Abstract The vertebrate faunal assemblage associated with the Neogene deposits in the Acre region (southwestern Amazonia) is secured as Late Miocene on the basis of a correlation with the ‘Mesopotamian’ faunal assemblage from the Paraná region of Argentina and Uruguay. Both assemblages occur in the time span of the Huayquerian South American Land Mammal Age (SALMA). The Acre, ‘Mesopotamian’ (Argentina and Uruguay), and Urumaco (Venezuela) assemblages are considered faunistically correlated and contemporaneous on the basis of their shared amniote taxa. The Laventan assemblage from Colombia has important faunistic affinities with the previous three but is older. A paleogeographic scenario is proposed to explain the long-distance correlation among those assemblages. On the basis of data from geology, field observations, fauna, and palynology, the validity of the Ucayali unconformity as a time marker along all of western Amazonia is rejected.