Earliest Fossil Fungi

Although fungi were probably present during the Precambrian, the earliest fossil record of them has been difficult to interpret (Brunel et al., 1984). There are spore-like bodies and filaments in Precambrian rocks that may represent the remains of fungi, but the affinities of some of them remain equivocal. For example, although initially described as

FIGURE 3.3 Arcyria sulcata (slime molds), capillitium composed of capilitial threads (see FIG. 3.2) (Eocene). Bar = 300pm. (Courtesy A. Schmidt.)
Slime Molds Capillitium
FIGURE 3.4 Protophysarum balticum showing three sporocarps emerging from a plant fragment (Eocene). Bar =100 pm. (Courtesy A. Schmidt.)

septate fungal filaments, Eomycetopsis robusta (Schopf, 1968), (FIG. 2.32) from the Neoproterozoic Bitter Springs Formation (830-800Ma) of Australia is now regarded as a cyanobacterial sheath. Tappania (FIG. 3.9) is a Neoproterozoic acritarch (1.43 Ga) from marine carbonates and shales that has been suggested to be fungal (Butterfield, 2005). Extending from the spherical main body of this organism is what are interpreted as septate hyphae, some

FIGURE 3.5 Zoosporangium with discharge opening found in phloem cells of a Carboniferous fern. Bar = 20 |im.
FIGURE 3.6 Tracheid from Sphenophyllum containing fungi (Pennsylvanian). Bar = 500|im.

of which anastomose. Other organisms have been described from late Mesoproterozoic rocks as possibly fungal in origin (Hermann, 1979). Convincing evidence that any of these are some type of reproductive organ from a marine fungus, however, has not yet been forthcoming. Several structures regarded as fungal have been reported from the Lakhanda Series of Siberia dated at ~1 Ga (Hermann and Podkovyrov, 2006). Although the specimens are hypothesized to be possible zygomycetes, their preservation on organic sapropelic films makes assignment difficult. An even earlier putative fungus from Kola Peninsula (northwestern Russia) is dated at ~2Ga (Belova and Akhmedov, 2006). Petsamomyces varies in morphology and bears what are termed hyphal-like

FIGURE 3.7 Chlamydsospore with chytrids on the surface (Devonian). Bar = 300 |im.
FIGURE 3.8 Cortical cell filled with fungal hyphae (Pennsylvanian). Bar = 10|im.

appendages. Like many of these organic-walled microfossils, the affinities of these structures remain problematic. Despite these uncertainties, the presence of fungi in the Proterozoic has been indirectly inferred on the basis of divergence times using molecular clock assumptions, but see Taylor and Berbee (2006) on problems associated with many of

FIGURE 3.9 Tappania (Neoproterozoic). Bar = 100 pm. (From Butterfield, 2005.)

these estimates. Based on these calibrations it is suggested that fungi may have diverged from metazoans about 1.5 Ga (Hedges et al., 2004; Taylor and Berbee, 2006). If these estimates are even close to being accurate, then it would be expected that more fossil fungal remains will be described from these ancient sediments in the years ahead.

Some of the oldest fossil remains that are convincingly fungal in origin occur in Early Silurian (Llandoverian) rocks of Virginia, USA (Pratt et al., 1978). The bulk maceration of these terrestrial rocks produced small (6 pm wide), septate, and branched filaments (FIG. 3.10). Some filaments had specialized cells morphologically identical to those of fungi that produce endogenously formed chains of conidia (FIG. 3.11). To date, the single most important source of ancient fungi is the Early Devonian (Pragian—earlist Emsian) Rhynie chert from Aberdeenshire, Scotland. This Lagerstätte represents an entire ecosystem that is petrified in silica—plants, animals, and microbes are all present. Numerous spores (FIG. 3.12), hyphal filaments (FIG. 3.13), and sporocarps have been described from the silicified matrix and from tissues of the land plants Asteroxylon mackiei, Rhynia gwynne-vaughanii, Aglaophyton, Nothia, and Horneophyton. Rhynie chert fungi, such as Palaeomyces (FIG. 3.14) (Kidston and Lang, 1921a),

FIGURE 3.10 Branching hypha (Silurian). Bar Sherwood-Pike and Gray, 1985.)
FIGURE 3.11 Septate conidium (Silurian). Bar = 15 pm. (From Sherwood-Pike and Gray, 1985.)
Saprophyte Microscopy
FIGURE 3.12 Section of Aglaophyton major axis with two clusters of fungal spores (Devonian). Bar = 300 pm.
FIGURE 3.14 Palaeomyces sp. spores within large resting spore (Devonian). Bar = 75 pm.
FIGURE 3.13 Hyphae and chlamydospores in Rhynie chert plant tissue (Devonian). Bar = 120 pm.

Palaeoblastocladia (Remy et al., 1994), and Glomites (Taylor et al., 1995), are but a few of the morphotaxa known from the Rhynie chert ecosystem. These are described later in the sections "Glomeromycota" and "Chytridiomycota" (Palaeoblastocladia ).

Other fungi in the Rhynie chert cannot yet be assigned to major groups. They consist of non-septate hyphae that branch at irregular intervals, as well as hyphae that are distinctly septate, with the central region of the septum slightly thickened (Kidston and Lang, 1921a). At various points along the hyphae, ovoid to pear-shaped vesicles occur which are believed to have developed into large (250 pm), thick-walled sporangia. In other thin-section preparations, especially those of the chert matrix, larger sporangia occur, but these have not been found attached to hyphae. Those with stratified walls were named Palaeomyces gordonii var. major and are now thought to be members of the Glomeromycota (discussed later). Other sporangia contained a variety of thick-walled structures termed resting spores or resting sporangia. There is no doubt that several different natural forms are represented by these Rhynie chert fungi, but they cannot yet be assigned to a particular clade with certainty, as important parts of their life cycles have not yet been discovered. Recent work indicates that there is not only a considerable diversity of fungi within the Rhynie chert but also that these specimens offer the opportunity to examine the life history of some of these early terrestrial microorganisms as well as their biological interactions with other components of the ecosystem.

The biological relationships of the Rhynie chert fungi, especially those preserved within or on land plants in the chert, have been variously interpreted historically. Kidston and Lang (1921a) suggested that the nutritional mode of most of these fungi was saprophytic, but that some may have represented symbionts. Other workers (Boullard and Lemoigne, 1971) hypothesized that some of the Rhynie chert fungi were parasitic. Further work has shown that some of the Rhynie plants exhibit host response features and these can provide clues to the nutritional mode of certain fossil fungi. The presence of fungi in two different taxa from this site, Rhynia gwynne-vaughanii and Aglaophyton major (formerly Rhynia major), was at one time used to support the suggestion that these two different plants represented independent phases of the same life cycle. As will be discussed in Chapter 8, we now

FIGURE 3.15 Multicelled fungal spore (Cretaceous). Bar = 10 pm. (Courtesy J. M. Osborn.)

know that R. gwynne-vaughanii and A. major are not only different organisms, but probably belong to different clades.

Fungal spores represent one of the most common examples of fungi in the fossil record, and are found in a variety of facies from the Paleozoic (Pirozynski, 1976a; Ediger and Alisan, 1989) to the recent (see section "Fungal Spores" t . The identification of fungal palynomorphs (FIG. 3.15) is difficult, but there are now several glossaries of descriptive terms relating to spore and thallus morphology and structure (Elsik et al., 1983; Kalgutkar and Jansonius, 2000).

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