Above: Fungal hyphae and spore cysts in a stem
of Asteroxylon mackiei. Two species of fungi appear to be present, the
smaller cysts (bottom left) are probably Palaeomyces asteroxyli, the
larger thick-walled cysts probably belong to Palaeomyces gordoni (scale
bar = 200µm).
Fungi are an important group of multicellular eukaryotic
organisms comprising a meshwork of thread-like filamentous cells joined
end-to-end. The whole body of a fungus is called the mycellium. Fungi are usually classified into four main divisions; the chytridiomycetes
or chytrids (tiny, predominantly aquatic fungi), the zygomycetes (bread
molds and mycorrhizae), the ascomycetes
(terrestrial, mostly saprophytic fungi, including cup fungi and yeasts), and the
basidiomycetes (club fungi, including toadstools and mushrooms; entirely
terrestrial); the differences made primarily on the
process of reproduction and the morphology of the sporangia
since the reproductive structures are generally more diverse than the
- Chytrids have motile sexual and asexual spores with posterior flagella.
- Zygomycetes have thick-walled resting sexual spores called zygospores,
their asexual spores are produced in a sporangium and called sporangiospores.
- Ascomycetes produce sexual spores (ascospores) in a sack-like body called an ascus,
their asexual spores are produced externally, borne on a conidiophore.
- Basidiomycetes produce their spores externally from a club-like structure
called a basidium, these generally have no asexual spores.
Fungi may reproduce sexually or asexually and like plants show alternations
in their life cycle.
Fungi are unable to build structural materials by photosynthesis (they are heterotrophic
organisms). They live
on organic material from other living things, alive or dead and are therefore
generally parasitic or saprophytic. Some types of fungi, however, form symbioses
with plants. Certain fungi are mutualists forming symbioses with a green alga (chlorophyte) or a cyanobacterium
to create lichens. Some types of symbiotic zygomycetes live within plants and are called endotrophic
mycorrhizae (literally "fungus-root"). The latter are fungal hyphae that grow into the cells of
plants, branching within them to form arbuscles where the exchange of
nutrients takes place. The hyphae also occasionally grow to form thick
swellings or vesicles and are thus often termed vesicular-arbuscular
mycorrhizae. In both cases of symbiosis the fungus supplies the plant
'partner' with water and mineral nutrients, especially if these are in short supply in the
soil; whereas the plant supplies the fungus with carbohydrates from
Fossil fungi tend to be microscopic and not always found with their
reproductive structures attached, therefore positive identification is often
extremely difficult. They are not especially rare as fossils, though the best
preserved examples are generally found in amber, for example the Oligocene
Baltic ambers and Cretaceous ambers from northern France.
The earliest record of fungi in the fossil record are of probable
chytridiomycetes or chytrids from Vendian strata (Late Precambrian, 650 to 544
million years ago) of northern Russia. The Early Devonian Rhynie chert is host
to a plethora of fossil fungi including various endotrophic mycorrhizae, the
earliest ascomycetes, several chytridiomycetes and various other undescribed fungi (see Kidston
& Lang 1921b; Hass & Remy 1992; Taylor
et al. 1992a & b; Hass
et al. 1994; Remy et al.
1994a & b; Taylor
et al. 1994, 1995, 1999,
For the purposes of this resource, the morphology and palaeoecology a small
selection of different types of fungi from the chert are outlined below:
|As we have seen above, endotrophic mycorrhizae are fungal
hyphae that grow within cortical cells of many plants (see
inset right) with which they share a symbiotic relationship (see also the
section on Evidence for plant/animal
interactions). These have been found within many of the well preserved
Rhynie plants, though to date the only vesicular-arbuscular
mycorrhizae formally described have been those found in the outer cortex of
stems of Aglaophyton major and named Glomites rhyniensis
(Taylor et al. 1995b) (see
insets right and below right).
Above: Fungal hyphae (f) penetrating the outer
cortex of an Aglaophyton major stem (scale bar = 100µm) (Copyright owned by University Münster).
|Fungal hyphae of these mycorrhizae are often found to crowd
the intracellular spaces in the cortex of the plant. Many thin sections
show where the fungus has penetrated and entered the cells, in which they
have formed the tree-shaped arbuscles (see inset right).
