The Ar-40/Ar-39 Dating Of The Early Jehol Biota From Fengning, Hebei Province, Northern China

The bird fossil-bearing deposits at the Jiecaigou section, correlative to the Dabeigou Formation, in Fengning, Hebei Province, northern China, is well known for yielding a fossil assemblage representing the earliest evolutionary stage of the Jehol Biota. The precise age of the fossil-bearing deposits, however, is unknown. The Ar-40/Ar-39 age spectrum obtained on bulk K-feldspars from the tuff layer about 2 m below the bird fossil-bearing layer gave a plateau age of 129.0 +/- 1.3 Ma (2 sigma, full external error) and an isochron age of 132.3 +/- 4.5 Ma (2 sigma, full external error). Seventeen total-fusion Ar-40/Ar-39 ages on K-feldspars from the interbedded tuff about 6 m below the fossil-bearing layer resulted in a weighted mean of 130.7 +/- 1.4 Ma (2 sigma, full external error). These dates suggest an age of similar to 131 Ma for the early Jehol Biota and combined with previous dating indicate that this biota lasted at least from 131 Ma to 120 Ma (Late Hauterivian to Aptian). These dates also represent the earliest absolute age for known enantiornithine birds in the world.