PHYTOLITH EVIDENCE FOR THE ROLE OF GRASSES IN EARLY MIOCENE VEGETATION OF EAST AFRICA (Invited Presentation)

Pollen data indicate that C₃ grasses (Poaceae) have existed in Africa since the Maastrichtian. However, although floras pointing to open, dry vegetation are known from the Eocene onward, pre-Miocene evidence for grass-dominated vegetation is lacking. In East Africa, pollen and stable isotope data from paleosol carbonates suggest a significant expansion of (C₃) grass-dominated vegetation during the mid-Miocene (15-13 Ma). Carbon isotope ratios of mammalian tooth enamel and paleosol carbonates indicate a dominance of C₄ plants by the late Miocene. A limitation of these reconstructions is their reliance on plant fossils (e.g., pollen) and proxies that infer grass community composition (e.g., C₃ vs. C₄) and abundance in past vegetation without direct observations.

We performed phytolith analyses in a series of terrestrial sediment and paleosols collected from two early Miocene sites: the Mush Valley locality in Ethiopia (22 Ma) and the R3 locality from Rusinga Island (~18 Ma) in Kenya. We aimed to test (a) when open-habitat grasses rose to dominance in East Africa and (b) what role these grasses played in ecosystems prior to their expansion.

Our preliminary analyses indicate that silica short cell phytoliths (GSSCPs) - diagnostic of grasses - account for <11% of the total phytolith content in the Mush Valley samples, while rugose globular morphotypes - forest indicators - are largely dominant the assemblages (>72%). In contrast, GSSCPs represent between 9 and 59% of the R3 phytolith assemblages. The remainder of these assemblages consist of a variety of morphotypes with unclear origin, mostly reflecting poor preservation and insufficient modern reference material. Silica bodies typical of current C₄ PACMAD grasses are present in the Rusinga deposits, although it is not yet clear how abundant they were in the vegetation. The previously documented occurrence of tree stumps in the same paleosol as the phytoliths and leaf remains in the overlying strata at R3 have led to the interpretation of a closed-canopy forest; our new data show that grasses were also important in the understory or in forest openings. Furthermore, our study provides the oldest direct evidence for C₄ grasses in Africa so far and support the notion that C₄ grasses evolved several million years before they became dominant in vegetation.