EXCEPTIONAL PRESERVATION IN NON-MARINE SETTINGS AT 1 GA

The Torridonian Sequence (1199±70 Ma to 977±39 Ma) in the northwest Scottish Highlands contains well-preserved assemblages of palynomorphs extracted from gray shales and siltstones considered to be lacustrine in origin. The approximately coeval Nonesuch Shale (1100 Ma) in Michigan also contains diverse associations of exceptionally well-preserved prokaryotic and eukaryotic microfossils. The Diabaig Formation (977±39 Ma) at the base of the Torridon Group contains fossiliferous phosphatic nodules associated with mud-cracked and reticulate MISS (Microbially induced Sedimentary Structures). This particular deposit, as a Lagerstätte, represents a rare opportunity for comparison and reconstruction of the biology and ecology of freshwater settings near the beginning of the Neoproterozoic. Phosphatic nodules, which preserve sub-cellular details, allow direct comparisons with the dispersed palynoflora, enhancing the morphological interpretation of palynomorphs recovered through maceration. Nodules also preserve primary laminations in the siliciclastic mats; however, evidence of in situ microfossils is missing in the MISS in these deposits. For example, filamentous microfossils studied in bedding parallel thin sections are oriented randomly – they do not show any evidence of having constructed reticulated microbial mat fabrics. Macerated phosphatic nodules also produce intact palynomorphs retaining both cyst and vegetative cell walls, some of which appear to preserve stages of biological decay. Framboids of haematite compare favorably with organically preserved balls of cells raising questions about the role of sulfur in some ancient freshwater settings. Populations of balls of cells surrounded by structurally complex walls provide intriguing evidence of early forms of multicellularity. We interpret these microfossils as the palintomic stage of unicellular protoctists, providing direct support of the synzoospore theory of metazoan origins. These insights indicate that non-marine habitats may have acted as potential cradles of evolutionary innovation during Precambrian time, an idea supported by recent genomic studies (Carrine Blank, University of Montana) indicating a freshwater origin to the basal cyanobacteria.