Rapid preservation of Jehol Biota in Northeast China from high precision 40Ar/39Ar geochronology

Abstract

In Liaoning Province, Northeastern China, the Early Cretaceous Jehol Biota records significant evolution and radiation of fauna in terms of diversity, abundance and complexity. The alluvial-to-fluvial Lujiatun and lacustrine Jianshangou beds of the lower Yixian Formation yield exceptionally well-preserved three-dimensional and articulated dinosaur and mammal fossils, and two-dimensional primitive birds/feathered dinosaurs, respectively. Published 40Ar/39Ar ages for the two fossil beds have uncertainties of ± 0.4 to 1.0 Ma (± 2σ analytical uncertainty), and these reflect the measurement of large, and potentially heterogeneous, populations of feldspar crystals. More precise constraints on the timing and duration of biological radiation recorded by this sequence can further quantify evolutionary patterns and rates of diversification of important Early Cretaceous animals and plants. Single crystal 40Ar/39Ar fusion experiments on sanidine from intercalated ash beds yield dates that constrain the deposition of the Jianshangou and Lujiatun fossil beds to between 125.374 ± 0.085 and 125.572 ± 0.120 Ma, and 125.541 ± 0.099 and 125.793 ± 0.127 Ma, respectively. These ages indicate that the lower part of Jianshangou fossil bed was deposited nearly contemporaneously with the Lujiatun fossil bed. Thus, mass mortality events reflected by dinosaur fossil concentrations in the Lujiatun fossil bed and feathered dinosaur/primitive bird fossils in the Jianshangou fossil bed were nearly coeval and short-lived. These 40Ar/39Ar ages, combined with astrochronologic and paleomagnetic results from the lacustrine sediments in the Jianshangou fossil bed, not only refine the duration and rapid radiation of primitive birds and feathered dinosaurs in the deposit, but also suggest that the distribution of magnetic polarity chron boundaries within the Barremian stage in the current geologic time scale (GTS 2020) requires a revision.