FOSSIL PLANTS/Fungi and Lichens 441 some other type of life form with an unusual combination of features, perhaps not even remotely related to any modern organism, it must have presented an imposing structure, extending more than m above the very small plants that made up the Early Devonian landscape Lichens Lichens may be interpreted as ‘dual organisms’ since they not represent a single organism, in the way that most other life forms on Earth do, but rather a combination of two or three entirely different organisms: a fungus (mycobiont) that lives in an obligate mutualistic association with a green alga or cyanobacterium or both (photobiont) The lichen symbiosis is unique since it represents a new ‘hybrid’ life form with a distinct body plan (lichen thallus) and structural and physiological properties that are different from those of either of the partners An estimated 13 000–17 000 species of lichen exist today, extending from the tropics to the polar regions and growing on a highly diverse array of substrates, including soil, bark, bare rock surfaces, leaves of vascular plants, barnacle shells, and other lichens The vast majority of lichen-forming (lichenized) fungi are members of the Ascomycetes, but some 20 species of basidiomycete and one zygomycete are also known to enter into lichen symbioses In contrast to the many thousands of lichenized fungi, there are only about 100 different photobionts, most of which are unicellular green algae (Chlorophyta) It is still not fully understood how this unique assemblage of organisms evolved The failure to resolve more fully the evolutionary history of lichens is due primarily to a meagre fossil record Moreover, in order to establish the existence of a fossil lichen, it is necessary to demonstrate not only the individual partners but also that there is some degree of interdependence between the separate organisms Thus, even if an entire association is well preserved, it may be difficult to determine whether it is a lichen or simply an asymptomatic aggregate of algae or cyanobacteria and fungi that are closely associated but not interact Alternatively, the aggregation may represent a fungus that has conveniently parasitized an alga or cyanobacterium Molecular and genetic studies of extant lichens suggest that the lichen symbiosis has arisen several times over the course of geological time Fossil Lichens It has been hypothesized that one of the first steps in the colonization of land by eukaryotes may have been the formation of a lichen symbiosis However, compelling fossil evidence supporting this hypothesis remains elusive The earliest fossil (from the Early Proterozoic Witwatersrand Group, South Africa) that has been considered to be a lichen is a thallus-like structure comprising erect columns made up of branched and apparently septate filaments The lichen nature of this fossil is, however, questionable since remains of the photobiont are not observed Moreover, the filaments are much smaller than most fungal hyphae and are in the size range of certain filamentous bacteria Some believe that the filaments are not fossils because similar objects can be produced artificially in the laboratory Other Precambrian life forms, which have been suggested to be lichens, are the so-called ‘Ediacara fossils’ or ‘Vendobionta’ (see Precambrian: Vendian and Ediacaran) These unusual 600 Ma old organisms, which presumably lived on soft mud or sands at the bottom of shallow coastal seas, are usually considered to be primitive animals, but one hypothesis considers them to be lichens or lichen-like based on the fact that they were relatively resistant to being compressed after burial This idea has not received much support from either the palaeontological community or the lichenological community Several groups of Palaeozoic fossils have variously been interpreted as lichens or lichen-like associations One group is the Nematophytes, which occurred from the Late Ordovician to the Late Devonian The most impressive Nematophyte, Prototaxites (Figure 2A), is usually considered to be a giant fungus (see above), but some workers have speculated that it may have been a lichen-like association of two or more different kinds of organism A second member of the Nematophytes, Nematothallus, was a small foliose organism that consisted of intermixed large and small tubules, covered by a thick cuticle-like layer Nematothallus possesses a number of striking features that are also found in certain extant lichens The cuticle-like surface layer, for example, displays impressions of a distinct cell pattern, which is virtually indistinguishable from that seen on the thallus surface in certain extant cyanolichens Unfortunately, as is the case with other early lichen-like fossils, the affinities of Nematothallus cannot be established with certainty because a photobiont partner has not yet been discovered Another example of a Palaeozoic life form, the identity of which is still being debated, is Spongiophyton (Middle Devonian) This thalloid organism is typically preserved as a flattened cuticle-like sheath with circular pores on one surface Some have suggested that Spongiophyton was an early bryophyte (perhaps a primitive liverwort), whereas others speculate that it may have been a lichen The most convincing fossil example of a Palaeozoic lichen comes from the Early Devonian Rhynie Chert