Biogeographic and Evolutionary Implications of an Extinct Late Pleistocene Impala from the Lake Victoria Basin, Kenya

This study contributes to the growing complexity of the impala fossil record through a morphological description and analysis of Aepyceros fossils from late Pleistocene deposits in Kenya's Lake Victoria Basin. We show that the Lake Victoria impala belongs to an extinct species that differs from modern impala and its fossil predecessors by a combination of exceptionally deep mandibles and teeth characterized by greater hypsodonty and occlusal lengths. Whereas modern impala (A. melampus) displays substantial ecological flexibility, these traits in the extinct species suggest a more dedicated adaptation to grazing in open and dry environments. Previous phylogeographic observations indicate that A. melampus was extirpated from East Africa, perhaps during the middle-to-late Pleistocene, and later recolonized from southern Africa. The Lake Victoria impala raises the possibility that the evidence interpreted as extirpation may instead reflect speciation, with A. melampus giving rise to a novel East African species while persisting unchanged in southern Africa. Increased rainfall and rising atmospheric CO₂ concentrations at the end of the Pleistocene may have played a role in the disappearance of the extinct form via habitat loss and possibly competition with the more versatile A. melampus.