Patterns of abundance and diversity in late Cenozoic bovids from the Turkana and Hadar Basins, Kenya and Ethiopia

Abstract

After decades of fieldwork spurred by the search for human ancestors, paleontologists in East Africa are compiling networks of databases to address questions of long-term evolutionary, environmental, and ecological change. Paleontological databases from the Turkana Basin of Kenya and Ethiopia (East Turkana, West Turkana, Kanapoi, Lothagam, and Omo) and the Hadar Basin of Ethiopia’s Afar region consist of nearly 70,000 specimens of fossil vertebrates (mostly mammals) that date from the late Miocene to the Pleistocene. Here we focus on the most abundant family of fossil mammals, the Bovidae (N = 8213 specimens), and illustrate patterns of taxonomic abundance and diversity from about 7 Ma (million years ago) to about 1 Ma. The key questions we address are the following: How much variation in patterns of faunal change is there within different areas of a large sedimentary basin? How much variation is there between basins? How are these patterns related to broad signals of climatic change? What are the implications of the bovids for East African environments and for hominin evolution in the late Cenozoic? A correspondence analysis of bovid tribes indicates that important differences in taxonomic abundance existed among different areas of the Turkana Basin, and that some of these differences had environmental implications. The lower Omo Valley appears to have remained distinct from other parts of the Turkana Basin between 3 and 2 Ma, with consistently higher proportions of Tragelaphini and Aepycerotini, and at times of Reduncini and Bovini. These bovids are indicative of woodlands or forests (Aepycerotini and Tragelaphini) or of moist grasslands near wooded habitats (Reduncini and Bovini). An analysis of bovid tribes indicative of open and seasonally arid grasslands (Alcelaphini, Antilopini, and Hippotragini) shows relatively high proportions of these bovids in the West Turkana areas, but very low proportions in the Omo, especially prior to about 2 Ma. This indicates that the Omo remained wetter and more wooded than other parts of the Turkana Basin for much of the Plio-Pleistocene, while the West Turkana area appears to have been more open than other parts of the basin, and East Turkana had conditions intermediate between those at West Turkana and those in the Omo. Fossil bovids from the Hadar Basin suggest diverse environments including woodlands, wet grasslands, and drier savanna grasslands. An increase in the abundance of arid adapted bovids in the late Pliocene and early Pleistocene of the Turkana and Hadar Basins provides evidence that faunal changes in these different areas were driven by common factors consistent with the known record of climatic change. Analyses of species diversity among bovids show three peaks of richness in the Pliocene and Pleistocene. The first peak occurred at about 3.8–3.4 Ma, the second at 2.8–2.4 Ma, and the last from about 2.0 to 1.4 Ma. The last two of these peaks coincide with previously identified periods of high faunal turnover in East Africa. Although climate appears to have shaped major patterns in the evolution of bovids, the fact that different areas of a single sedimentary basin show distinct responses highlights the complexities involved in establishing causal links between paleoclimate and evolution.