Preliminary geology and paleontology of new hominid-bearing Pliocene localities in the central Afar region of Ethiopia

The Woranso-Mille paleontological study area is located in the central Afar region of Ethiopia, about 360 km northeast of the capital, Addis Ababa. Some parts of this area have been paleontologically known since the 1970s. However, most of the fossiliferous areas were discovered by surveys conducted in the region between 2002 and 2004. By the end of the 2006 field season, a total of 17 vertebrate localities had been designated, and more than 1000 vertebrate fossil specimens collected. Among these specimens, there are more than 20 fossil hominid craniodental and postcranial remains, including one partial skeleton, of Pliocene age (3.5-3.8 Ma). Research at this study area has thus far focused on the geology and paleontology of the early Pliocene deposits along the Mille river and adjacent areas located between the towns of Mille and Chifra. Exposed sediments in the new fossiliferous area are mostly silty sand and silty clay horizons interbedded with a number of volcanic tuffs and basaltic flows suitable for 40Ar/39Ar radioisotopic dating. The total stratigraphic section is about 50 in thick with a minimum age of similar to 3.5 Ma. The study area also has deposits of early to middle Pleistocene age, although no locality has been designated to date. The new Woranso-Mille paleontological study area provides a crucial temporal window into the time during which Australopithecus anamensis (3.9-4.2 Ma) appears to have given rise to Australopithecus afarensis (3.0-3.6 Ma). Radiometric dates have thus far yielded a minimum age of 3.5 Ma for the hominid localities and this conforms well with a biochronological age estimate of 3.6-3.8 Ma. The associated fauna, particularly the abundance of fossil cercopithecids and presence of diverse aquatic fossil vertebrates, indicates a relatively closed, wooded habitat probably associated with a paleo-Mille river.