RICHNESS, DIVERSITY, AND COMPOSITION IN AFRICA'S LATE OLIGOCENE TROPICAL FORESTS: EXAMPLES FROM CHILGA, NORTHWESTERN ETHIOPIA

Several studies have indicated modern African tropical forest plant species richness is less than that of South America or Southeast Asia, but how and when these differences developed are open to debate. The Chilga region of northwestern Ethiopia is characterized paleontologically by numerous and rich plant fossil localities, and commonly occurring vertebrate fossils. These fossils are constrained to 28 – 27 Ma by radiometric and paleomagnetic methods, placing them securely in the Late Oligocene, after flood basalt eruption and global climate changes of the Paleocene, Eocene, and Early Oligocene, but before influx of Eurasian fauna or development of the East African Rift. Thus, Chilga fossil assemblages afford us the opportunity to document richness and diversity after significant global climate changes, but before tectonically induced physical and biotic disruptions. In this study we focus upon two contemporaneous (stratigraphically equivalent) plant localities, approximately 1.5 km distant from each other, to assess species composition, richness, and diversity in the context of local depositional and ecological settings. Analyses indicate they are compositionally different from each other, approximately equal in richness, but are different in diversity (evenness). Assessment of composition indicates that one of these localities is likely to be in an earlier stage of succession, thus accounting for the diversity difference, and the other is under-sampled relative to total richness. Estimated species richness is compared with living analogous African, Asian and South American tropical forests to ascertain whether Late Oligocene forest communities were comparable to their living African analogues (and thus poorer than modern Southeast Asian or South American forests), or were richer than living African tropical forests. This has important implications for how and when African tropical forests became relatively species poor.