CENOZOIC VEGETATION CHANGE IN AFRICA: A LARGE-SCALE VIEW OF A SMALL-SCALE PROCESS

Relying on plant fossils to represent the evolution of Cenozoic African biomes renders a view of past vegetation biased toward geographic areas and time intervals that preserve the most specimens or had the most research. Africa is roughly three times the size of the U. S. yet is documented by only a handful of Paleogene plant localities, and has a Neogene record biased toward the depositional basins of the East African Rift. Placing all known paleobotanical sites in their correct location on paleogeographic maps helps illustrate the varied data coverage. Efforts to improve this record, which is also uneven with respect to time control, have produced Eocene, Oligocene, and Miocene sites in Tanzania, Ethiopia, and Kenya. These enrich our understanding of the history of modern plant communities, illustrate how they may have differed in the past, and provide dated paleoclimate for these intervals.

A middle Eocene (46 Ma) site demonstrates that northern Tanzania (12º S paleolatitude) had a dry climate similar to today, and woodland communities dominated by microphyllous, caesalpinioid legumes – structurally similar to modern woodland communities. Paleobotanical sites on the northwestern Ethiopian Plateau (11º N paleolatitude) dated at 28-27 Ma document forest communities with genera found today in West Africa, and the coastal and Eastern Arc Mountains of Kenya and Tanzania, but now absent from Ethiopia. The fossils document a biogeographic link between these now disjunct genera. But, the common occurrence of palm fossils indicates these forests differed from living counterparts, where palms are always absent or species-poor. Thus, a decline in the ecological role for palms took place after 27 Ma. A decrease in palm diversity seems to have occurred before 28 Ma as the palm genera identified are found in Africa today. The Miocene record from the Tugen Hills, Kenya, indicates considerable variation in environments between 12.6 and 7 Ma, ranging from lowland forest with West African botanical affinities (including absence of palms), to seasonally dry, legume-dominated woodland or wooded savanna. These sites demonstrate the overall trend toward increasing aridity and spreading grass-dominated environments during the Neogene was complicated by smaller-scale variations in landscape and vegetation in the East African rift.