These fungi were terrestrial symbionts, though in part may also have
been saprophytic after the death of the plant partner.
Right: Cortical cells of Aglaophyton major with
well developed arbuscles. The places where the fungus penetrated the
cell wall are shown (c) (scale bar = 20µm) (Copyright owned by University Münster).
|The earliest unequivocal ascomycetes have been described from the
Rhynie chert by Taylor
et al. (1999). The flask-shaped ascocarps or peritheca (fruiting bodies)
of these fungi have
been found just below the epidermis of partially decayed stems, rhizomes and microphylls of the
lycophyte Asteroxylon mackiei. The peritheca are
on average 400µm in size and often appear to have developed in the stomatal
chambers of the plant (see inset right). The walls of each perithecium comprise two layers of
hyphae forming a thickened wall. In mature samples the asci,
approximately 50µm in length, appear to arise from the inner wall of hyphae.
Each ascus contains between sixteen and thirty two ascospores, these are
approximately 5µm in length.
These ascomycetes were terrestrial and probably saprophytic.
Right: Mature perithecium in a stomatal chamber of Asteroxylon.
This shows the two layers of the wall (w) and the opening through
which the ascospores are released (n). Click on the image for a
close up! (scale bar = 100µm) (Copyright owned by University Münster).
Read more about the Rhynie chert ascomycetes on the web site of the
Palaeobotanical Research Group, University of Münster here.
The formal description and diagnosis for the fossil ascomycete is currently
awaiting publication (Taylor et al. in
The tiny, predominantly aquatic chytrids are the most common forms of fungi
found in the Rhynie chert. Most resemble fungi from either of two extant orders,
the Blastocladiales and the Spizellomycetales. Both orders include
saprophytic and parastic fungi, though living spizellomycetales are mostly found
in damp and water-logged soil.
Above: Transverse section of an Aglaophyton
stem with fungal tufts of Palaeoblastocladia milleri (arrowed) on the
epidermis. Click on image for a close up! (scale bar = 200µm) (Copyright owned by University
One of the Rhynie chytrids, Palaeoblastocladia milleri, (Remy,
et al. 1994) is a blastocladalean. It occurs as tufts that arise
from the stomata or from between the cuticle and epidermis of Aglaophyton
major stems (see inset above). The non-septate hyphae of the fungus form two types of mature
thalli. The sporothalli (the sporophyte
generation of the fungus) show repeated dichotomous
branching and bear terminal globose zoosporangia and pitted, thick-walled
resting sporangia (see inset right). The gametothalli (the gametophyte
generation of the fungus) also show dichotomous branching and bear terminal
chains of two or three globose gametangia
(see inset right).
This chytrid was aquatic and a saprophyte.
Right: Idealised reconstruction of Palaeoblastocladea
milleri. A: Sporothallus showing terminal zoosporangia (z) and
resting sporangia (r) (scale bar = 40µm). B: Part of a
gametothallus showing terminal gametangia (g) (scale bar = 20µm) (after Remy
et al. 1994).
Other Rhynie chert chytrids were clearly aquatic parasites, three types have been described
as parasites on the probable charophyte Palaeonitella cranii by Taylor
et al (1992). Below are images of two of these, Milleromyces
rhyniensis (inset below left) and Lyonomyces pyriformis
(inset below right).
Above: The zoosporangium of Milleromyces rhyniensis
within a Palaeonitella cell. The cell wall of the alga (c)
is penetrated by a cylindrical discharge tube (d). The globose
zoosporangium inside the cell (z) is empty and partially
collapsed (scale bar = 20µm) (Copyright owned by University Münster).
Above: Two spherical thalli of Lyonomyces
pyriformis (t) attached to the outside of a Palaeonitella
cell. The cell wall (c) is penetrated by two collapsed rhizoids (r)
(scale bar = 10µm) (Copyright owned by University Münster).
A review of the Rhynie chert fungi is currently awaiting publication (Taylor
et al. in press).