THE MESOPROTEROZOIC RUYANG GROUP, CHINA: A HOTSPOT OF EARLY EUKARYOTE BIODIVERSITY

Mesoproterozoic Era was a critical time for the initial diversification of eukaryotic groups and the appearance of the first complex morphologies. While eukaryotes evolved around 2.4 Ga, the first microfossils with ornamentation and sculpture occur in the 1.8-1.6 Ga successions worldwide. Shales and siltstones of the Ruyang Group, Shanxi Province, North China Craton, record a high diversity of such organic-walled microfossils.

Studied samples derive from several organic-rich shale intervals through the Baicaoping and the Beidajian formations, deposited in shallow marine environments. Acetolysis-resistant palynomorphs were extracted from the rock via dissolution in hydrofluoric acid and studied with light, scanning, and transmission-electron microscopy. Recently, the depositional ages of the Ruyang successions and the overlying Luoyou Group have been constrained to 1.75-1.40 Ga via zircon U-Pb dating. This dating extends back the time of the first appearance of complex eukaryotic characters (e.g. processes, complex wall ultrastructure) in the fossil record. Novel morphologies among the unicellular Ruyang biota include a variety of processes, from tube-like extensions to hirsute spines, vesicles with velutinous and spongy outer membranes, as well as numerous specimens with internal bodies of varying sizes, up to 87% of the vesicle diameter. We have also recorded the globally distributed Mesoproterozoic taxa Dictyosphaera, Shuiyousphaeridium, Spiromorpha and Tappania. Fossil diversity is highest in the middle Baicaoping Formation and declines in the lower-middle Beidajian Formation.

Key characters displayed by the Ruyang biota are consistent with reproductive structures (especially cysts) among modern protists. A significant degree of intracellular complexity would have been required for the intricate morphology observed in these microfossils. Thus, the Ruyang biota supports the previous evidence for the evolution of certain organelles and a cyst-forming mechanism by 1.7-1.4 Ga. Although the biological affinity of new Ruyang microfossils is not fully resolved, collective evidence from size and morphology to co-occurrence with members of diverged eukaryotic lineages like Shuiyousphaeridium (Chloroplastida), positions them within the crown group Eukarya.