Age Ranges of Australopithecus Species, Kenya, Ethiopia, and Tanzania

Abstract

Australopithecus anamensis, Australopithecus afarensis, Australopithecus bahrelghazali, Australopithecus garhi, and Kenyanthropus platyops have all been described from eastern Africa and Chad. Principal results presented are the age of specimens assigned to these taxa that derive from sedimentary formations of the Omo Group in the Omo-Turkana Basin of Kenya and Ethiopia. Also included are ages of relevant fossils from various sites in sediments of similar age preserved in the Ethiopian Rift Valley (e.g., Hadar, Asa Issie, Aramis, Maka, Bouri), and at Laetoli in Tanzania. All 40Ar/39Ar ages were recalculated to a common age for the Fish Canyon sanidine fluence monitor (FCs) to eliminate small differences in age caused by different choices for this value. The value chosen for the age of the Fish Canyon sanidine monitor (28.10 Ma) is that of Spell and McDougall (2003). The overall effect is to increase ages computed using 27.84 Ma for the age of the monitor by 0.93 %, and to increase ages computed using 28.02 Ma for the age of FCs by 0.29 %. An age of 4.000 Ma using the 27.84 Ma age for FCs is thus increased to 4.037 Ma; whereas the same age computed using 28.02 Ma is increased to 4.011 Ma. Thus the differences in the stated ages are on the order of 0.02 Ma--up to about twice the length of a precessional orbital cycle. Excellent age information is available on most specimens principally due to the efforts of Paul Renne and coworkers at the Berkeley Geochronology Center (BGC), and Ian McDougall and coworkers at the Research School of Earth Sciences, Australian National University; some other information (e.g., Walter and Aronson 1993) is also useful, but less extensive than the results obtained by the workers mentioned above. Show more