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This study uses the interpretation of paleosol features at Kanapoi, Kenya (4.2-3.4Ma) to reconstruct the ecosystem occupied by Australopithecus anamensis. The paleosols at Kanapoi provide a unique and fortuitous opportunity, in that the bulk of the hominid specimens derive from paleosols, providing direct evidence of the environment that the Kanapoi hominids occupied. Seven named types of paleosols are recognized at Kanapoi, each representing a trace fossil of the local ecosystem during soil formation. The hominid-bearing Dire paleosols provide evidence that A. anamensis inhabited areas of semi-arid, seasonal climate regimes with mean annual precipitation ranging from about 350-600 mm. The in situ hominid collections from Dire paleosols show that A. anamensis at least occasionally occupied relatively open low tree-shrub savanna vegetation formed in well drained settings, and may have preferred these conditions over other poorly drained soils. The relatively open conditions of Dite paleosols existed within a spatially variable ecosystem, characterized by a mosaic of environments, ranging from forb-dominated edaphic grassland to gallery woodland, providing a larger view of the mixed ecosystem in which A. anamensis lived. Synthesis of paleoenvironmental indicators of A. anamensis at Kanapoi and Allia Bay, Kenya suggests that as early as 4 Ma hominids thrived in varied ecosystems. (C) 2000 Academic Press.