Discovery of water fern megaspore Ghoshispora and new dating for the Upper Cretaceous Yong'ancun Formation in Jiayin, NE China.

The Upper Cretaceous Yong'ancun Formation of the Jiayin area, outcropping along the Heilongjiang (Amur) River in China and yielding fossil plants and dinosaurs, has been considered Santonian in age mainly based on palynostratigraphic studies. Until now, there is no document on isotopic dating and guide fossils from this formation, limiting stratigraphic correlation with its equivalents from adjacent basins. This study reports a new age (84.64 ± 0.65 Ma, late Santonian) for the Yong'ancun Formation based on zircon LA-ICP-MS U–Pb dating for the first time from a rhyolitic tuff in the top of the unit. Ghoshispora , a fossil genus of megaspore of water fern is also reported for the first time, co-occurring with other valuable guide fossils such as G. zhaoi and G. triangulata. Based on the presence of Ghoshispora and plant megafossils, the flora of the Yong'ancun Formation is now interpreted to reflect a moist and warm climate, implying that the temperature dropped from high (mid-Cretaceous thermal maximum, KTM) to moderate warm and wetter conditions, at least in the Heilongjiang (Amur) inland region, close to the paleo-Pacific Ocean. The new discovery and dating evidence could provide valuable data for the correlation with the Yaojia Formation of the Songliao Basin, China. • LA-ICP-MS U–Pb date (84.64 ± 0.65 Ma) from the rhyolitic tuff of the Yong'ancun Formation reported for the first time. • The newly found water fern megaspore Ghoshispora is a valuable stratigraphic and palaeoenvironmental fossil. • The Yong'ancun Formation is the equivalent to the Yaojia Formation of the Songliao Basin (middle to late Santonian). [ABSTRACT FROM AUTHOR]