New Australopithecus boisei specimens from east and west Lake Turkana, Kenya

New specimens of Plio-Pleistocene Australopithecus boisei are described from east and west Lake Turkana in Northern Kenya. These include a cranium and partial mandible from deposits close to 2.5 Myr and two partial crania and two mandibles from later horizons. The earlier fossils enable us to decipher, for the first time, some of the in situ evolution of this species within the Turkana Basin. The following are among the important changes in the cranium through time: 1) increase in size and change in shape of the braincase, 2) changes in the meningeal vessel pattern and possibly in the venous drainage pattern, 3) increased flexion of the cranial base and decreased prognathism, and 4) changes in the temporal bone to bring about a more vertical posterior face of the petrous pyramid and the development of a strong articular eminence.