Y. Sawada, M. Saneyoshi, K. Nakayama, T. Sakai, T. Itaya, M. Hyodo, Y. Mukokya, M. Pickford, B. Senut, S. Tanaka, T. Chujo, and H. Ishida. The Ages and Geological Backgrounds of Miocene Hominoids Nacholapithecus, Samburupithecus, and Orrorin from Kenya. Human Origins and Environmental Backgrounds, Springer US, pages 71-96, 2006.