386


Journal of Paleontology 91(3):369–392


Figure 13. Diameter-frequency distributions of isolated individuals of coccoidal microfossils. Gray bars indicate spherical microfossils <60 μm in diameter; black bars indicate spherical microfossils >60 μm in diameter.


Formation from Siberia, the 1450 Ma Roper Group from Australia, the >1400 Ma Ruyang Group from North China, and the 1400Ma Greyson Formation from United States (Xiao et al., 1997; Javaux et al., 2001; Javaux andMarshal, 2006; Peng et al., 2009; Vorob’eva et al., 2015; Adam et al., 2016). These strata (the Kotuikan Formation from Siberia is an exception) are com- parable with each other for bearing the same microfossils. The underlying Changcheng Group of the Dahongyu Formation in the Jixian section could also be considered comparablewith the McArthurGroup because they share Valeria lophostriata,similar age, and lithology (Yan and Liu, 1993; Javaux and Marshal, 2006).


However, the Dahongyu microbiota, together with sub-


sequent microbiotas of the Jixian Group in the whole Yanshan Basin, exhibit totally different microbiotic components. Although, tens of thousands of microfossils have been reported from the Proterozoic Yanshan Basin (Sun, 2006; Shi et al., 2014), microfossils with complex morphology or ornament are still rare or debatable. Complex eukaryotes, such as Tappania, Satka and Shuiyousphaeridium, which are abundant micro- fossils in several locations (Xiao et al., 1997; Javaux et al., 2001;


Javaux and Marshal, 2006; Peng et al., 2009; Vorob’eva et al., 2015; Adam et al., 2016), were absent in the Dahongyu For- mation and any strata in the entire Yanshan Basin. Even Valeria lophostriata, which emerged in the underlying Changcheng Group, is absent in the subsequent strata. Although some allochthonous forms from the Dahongyu microbiota possess ornaments or odd morphology, there is a long path to follow before their eukaryotic nature can be proven because they are either too simple or too rare. In the Proterozoic Yanshan Basin, carbonates initially


emerged in the upper part of the Changcheng Group and dominated the sediments in the Jixian Group. Microbiotas, such as the Dahongyu microbiota, became the dominant or only type of microbiota in these carbonate-dominated strata of the Yanshan Basin (Shi et al., 2014; Fig. 5). However, based the record of several potential eukaryotes in the newly studied Dahongyu microbiota and Valeria lophostriata in the under- lying Changcheng Group (Peng et al., 2009), it could be inferred that eukaryotic life was already well established in the transi- tional time and had undergone a moderate diversification. Among reported potential eukaryotes in the Yanshan Basin, the


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