Diversity of organic-walled microfossils from the early Mesoproterozoic Ruyang Group, North China Craton - A window into the early eukaryote evolution

- The Ruyang assemblage comprises 26 organic-walled microfossil taxa of 16 genera.
- Microfossils exhibit disparity of body plans, some appearing earlier than previously documented.
- Large size and diverse forms of vesicle ornamentation indicate eukaryotic affinity.
- Within-assemblage diversity in the Ruyang Group is high by pre-Ediacaran standards.
- Diversification of Eukarya was underway by 1.7-1.4 billion years ago.

Mesoproterozoic Era was an important time for initial diversification of eukaryotic clades and appearance of the earliest complex cell morphologies. Shales and siltstones of the Ruyang Group, Shanxi Province, North China Craton, record such diverse organic-walled microfossils, in addition to novel morphotypes. Recently, the depositional age of this succession has been constrained to 1744 ± 22 to 1411 ± 27 Ma via zircon U-Pb and Hf isotopic dating. This dating extended back the time of the first appearance of complex eukaryotic characters (e.g. processes, sculpture, layered wall structure) in the fossil record. We have conducted a biostratigraphic investigation of the samples throughout the fossiliferous Ruyang Group to provide an estimate of the relative early eukaryotic diversity in the Mesoproterozoic. Light- and scanning electron microscope observations have documented 26 species, including several that are reported for the first time, and some that were previously known only from younger, Neoproterozoic strata. Diversity was high in the upper Baicaoping Formation, declined in the middle and reached its peak in the upper Beidajian Formation. Novel morphologies among the unicellular Ruyang biota included a variety of processes, from tube-like extensions to hirsute spines, vesicles with velutinous outer membranes, as well as numerous specimens with internal bodies of varying sizes. We have also recorded the globally distributed Mesoproterozoic taxa Dictyosphaera and Tappania, and several morphotypes of Shuiyousphaeridium. Key characters displayed by the Ruyang biota are consistent with reproductive structures (especially cysts) like those among modern protists. These microfossils provided an additional evidence for the ongoing first radiation of Eukarya by at least 1400 Ma.

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