Early Miocene fossils from Rusinga Island, Kenya, provide some of the best evidence for catarrhine evolution and diversification, and, together with more than eighty-five other mammalian species, form an important comparative reference for understanding faunal succession in East Africa. While there is consensus over the stratigraphic position of most of Rusinga's volcaniclastic deposits, the lacustrine Kulu Formation has been placed in various parts of the geological sequence by different researchers. To resolve this discrepancy, we conducted detailed geological analyses which indicate that the Kulu Formation was formed in the Early Miocene during a period of volcanic inactivity and subsidence following the early, mainly explosive hyper-alkaline phase of the Kisingiri complex and prior to the final eruptions of nephelinitic lavas. The underlying Hiwegi and older formations were locally deformed and deeply eroded before sedimentation began in the Kulu basin, so that the Kulu sediments may be significantly younger than the 17.8 Ma Hiwegi Formation and not much older than the overlying Kiangata Agglomerata-Lunene Lava series, loosely dated to ca. 15 Ma. The overall similarities between Kulu and Hiwegi faunas imply long-term ecological stability in this region. Our stratigraphic interpretation suggests that the Kulu fauna is contemporaneous with faunas from West Turkana, implying that differences between these assemblages-particularly in the primate communities-reflect paleobiogeographic and/or paleocological differences. Finally, the position of the Kulu Formation restricts the time frame during which the substantial faunal turnover seen in the differences between the primate and mammalian communities of Rusinga and Maboko Islands could have occurred.
Bibliographical noteFunding Information:
This research would not have been possible without generous and continued support from the Leakey Foundation. We gratefully acknowledge the Kenyan government and National Museums of Kenya for facilitating our research, and thank especially Drs. Emma Mbua and Fredrick Kyalo Manthi for project support, and Blasto Onyango and Samuel Muteti for vital assistance in the field. We are deeply indebted to Wayne and Tanya Powell and to the staff of the Rusinga Island Lodge for their gracious hospitality. We also thank the British Institute in Eastern Africa for providing us with many of the necessities for our field work. Many thanks to Eric Delson, Alan Walker, and three anonymous reviewers for comments on this manuscript, and to Thomas Lehmann, Lars Werdelin, Ellen Miller, Stephane Ducrocq, and Bill Sanders for aiding in updates to the faunal list. We also thank the University of Minnesota and NYCEP (students funded by NSF IGERT award 0333415) for additional assistance and additional assistants.
- Catarrhine evolution
- Early Miocene
- Kulu Formation
- Rusinga Island