalosa Fm and a review of the fossil record of trichechines further support the notion that manatees have had a close association with freshwater systems since early in their evolutionary history, and that reinvasion of marine ecosystems did not occur until much later.