Age of Lucy and the first family; single-crystal (super 40) Ar/ (super 39) Ar dating of the Denen Dora and lower Kada Hadar members of the Hadar Formation, Ethiopia

Single-crystal laser-fusion (super 40) Ar/ (super 39) Ar analyses on previously undatable tephra deposits provide the first reliable dates for the important hominid fossils popularly called "Lucy" and the "First Family," here dated to 3.18 and 3.20 Ma, respectively. These results help to establish an age framework for hominid and biostratigraphic evolution for the fossil-rich Hadar Formation. The abundance of well-preserved vertebrate fossils at Hadar can be attributed, in part, to an unusually high sediment-accumulation rate of approximately 80 cm/ka, which might be due to increased tectonic subsidence of the Hadar basin between 3.4 and 3.2 Ma. The new dates also provide supportive evidence that the boundary ages for the Kaena and Mammoth subchrons, of the geomagnetic polarity time scale, need to be increased by as much as 4% to 5%.