Jehol group

The Jehol Biota (pinyin: Rèhé Qún) includes all the living organisms – the ecosystem – of northeastern China between 133 and 120 million years ago. This is the Lower Cretaceous ecosystem which left fossils in the Yixian Formation and Jiufotang Formation. It is also believed to have left fossils in the Sinuiju series of North Korea. The ecosystem in the Lower Cretaceous was dominated by wetlands and numerous lakes (not rivers, deltas, or marine habitats). Rainfall was seasonal, alternating between semiarid, and mesic conditions. The climate was temperate. The Jehol ecosystem was interrupted periodically by ash eruptions from volcanoes to the west. The word "Jehol" now said to refer to a mythical land of the past in Chinese folklore, was the name given during Japanese occupation of the former Rehe Province.

Some scientists have argued that the Jehol Biota evolved directly from the preceding Daohugou Biota without any strongly defined division. However, the absolute dating of the Daohugou beds has been the subject of divergent opinion: in 2006, Wang et al. found an overall similarity between the fossil animals found in the Daohugou Beds and the "Jehol Biota" from the Yixian Formation. Several other research teams, including Liu et al., have attempted to disprove this reasoning by using Zircon U-Pb dating on the volcanic rocks overlying and underlying salamander-bearing layers (salamanders are often used as index fossils). Liu et al. found that the Daohugou beds formed between 164–158 million years ago, in the Middle to Late Jurassic. Later, Ji et al. argued that the key indicator of the Jehol biota are the index fossil fishes Peipiaosteus and Lycoptera. Under this definition, the earliest evolutionary stage of the Jehol Biota is represented by the Huajiying Formation.

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