High-precision age constraint for the Xiyingzi Bed of the Jiufotang Formation in western Liaoning, Northeast China

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The Jehol Biota includes exceptionally well-preserved fossils that are major components of Early Cretaceous land ecosystems. The youngest example of the Jehol Biota occurs in the Jiufotang Formation, which potentially records palaeoenvironmental factors triggering evolutionary turnover. However, the lack of stratigraphic correlations and uncertain age horizons for many fossil taxa lead to conflicting interpretations of events. This study describes a new significant fossiliferous bed (Xiyingzi Bed) from the upper Jiufotang Formation of the Baomayingzi-Yuanjiawa outcrop in western Liaoning, Northeast China. High-precision CA-ID-TIMS U-Pb analysis of a tuff sample from the Xiyingzi Bed provides an age of 121.272 ± 0.040/0.068/0.15 Ma, representing the first reliable age for the upper Jiufotang Formation in western Liaoning.Vertebrate fossils preliminarily reported from this bed include the confuciusornithid and enantiornithine birds, the istiodactylid pterosaur, the non-hadrosaurid iguanodontian dinosaur, the long-necked choristoderan reptiles, the sinemydid turtles, and the polyodontid and sinamiid fish. These fossils were previously reported from the lower horizons, indicating stable development of the late Jehol Biota. New results from the Xiyingzi Bed provides a glimpse of the late Jehol biota before transforming into a typical mid-Cretaceous biota in western Liaoning. Systematic biostratigraphic and chronostratigraphic analysis of the Xiyingzi Bed helps the evolutionary interpretations of the late Jehol Biota and the correlation of the Jiufotang Formation within different basins of western Liaoning.