EVIDENCE OF DIVERSE TERRESTRIAL PRECAMBRIAN ECOSYSTEMS

Precambrian fossils and microfossils are almost universally considered to be of marine origin. The few isolated reports of non-marine Precambrian fossils do not form a coherent picture of non-marine ecosystems at any point in Precambrian time. We have now recovered organic-walled microfossils from over 50 horizons throughout the Torridonian sequence (Sleat, Torridon and Stoer Groups) of northwest Scotland. These rocks, which range in age from 1199±70Ma to 977±39Ma, were deposited in non-marine settings. Acid maceration of grey laminated siltstones yields populations of eukaryotic and prokaryotic cells of mixed provenance. Excellent preservation allows us to demonstrate that both vegetative and encysted cell walls have been preserved. Lophosphaeridium and other sphaeromorphs with low-profile sculpture are common, but there are no true acanthomorphic acritarchs in these palynofloras. Large sphaeromorphs, similar to Chuaria, occur in both the Sleat and Torridon Groups, long before the gigantism of the Ediacaran Period. Palynomorphs extracted from microbial mat lithofacies and from sediments infilling desiccation cracks, include wefts of sheathes and populations of small cells similar to the Halothece/Euhalothece Group of extant halotolerant, mat-dwelling cyanobacteria. Structural elements (spines, cuticles and complex tissues) indicate the presence of larger, likely multicellular organisms which are clearly not algal. Overall, organic remains from Torridonian shales demonstrate the existence of a diverse and complex biota adapted to subaerial habitats by around 1 Ga. This indicates a high probability that habitat heterogeneity that is represented in the terrestrial landscape was a selective factor in evolution by the beginning of the Neoproterozoic. General evolutionary models of Precambrian life based solely on selection in marine habitats may need to be reëvaluated.