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New total-fusion K-Ar ages indicate that all of the fossiliferous formations that make up the lower part of the Early Miocene Kisingiri sequence in western Kenya at Rusinga Island, Mfwangano Island, and Karungu were deposited during an interval of less than 0.5 million years at c. 17.8 Ma ago. This contrasts markedly with K-Ar ages previously published from these detrital-tuffaceous formations, which suggested that they were deposited over an interval of as much as 7 million years between 23 and 16 Ma, overlapping the age-ranges of all other East African Early Miocene sites including Koru, Songhor, Napak, Bukwa, Loperot, Muruarot and Buluk. In addition, the analytical problems revealed by the new Kisingiri results cast doubt on biotite ages which provide dating for the most important sites. Thus, the strong differences between the Kisingiri fauna and those of Koru, Sonhor and Napak, long held to be due to …