Precambrian life on land

Abstract

Although Precambrian landscapes have been regarded as barren as the surface of Mars, increasingly close inspection of fossil soils (palaeosols) is revealing a variety of fossils, comparable with those already documented in Cambrian to Ordovician (542–444 Ma) palaeosols. The biggest surprise was that some Ediacaran (550 Ma) fossils of South Australia grew in soils. Different kinds of palaeosols can be used to define Ediacaran terrestrial communities in Australia (550 Ma) and Newfoundland (565 Ma). Simple discoids such as Aspidella dominate communities of intertidal sulfidic palaeosols, whereas quilted forms such as Dickinsonia dominate communities of well drained palaeosols. The discoids may be simple microbial colonies, but complex quilted fossils may be lichenized fungi. Complex quilted fossils appear in palaeosols during the Ediacaran along with large “acritarchs” (such as Ceratosphaeridium, and Germinosphaera) comparable with fungal chlamydospores and vesicles like those of Glomales (Glomeromycota). Discoid fossils and microbial filaments also are found in Palaeoproterozoic palaeosols, for example, in the 2100 Ma Stirling Range Quartzite of Western Australia. Complex Palaeoproterozoic (2200 Ma) fossils in South African palaeosols include Diskagma, comparable with the living endocyanotic Geosiphon (Archaeosporales, Glomeromycota). Archaean (2800 Ga) palaeosols of South Africa contain fossils such as Thucomyces, comparable with modern columnar biofilms. Even older terrestrial fossils may be represented by un–named spindle–like fossils from the 3000 Ma Farrel Quartzite and 3420 Ma Strelley Pool Formation of Western Australia. These spindle–like forms are comparable in morphology with modern soil actinobacteria, such as Planomonospora. Life on land may extend well back into geological history. Positive feedback for soil stabilization by formation of clay and organic matter, and the metered supply of water and nutrients in soils, make soils attractive sites for theories concerning the origin of life.