Lichens are a group of non-vascular plants that are formed by a symbiotic association between a fungus (termed a mycobiont) and green alga or a cyanobacterium (termed a photobiont). The relationship is mutually beneficial since the photobiont can obtain water and nutrients as well as some degree of protection from the fungus whereas the mycobiont gains a source of carbon. Today lichens are relatively diverse and have adapted to a number of different habitats, for example, being found in mountain, desert and tundra regions.
Lichens are not common in the fossil record. This partly reflects the fact that the habitats in which they are often found, are rarely conducive to fossil preservation. A few fossil lichens have been described from Mesozoic and Cenozoic rocks, most notably from Oligocene Baltic amber (Larson 1978; Garty et al. 1982). It has been suggested that a number of the Precambrian Ediacara fossils may represent lichens (Retallack 1994) though this remains rather suspect. It is evident that complex microbial communities were present even 3,500 million years ago, however, whether any of these comprised physiological symbioses is uncertain. Thus Winfrenatia reticulata, from the Rhynie chert is considered the oldest known lichen. The morphology of Winfrenatia is outlined below.
As with modern cyanolichen equivalents, during the early Devonian at Rhynie Winfrenatia was most likely an early coloniser of hard substrates. Degrading sinter surfaces could have provided a suitable substrate. Similarly it may have been able to weather the rock surfaces it was colonising and thus contributing to soil formation.
The presence of a cyanolichen in the early Devonian is thus
important in understanding the ecology of early terrestrial communities as well
as the evolution of land plants.