Taphonomy of phytoliths and macroplants in different soils from Olduvai Gorge (Tanzania) and the application to Plio-Pleistocene palaeoanthropological samples

The abundance and types of phytoliths in the fossil record are taphonomically biased and do not correspond with the macroplant record. To better understand the bias and improve the interpretation of samples from Olduvai Gorge, Tanzania, we analysed the phytoliths from three sample sets: modern grasses, sedges, palm and dicots in the area; modern soils in the area; and the fossil soils from the eastern palaeolake margin in lowermost bed II. Fourier transform infrared spectrometry analyses were performed in parallel to compare the mineral composition of both modern soils and fossil soils. We found that the abundance of phytoliths is greatly reduced in soil samples compared with modern plant material, but the morphotypes can be reliably interpreted. Dicotyledonous wood/bark phytoliths appear to be over represented in the soil types. Grass phytoliths, and sedges to a lesser degree, are preserved in the soils but in lower abundances resulting in these groups being under-represented. The macroplant fossils are fragments of grass and sedge aerial culms, and dicotyledonous stems that are preserved in fluvial and lacustrine deposits. It is suggested that phytoliths represent the continuous flora whereas macroplant fossils represent the more robust tissues of selected plants preserved under more catastrophic conditions. A post-depositional model is proposed and the palaeovegetation is reinterpreted as being richer and more complex than indicated by other fossil data.