PALEOCLIMATE AND PALEOECOLOGICAL RECONSTRUCTIONS OF THE EARLY MIOCENE HIWEGI FORMATION, RUSINGA ISLAND, LAKE VICTORIA, KENYA

The Early Miocene of Rusinga Island (Lake Victoria, Kenya) is best known for its vertebrate fossil assemblage, and particularly its remarkable record of early hominoids and catarrhines. In addition to the vertebrate fauna, there are also multiple stratigraphic intervals with well-preserved fossil leaves that have received much less attention. Within the Hiwegi Formation there are three fossil leaf-rich intervals: Kiahera Hill (~18.3 ma), R5 (~18.1 ma), and R3 (~17.8 ma). Here we present preliminary paleoclimate estimates for Kiahera Hill and R3 using the leaf physiognomic methods leaf margin analysis and leaf area analysis to reconstruct mean annual temperature (MAT) and mean annual precipitation (MAP), respectively. The MAT and MAP of Kiahera Hill is reconstructed to be 31.7 °C and 1183-2425 mm/yr, respectively, and the MAT and MAP of R3 to be 17-23.5 °C and 858-1760 mm/yr. The estimates for R3 overlap within uncertainty of previous estimates but are slightly cooler and drier. These paleoclimate estimates indicate that the Kiahera Hill site falls within a tropical seasonal forest biome, and the R3 site within a transitional woodland-tropical seasonal forest biome. The flora of Kiahera Hill had few monocots or herbaceous taxa and was dominated by large leaves, which also suggests a warm, forested environment. Collections from the R3 flora indicate it was spatially heterogeneous with monocots and herbaceous taxa found dominating patches on the landscape. In addition, the presence of reed-like monocots and emergent aquatics (e.g., aff. Potamogeton) indicates at least periodic standing water. Interestingly, the Kiahera Hill and R3 floras have significantly different taxonomic compositions. Further, the Kiahera Hill flora had a higher species richness and was more even than the R3 flora. These results demonstrate that there was a considerable change in both climate and vegetation over an ~500 kyr interval between the Kiahera Hill and R3 floras. Thus, this work, coupled with previous research, suggests that the Hiwegi Formation on Rusinga Island samples multiple environments in the Early Miocene, which in turn likely influenced the evolution and habitat preferences of early hominoids.