FOSSIL PLANTS/Fungi and Lichens 437 understanding the evolution of other organisms and the ecosystems in which they lived The extant fungi are classed into four major groups: chytridiomycetes, which are microscopic forms that occur in both terrestrial and aquatic habitats and produce motile cells; zygomycetes, which form thickwalled spores as a result of sexual reproduction and are constructed of coenocytic hyphae (included in this group are the mycorrhizae, which have formed intricate symbiotic associations in the roots of most terrestrial plants); ascomycetes, which have specialized cells (asci) that produce internally formed spores (ascospores); and basidiomycetes, which produce spores externally on a specialized cell (basidium) Although other structural, biochemical, and physiological characters are used to subdivide these major groups further, the recognition of fossil fungi is based on certain morphological features and, where sufficiently preserved, the symptoms they cause Fossil Fungi Although evidence of life extends well back into the Proterozoic, the first unequivocal fungal remains come from the Palaeozoic These consist of branched tubular hyphae with specialized cross-walls and multicelled spores recovered from Lower and Middle Silurian rocks Based on similarities to structures seen in modern fungi, the Silurian remains have been suggested to have affinities with the ascomycetes These early reports are based on fossils that were discovered after macerating rock fragments, and thus nothing is known about the role that the fungi played in the ecosystem in which they lived Fungal remains in the form of spores have also been reported from the Ordovician; however, these spores look so modern that there is some question as to whether they may be contaminants of living fungi The Early Devonian Rhynie Chert ecosystem in Scotland has provided the most complete view of Palaeozoic fungi to date Because this freshwater hot-springs ecosystem is nearly completely preserved in situ in a siliceous matrix, it contains a large number of different types of fungi, which were present at the time of fossilization; many of these fungi demonstrate a wide variety of interactions with other organisms The most diverse group of fungi in the Rhynie Chert are the chytridiomycetes, which were probably the principal decomposers of organic matter in the Early Devonian Like their modern equivalents, the chytridiomycetes from the Rhynie Chert include forms that are found on the surfaces of other organisms as well as inside cells Because of their small size and great abundance, it has been possible to detail not only the morphology but also the life history of several forms Other Rhynie Chert fungi (members of the Zygomycetes) formed symbiotic associations with a number of land plants as vesicular–arbuscular mycorrhizae (Figures 1E and 1F) In these associations, the fungus (Glomites sp.) has the ability to penetrate certain cells of the plant to form highly branched absorbing structures termed arbuscules (Figure 1F), which act as physiological exchange sites where the fungus obtains carbon The host plant benefits from the greater ability of the fungus to obtain nutrients and water Many believe that this fungus–land-plant relationship allowed plants to become established in the terrestrial realm, since it is estimated that today more than 90% of all land plants share a beneficial relationship with certain fungi A large number of extant fungi are parasites and pathogens of other organisms, and some of these associations are represented in the Rhynie Chert Plant-parasitizing fungi in the Rhynie Chert ecosystem typically invaded the softer tissues of the plants, and it is possible to observe areas where, as a result of fungal infection, cells are broken down, sometimes resulting in easily observable necroses Other parasitic fungi (predominantly chytridiomycetes) in the Rhynie Chert ecosystem colonized the spores of land plants (Figure 2B); as a result, the spores lost their capability to germinate and produce a gametophyte Still other fungi were mycoparasites, i.e they obtained nourishment from other fungi and/or fungal spores (Figures 1A and 1B) In some instances, when a plantparasitizing fungus attempted to enter a cell, the plant responded by increasing the size of the cell, increasing the number of cells in the immediate region, and/or producing chemical deterrents in order to encapsulate or ward off the infection Responses of this type are common in plants today and indicate not only that the Rhynie Chert plants were alive when the fungi attacked but also that the signalling mechanisms between these two organisms were well-established more than 400 Ma ago To date, three of the four major groups of fungi have been found to be represented in the Rhynie Chert; only the basidiomycetes are not present or have not yet been discovered Especially noteworthy is the occurrence of an advanced saprophytic or parasitic perithecial ascomycete Beneath the epidermis of upright stems and rhizomes of the land plant Asteroxylon (an early representative of the lycophytes) occur spherical to flask-shaped opaque structures (Figure 1C), which represent the reproductive structures (perithecia) of this fungus The perithecia are characterized by short ostiolate necks that typically protrude from the epidermis of the host plant through stomatal openings Lining the interior of the perithecium are elongate thin-walled hairs interspersed with