The Fort Ternan hominoid site, Kenya: Geology, age, taphonomy and paleoecology

Fort Ternan is of unusual interest, both because it is the oldest, directlydated site containing remains of Ramapithecus and because the well-known Early Miocene sites in the area afford the opportunity to trace hominoid evolution in one area over time. A phonolitic lava flow destroyed the lowland forest that had blanketed western Kenya since the Oligocene at about 15 Ma (date obtained by the ³⁹Ar/⁴⁰Ar step-heat method). The site itself is an inverted basin developed by erosion of a horst upthrust near the foot of the volcano, Tinderet. At 14 Ma, volcanic activity associated with the developing Western Rift Valley produced tuffs that washed into the basin and weathered into paleosols, in which the fossils are preserved. Mica flakes from the upper paleosol gave a date of 13·9 Ma, so the time span sampled is relatively brief. Evidence of the relative abundance and diversity of species and skeletal elements, the state of preservation of the fossils and their spatial distribution suggests that: (1) most of the fauma was locally derived; (2) only a small percentage of the species were derived from the forests on Tinderet; (3) Ramapithecus shared a common taphonomic history with the open-country animals and probably lived with them or in the ecotone; (4) the appearance of Ramapithecus and other new species may have been related to the ecological shift towards more open country.