40Ar/39Ar age constraints on the Haifanggou and Lanqi formations: When did the first flowers bloom?
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Well-preserved Mesozoic terrestrial fossils were discovered in the Haifanggou Formation and the overlying Lanqi Formation (or their correlative strata) in NE China. The recent discoveries of Schmeissneria sinensis and Xingxueanthus sinensis from the middle and upper Jurassic Haifanggou Formation provide evidence that the origin of angiosperms could be predate the Early Cretaceous. In addition to the finding of pre-Cretaceous angiosperms from the Haifanggou Formation, the overlying Lanqi Formation yields a rich and varied terrestrial flora. The high diversity and abundance of the palaeoflora from these formations provide a unique window to understand floral evolution and its diversification in the Mesozoic. Two tuff samples and one andesite sample collected from the Haifanggou and Lanqi formations near Beipiao City, Liaoning, NE China yield robust 40Ar/39Ar age results. Our 40Ar/39Ar age of 166.7 ± 1.0 Ma for plagioclases from one tuff interbedded in the fossiliferous horizons of the middle Haifanggou Formation provides accurate age calibration for the pre-Cretaceous angiosperms for the first time. Moreover, our age results for these fossil-bearing formations will improve our knowledge of the Jurassic environment in general, including the link between plants and atmospheric CO2.