Abstract

The Nsungwe Formation in the Rukwa Rift Basin provides a key window into mid-Cenozoic faunal evolution from the Western Branch of the East African Rift System. In this rift segment, proximal alluvial fan systems transitioned into a complex, volcanically-influenced landscape of fluvial, alluvial and lacustrine depositional environments. Dated at ~26-24 Ma via high-precision U-Pb and Ar/Ar geochronology of intercalated volcanic tuffs, sedimentological data from fossil-bearing sequences suggest seasonal aridity with perennial availability of water. Faunal analyses reveal a distinctive assemblage, with significant primate discoveries including the earliest evidence of the split between cercopithecoids and hominoids, the latest record of parapithecid anthropoids, and the first Paleogene lorisiform from eastern Africa south of the equator. Non-primate mammals span a diverse range of body sizes and ecological specializations. Other vertebrate and invertebrate clades are also well represented, offering a novel glimpse into the evolutionary history of terrestrial and freshwater habitats in eastern Africa and providing data on the complex tectono-sedimentary history of the Rukwa Rift Basin. Continued exploration refines perspectives on the Palaeogene-Neogene transition on continental Africa, with expanded opportunities for recognizing trends in faunal diversity across habitat types and through time.