[1] The timing of the Jiufotang Formation remains speculative despite recent progress in the study of the Jehol Biota. In this paper we contribute to this topic with 40 Ar/ 39 Ar dating on K-feldspar (sanidine and orthoclase) from tuffs interbedded within the fossil-bearing shales of the Jiufotang Formation, from the upper part of the Jehol Group in Chaoyang, Liaoning, northeastern China. 40 Ar/ 39 Ar step heating analyses of K-feldspar and the SHRIMP U-Pb zircon data indicate that tuffs at the Shangheshou section erupted at 120.3 ± 0.7 million years ago. This result confirms an Aptian age for the Jiufotang Formation that was mainly based on biostratigraphic evidence. It also places stringent controls on the age of the fossils from the formation, providing a minimum age (120 Ma) for the four-winged dinosaur, Microraptor, and the seed-eating bird, Jeholornis.

The 40Ar/39Ar step heating analyses of K‐feldspar derived from the ignimbrite in Inner Mongolia, China indicates that they were formed at 159.8 ± 0.8 Ma, which provides a maximum age for the overlying fossil‐bearing lacustrine deposits (Daohugou Bed). This result favors a post‐Middle Jurassic (Late Jurassic or younger), rather than the Middle Jurassic age for the Daohugou Bed. Such a result is generally consistent with vertebrate biostratigraphic evidence, providing a maximum known age for the first appearance of several major animal groups such as Cryptobranchidae of Urodela (salamanders) and Maniraptora (birds and their closest dinosaurian relatives).

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 40Ar/39Ar 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 (2σ, full external error) and an isochron age of 132.3 ± 4.5 Ma (2σ, full external error). Seventeen total‐fusion 40Ar/39Ar 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 (2σ, full external error). These dates suggest an age of ∼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.

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