The precursor environment for vascular plant colonization

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Abstract
There is evidence, although inconclusive, that a biologically active soil cover existed long before the late Silurian. The earliest vascular plants may have colonized a land surface containing well-developed soils which were functioning biologically and biochemically in similar ways to modern soils. In any discussion of the late Silurian-early Devonian ‘invasion of the land’, two basic questions arise in relation to the history of the land cover and its soils: (i) Did vascular plants colonize a barren landscape or did biologically functioning soils already exist? (ii) What changes did the vascular plant cover cause to the land surface and its soils? This latter question has been tackled by Retallack (this symposium) and the following is a discussion of some aspects of the former. Many soil-forming processes are purely physical or physico-chemical in origin and examples of such pedogenic modifications of the land surface have now been documented from the Precambrian and lower Palaeozoic (Retallack 1981). However, there is also some evidence that these early soils were biologically active and were associated with microbial communities. Golubic & Campbell (1979) have compared the mid Precambrian microfossil, Eosynechnococcus moorei Hofmann with the extant cyanobacterium Gloeothece coerulea Geitler, which is a subaerial form. They have suggested that prokaryotic communities may have colonized the land surface as long ago as the early Precambrian. Organic-rich palaeosols are known from Blind River Formation of Ontario (Campbell 1979) which is 2.4 Ga old.