AFRICA’S MIOCENE TECTONIC, PHYSIOGRAPHIC, AND CLIMATE EVOLUTION LED TO ITS LOW SPECIES RICHNESS AND ITS STATUS AS THE “ODD MAN OUT” AMONG THE WORLD’S TROPICAL REGIONS

Plant biogeographic studies of New and Old World tropical floras indicate Africa has the lowest species richness, and was dubbed “the odd man out” by P.W. Richards in 1952. The wettest tropical regions of Africa never reach the high mean annual rainfall or aseasonality of South America, where α diversity for tree species ≥ 10 cm dbh is higher. Explanations for this difference in richness have been considered, including the difference in climate, but if climate is the cause, then when and how did the disparity arise since the breakup of Gondwana?

The plant fossil record documents the history of Africa’s unique biogeography and climate evolution, and can address hypotheses derived from modern data. A review of the record by Morley showed declines in pollen assemblage richness at or near Cenozoic Epochal boundaries, and pointed to the loss of palm diversity since the Paleogene. Our work in the highlands of Ethiopia documents paleofloras from the late Oligocene Chilga and early Miocene Mush Valley localities. The sites are highly productive and the fossils are beautifully preserved. Thus, we can analyze plant species richness, and climates across the Paleogene – Neogene boundary. The boundary also approximates closure of the Tethys Seaway and development of a land bridge - factors that also could have contributed to species loss (or gain).

We document high species richness in the late Oligocene. A sample of 1079 leaves from 11 sublocalities across the same stratigraphic level yielded 75 morphotypes, and a steep rarefaction curve. At the early Miocene Mush Valley site, 2427 leaves from 6 sublocalities within 7 meters of lacustrine shales yielded 49 morphotypes and a rarefaction curve that is less steep and leveling off. Both paleofloras are forests assemblages, but we find 3 of 5 palm subfamilies at Chilga and none at the Mush Valley. Eight of 10 identified plant genera from Chilga are found together today only in the speciose Albertine Rift, where rainfall is high and the dry season is short. Leaf area calculations indicate rainfall at Chilga was similar to today, ~1100 mm/yr. At Mush Valley rainfall was ~1600 mm/yr, and seasonality is under study. These data are put into the context of relative species loss from Africa, given its unique tectonic, physiographic, and resulting climatic history, and explain why it is now “the odd man out”.