MAGNETOSTRATIGRAPHY OF THE EARLY MIOCENE HIWEGI FORMATION (RUSINGA ISLAND, LAKE VICTORIA, KENYA

The early Miocene fossils from Rusinga Island (Lake Victoria, Kenya) provide some of the best evidence for understanding early hominoid evolution and Miocene faunal succession in East Africa. The majority of the fossils from Rusinga have been recovered from several localities within the Hiwegi Formation. Determining accurate ages for these fossil sites is crucial for developing an understanding of East African paleobiology. Previous researchers have suggested that the major fossil localities within the Hiwegi Fm. are correlative and were deposited synchronously. However, due to the faulting and the lack of horizontal continuity between localities, correlating between the fossil sites is difficult and the age relationships of the fossil localities are uncertain. To address these problems, we conducted paleomagnetism analyses at four of the major Hiwegi Fm. fossil localities (R1, R2, R3 and R5). Our results document a series of reversals through the formation that can be used to correlate the sites. Importantly, fossil sites at the different localities are found within strata of different polarities. This indicates that there are multiple fossiliferous horizons within the Hiwegi Formation and that the fossils were not deposited synchronously. These results suggest that previous paleontological and paleoenvironmental studies that lumped all of the fossils collected from the Hiwegi into a single unit have time-averaged fossils from multiple intervals. Thus the models of Rusinga’s paleoenvironments and faunal communities based on analyses of fossils from the Hiwegi Formation may be oversimplified. Furthermore, this work highlights the importance of well-provenanced collections in paleontological research.