CYTOLOGICAL AND ECOLOGICAL COMPLEXITY IN THE EARLY MESOPROTEROZOIC

In combination, molecular phylogeny and biogeochemistry indicate that the eukaryotic clade differentiated early in Earth history. Sequence comparisons of SSU rRNA genes suggest a deep evolutionary divergence of Eukarya and Archaea, and steranes derived from C27-C29 sterols (synthesized by eukaryotes) and strong depletion of 13C (a biogeochemical signature of methanogenic Archaea) in 2700 Ma kerogens provide a minimum age for this split. Steranes, large spheroidal microfossils, and rare macrofossils of possible eukaryotic origin occur in Paleoproterozoic rocks, providing further hints of early and continuing eukaryotic participation in marine ecosystems. Nonetheless, until now evidence for morphological and taxonomic diversification within the domain has generally been restricted to latest Mesoproterozoic and Neoproterozoic successions. We now report morphologically complex, process-bearing protistan microfossils in early Mesoproterozoic shales from the Roper Group in Australia and consider their implications for eukaryotic evolution, paleoecology and biostratigraphy. The Roper Group consists of six highly asymetric depositional sequences bounded by conformable surfaces in the basin interior, dominated by deep basinal to distal shelf mudstones shallowing upward to coastal sandstones and intercalated shales. The Group is constained by an age of 1,492 ± 3 Myr (U-Pb SHRIMP analyses of zircons from an ash bed) at his base and a 1,429 ± 31 Myr Rb-Sr age near the top of the succession. The distribution of microfossils of probable eukaryotic origin shows a clear onshore-offshore pattern of decreasing abundance, declining diversity, and changing dominance. The Roper Group provides an unusually clear window on aspects of biology in the early Mesoproterozoic oceans, demonstrating that 1500 million years ago marine protists included cytologically sophisticated organisms that lived in ecologically differentiated communities.