AGE OF LUCY AND THE 1ST FAMILY - SINGLE-CRYSTAL AR-40 AR-39 DATING OFTHE DENEN DORA AND LOWER KADA HADAR MEMBERS OF THE HADAR FORMATION, ETHIOPIA/

Single-crystal laser-fusion Ar-40/Ar-39 analyses on previously undatab le tephra deposits provide the first reliable dates for the important hominid fossils popularly called ''Lucy'' and the ''First Family,'' he re dated to 3.18 and 3.20 Ma, respectively. These results help to esta blish an age framework for hominid and biostratigraphic evolution for the fossil-rich Hadar Formation. The abundance of well-preserved verte brate fossils at Hadar can be attributed, in part, to an unusually hig h sediment-accumulation rate of approximately 80 cm/ka, which might be due to increased tectonic subsidence of the Hadar basin between 3.4 a nd 3.2 Ma. The new dates also provide supportive evidence that the bou ndary ages for the Kaena and Mammoth subchrons, of the geomagnetic pol arity time scale, need to be increased by as much as 4% to 5%.