Implications of a New Aff. Hippopotamus Karumensis Mandible From the Koobi Fora Formation, Turkana Basin, Kenya

Abstract

The phylogeny of the East African Hippopotamidae is problematic. A particularly controversial relationship is that between aff. Hippopotamus protamphibius and aff. Hippopotamus karumensis from the Pliocene and Pleistocene of the Turkana Basin (Kenya and Ethiopia). Various hypotheses have been proposed, including (1) that the species are sister taxa derived from a common ancestral hexaprotodont species, (2) that they are time-successive segments of a single anagenetic evolutionary lineage, and (3) that aff. Hippopotamus karumensis represents a branch of an evolving aff. Hippopotamus protamphibius lineage. A major obstacle to determining which of these evolutionary scenarios is the most parsimonious is the unknown degree to which the two species co-occurred. Here we describe a new aff. Hippopotamus karumensis mandible from the Upper Burgi Member of Koobi Fora Formation, east of Lake Turkana (Kenya), that represents the earliest occurrence of this species yet to be documented. The presence of aff. Hippopotamus karumensis in this member implies a lengthy period of sympatry with aff. Hippopotamus protamphibius that makes an anagenetic relationship between these species improbable. It is also unlikely that the species derived from a common hexaprotodont since there is currently a lengthy time gap between the earliest occurrences of the two species. The most parsimonious interpretation is that aff. Hippopotamus karumensis represents a branch of an evolving aff. Hippopotamus protamphibius lineage