Fungi

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).

 

Introduction

Fossil Record

Morphology

 

Introduction

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 mycellium.

• 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 photosynthesis.

Fossil Record

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, in press).

For the purposes of this resource, the morphology and palaeoecology a small selection of different types of fungi from the chert are outlined below:

Morphology

Endotrophic Mycorrhizae

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).

Ascomycetes

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 press).

Chytridiomycetes

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 Münster).

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).