Tragelaphus nakuae: evolutionary change, biochronology, and turnover in the African Plio-Pleistocene

Abstract

Fossil Bovidae constitute one of the most significant proxy records for evolutionary and palaeoecological change in Africa. Tragelaphus nakuae is a regularly encountered antelope in the East African Plio-Pleistocene, and is a common component of hominin faunas. As previously understood, this species ranged for almost 2 million years, encompassed a large range of morphological variation, exhibited relative stasis in the face of environmental perturbations, and left no known living descendants. I here review and revise the fossil record of this tragelaphin bovid, finding that specimens older than ∼2.8 Mya and previously attributed to T. nakuae or a close form are in fact referable to a distinct, but ancestral, species. This new interpretation adds these fossil tragelaphins to the body of evidence supporting major faunal turnover occurring around 2.8 Mya in concert with global climatic change. I also document morphological changes that occur through the duration of T. nakuae, particularly after 2.3 Mya. These taxonomic revisions allow for refined biochronological estimates for several East African Plio-Pleistocene sites and specimen assemblages of uncertain age. A phylogenetic analysis suggests that the T. nakuae lineage is related to the extant bongo (Tragelaphus eurycerus), relating this living but enigmatic forest antelope to the fossil record. One resulting palaeoecological hypothesis is that the bongo's modern fragmented range represents the relicts of a much more widely distributed late Pliocene African forest belt. This study highlights the importance of specimen-based approaches for elucidating the pattern and timing of major evolutionary events. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 162, 699–711.