EARTH'S ADOLESCENT LIFE: INSIGHTS INTO THE PALAEOBIOLOGY OF EARLY EUKARYOTES FROM THE MESOPROTEROZOIC FOSSIL RECORD

In the aftermath of the Great Oxidation Event, the mid-Proterozoic interval is marked by tectonic and climatic stability, low primary productivity, and lower oxygen levels than the modern, predominantly anoxic ferruginous below the mixed layer. Apparent stability during this time prompted the moniker “Boring Billion”. However contrary to it, some of the major evolutionary events happened at this interval: the evolution and diversification of eukaryotes and the innovation of multicellularity. Early eukaryotes are represented in the rock record predominantly by organic-walled microfossils (OWM). Although they first appear around 1.65 Ga, it took until c. 1 Ga for their diversification and the onset of complex multicellularity. Biological affinities and ecology of this microfossil group are still poorly understood. The impact of environmental conditions like ocean oxygenation, often invoked as drivers of the eukaryotic evolution is thus also uncertain, e.g. if they were restricted to oxygenated surface or cyanobacterial pools, or also inhabited the ferruginous or euxinic parts of the water column, and if they predominantly occupied shallow or deep environments.

A compilation of recent progress as well as detailed studies of OWM assemblages from the Paleoproterozoic-Mesoproterozoic units of the North China Craton and the late Mesoproterozoic of Arctic Canada aims to discuss major trends in evolution and ecology of early eukaryotes, including the appearance of their crown group. Following the prokaryote-dominated assemblages of the Paleoproterozoic, the early Mesoproterozoic units record an increase in complexity of eukaryotic microfossils. Taxonomic turnover is observed around 1.2 Ga, with the disappearance of process-bearing Tappania and the increase of taxa with ornamented vesicle wall. Several case studies show that diverse OWM occur in a variety of oxygenated and ferruginous settings, as well as spatially heterogenous environments. These preliminary data also show that successful long-ranging species most commonly occur in shoreface to shelfal, shallow marine environment.