Daohugou Beds - Scientific Studies

Scientific Studies

A 2004 study by He et al. on the age of the Daohugou Beds found them to be Early Cretaceous, probably only a few million years older than the overlying Jehol beds of the Yixian Formation. The 2004 study primarily used radiometric dating of a tuff within the Daohugou Beds to determine its age. However, a subsequent study by Gao & Ren took issue with the He et al. study. Gao and Ren criticize He et al. for not including enough specifics and detail in their paper, and also take issue with their radiometric dating of the Daohugou tuff. The tuff, Gao and Ren argue, contains crystals with a variety of diverse radiometric ages, some up to a billion years old, so using dates from only a few of these crystals cannot determine the overall age of the deposits. Gao and Ren go on to defend a Middle Jurassic age for the beds based on biostratigraphy (the use of index fossils) and the bed's relationship to a layer that is known to mark the Middle Jurassic-Late Jurassic boundary.

Another study, published in 2006 by Wang et al., found that the Tiaojishan Formation (159-164 million years old, Middle-early Late Jurassic in age) underlies, rather than overlies, the Daohugou Beds. Unlike the earlier study by Gao and Ren, Wang et al. found an overall similarity between the fossil animals found in the Daohugou Beds and those from the Yixian Formation. The authors stated that

"vertebrate fossils such as Liaoxitriton, Jeholopterus and feathered maniraptorans show much resemblance to those of the Yixian Formation. In other words, despite the absence of Lycoptera, a typical fish of the Jehol Biota, the Daohugou vertebrate assemblage is closer to that of the Early Cretaceous Jehol Biota than to any other biota."

Wang et al. concluded that the Daohugou probably represents the earliest evolutionary stages of the Jehol Biota, and that it "belongs to the same cycle of volcanism and sedimentation as the Yixian Formation of the Jehol Group." However, a later study by Ji et al. argued that the key indicator of the Jehol biota are the index fossils Peipiaosteus and Lycoptera. Under this definition, the earliest evolutionary stage of the Jehol Biota is represented by the Huajiying Formation, and the Daohugou Formation is excluded due to the absence of Lycoptera fossils. Later in 2006, Liu et al. published their own study of the age of the Daohugou beds, this time 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 beds formed between 164-158 million years ago, in the Middle to Late Jurassic.