Anna K. Behrensmeyer Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0121, U.S.A. E-mail: behrensmeyer.kay@nmnh.si.edu Alan L. Deino Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, U.S.A. E-mail: adeino@bgc.org Andrew Hill Department of Anthropology, Yale University, Box 208277, New Haven, Connecticut 06520, U.S.A. E-mail: andrew.hill@yale.edu John D. Kingston Department of Anthropology, Emory University, 1557 Pierce Drive, Atlanta, Georgia 30322, U.S.A. E-mail: jkingst@emory.edu Jeffrey J. Saunders Illinois State Museum, RCC 1011, East Ash Street, Springfield, Illinois 62703, U.S.A. E-mail: saunders@museum.state.il.us Received 28 February 2001 Revision received 13 June 2001 and accepted 14 June 2001 Keywords: Kipsaramon, Miocene, Kenya, Tugen Hills, stratigraphy, geochronology, hominoid, Equatorius. Geology and geochronology of the middle Miocene Kipsaramon site complex, Muruyur Beds, Tugen Hills, Kenya The Muruyur Beds are a substantial sedimentary deposit within a middle Miocene sequence of mafic volcanic flows associated with early stages of rifting in the central Kenyan Rift Valley. They are best represented in the Muruyur region, near Bartabwa, north of Kipsaramon, where dates range from 16·0 to 13·4 Ma. At Kipsaramon, located about 10 km south of Muruyur along the crest of the Tugen Hills, the upper Muruyur Beds are absent and the lower part can be divided into three members. Important fossil sites within Member 1 are dated between 15·8 and 15·6 Ma, and within Member 3 between 15·6 and 15·4 Ma. BPRP#89, in Member 1, is a bonebed at least 2500 m 2 in areal extent and up to 30 cm thick, which constitutes one of the richest concentrations of in situ fossil vertebrate bones in eastern Africa. BPRP#91, at approximately the same level at BPRP#89, is the source of a hominoid talus and other mammal and bird fossils. In Member 3, BPRP#122 has produced specimens of at least five individuals of the hominoid Equatorius, including a partial skeleton. The Muyuyur Beds were deposited near the western margin of a lake that was formed during the early stages of faulting and volcanism in the African Rift system. The bonebed in Member 1 appears to represent the influx of fluvially transported vertebrate and plant remains into a shallow portion of the lake. Elements of the fauna as well as stable isotopes that indicate both forest and more open environments occurred in proximity to the lake during the time of deposition of Member 1. Journal of Human Evolution (2002) 42, 11–38 doi:10.1006/jhev.2001.0519 Available online at http://www.idealibrary.com on Introduction There are a number of very rich fossil sites in the area around Kipsaramon, Tugen Hills, in the Baringo District of Kenya. The fossil assemblages are concentrated primarily at two horizons within the Muruyur Beds, and contain a varied vertebrate fauna, including 0047–2484/02/010011+28$35.00/0