High-Precision 40Ar/39Ar Age Constraints on the Basal Lanqi Formation and Its Implications for Floral Evolution and Paleoenvironment

Well-preserved Mesozoic terrestrial fossils, mainly including plants and insects, were discovered in the Haifanggou Formation and the overlying Lanqi Formation in northeastern China. The recent discovery of Schmeissneria, previously identified as a member of Ginkgoales, from the Haifanggou Formation provided evidence that the origin of angiosperms could be earlier than previously believed and several theories regarding early seed plants should be reappraised. 92 taxa of plant fossils from the Lanqi Formation provide unique opportunities to understand floral evolution and its diversification in the Mesozoic. In addition to its importance for floral evolution, the fossil assemblage of silicified woods and compressed plants from the Lanqi Formation indicated a subtropical, humid and seasonal climate. Here we present robust high-precision 40Ar/39Ar data of 160.7 ± 0.4 Ma and 158.7 ± 0.6 Ma for two tuffs from the lowest part of the Lanqi Formation near the main outcrop of floral fossils in Beipiao City, Laioning, China. Our new precise 40Ar/39Ar age results indicate the whole Lanqi Formation was deposited in the Late Jurassic; consequently, the underlying Haifanggou Formation and Schmeissneria sinensis are at least Middle Jurassic in age. Two species of Schmeissneria, S. sinensis from the Haifangou Formtion in NE China and S. microstachys from the Liassic (Early Jurassic) of Germany and Poland, were reported. Because no basis for the Early Jurassic (Liassic) age ascribed to the Schmeissneria-bearing formations in Europe were provided, results presented here provide the first absolute age for Schmeissneria. Accepting Schmeissneria as an angiosperm pushes the origin of this group back in time by at least 40 Ma. Moreover, the new age data for the basal Lanqi Formation suggest that the paleoenvironment in the north margin of the North China Craton was dry and hot in the Late Jurassic.