PALEOBIOLOGY OF EARLIEST NEOPROTEROZOIC LAKES

Excellent palynological recovery combined with 3-dimensional preservation in phosphatic nodules provides a snapshot of life in two lacustrine systems at 1.0 Ga. Torridonian shales from the northwest Scottish Highlands preserve microbially-induced sedimentary structures (MISS), known as elephant-skin texture, within polygonal mud cracks. These sediments do not preserve in situ biologically-mediated fabrics, but are instead dominated by allochthonous remains. A century-old debate about raindrop impressions is revived to account for these reticulated MISS, which formed as rain fell on exposed sediment rich in exopolymeric substances (EPS). The biology of the ca. 1.1 Ga Nonesuch Shale, from the Midcontinent Rift System in Michigan, is dominated by eukaryotic remains which include the probable euglenoid, Moyeria, but lack any acanthomorphic acritarchs which might indicate adaptation to the planktic habitat. The Torridonian also lacks morphologies associated with phytoplankton found in lakes today. It seems that these organisms lived at the sediment-water interface and had yet to speciate within the water column. There are no unambiguous examples of fossils that match any specific chlorophyte morphologies. This leaves open the possibility that cyanobacteria remained the predominant source of primary fixed carbon during the earliest Neoproterozoic. Preliminary species richness assessments of fossil eukaryotes indicate that freshwater habitats were far more diverse than their marine counterparts at this time. Early lake systems could well have been centers of evolutionary innovation in eukaryotic lineages - providing habitat heterogeneity as a fundamental driver of Natural Selection, with the oligotrophic marine realm periodically re-populated by nonmarine species. This model would explain why some well-known marine taxa, Valeria lophostriata and Dictyosphaera sp., are also found in both the Torridonian and Nonesuch nonmarine assemblages.