THE EARLY MIOCENE FRUIT AND SEED FLORAS FROM RUSINGA AND MFWANGANO ISLANDS, KENYA

The rich fruit and seed flora from the Hiwegi Formation, Rusinga Island, Lake Victoria, provides a crucial window into floras and vegetation of the Kenyan early Miocene, an important setting for early hominoid evolution. Such floras from tropical palaeolatitudes are relatively rare. Some of the flora was published by Chesters in 1957 (Palaeontographica B). Our ongoing work is using extensive existing surface-picked collections (NHMUK and Nairobi) and new surface and in situ collections (Collinson et al. 2009, J. Hum. Evol.) to revise and reinterpret the floras.

Taphonomic studies of material from new excavations at Site R117 on Rusinga Island have enabled reconstruction of a closed woodland vegetation, recognition of undescribed new taxa, documentation of floristic variation between horizons and, by using sieving of surface and in situ collections, recovery of many small specimens. A number of familes in the flora have nearest living relatives in Africa that are woody vines or lianas, namely Connaraceae, Cucurbitaceae, Icacinaceae, Menispermaceae, and Vitaceae. Chesters (1957) did not record Vitaceae and some Menispermaceae were then known from only a single specimen. R117 has yielded two new taxa of Vitaceae and many small Menispermaceae that will be described. Rusinga and Mfwangano floras contain the only macrofossil record of these families in Africa.

Notably, key forest taxa, such as Entandrophragma and Sterculia, which occur in the Early Miocene of nearby Mfwangano Island, have been shown to be absent from Rusinga Island and no Rusinga taxa are unequivocal forest trees. At least 21 plant families are represented at R117. Some taxa occur in the area today including Zizyphus, Lannea, and Combretaceae whilst others (e.g. Berchemia and Cnestis) do not occur in Kenya but are present elsewhere in East Africa. Leakeyia (incertae sedis), not yet recognised in modern floras, is abundant in all horizons. There is only a single specimen possibly attributable to palms. Although legume pods occur in the flora Acacia seeds are absent. Thorns and thorny twigs occur in very low percentages and the thorns are short ‘rose-like’. Therefore, although the Miocene floras show some similarities to modern Kenyan vegetation, there are differences in composition, today's flora is much drier, and there is no single modern analogue.