Lichens

Winfrenatia thallus

Above: The thallus of the cyanolichen Winfrenatia reticulata (Copyright owned by University Münster).

Introduction

The earliest known fossil lichen, Winfrenatia reticulata (Taylor et al. 1995, 1997) has been described from the Rhynie chert.

Lichens are a group of non-vascular plants that are formed by a symbiotic association between a fungus (termed a mycobiont) and green alga or a cyanobacterium (termed a photobiont). The relationship is mutually beneficial since the photobiont can obtain water and nutrients as well as some degree of protection from the fungus whereas the mycobiont gains a source of carbon. Today lichens are relatively diverse and have adapted to a number of different habitats, for example, being found in mountain, desert and tundra regions.


Above: Modern lichens encrusting a rock surface near a hydrothermal area, Iceland.	Above: Diagrammatic section through a lichen showing the layered fungal hyphae (f) and alga or cyanobacterium (c).

Fossil Record

Lichens are not common in the fossil record. This partly reflects the fact that the habitats in which they are often found, are rarely conducive to fossil preservation. A few fossil lichens have been described from Mesozoic and Cenozoic rocks, most notably from Oligocene Baltic amber (Larson 1978; Garty et al. 1982). It has been suggested that a number of the Precambrian Ediacara fossils may represent lichens (Retallack 1994) though this remains rather suspect. It is evident that complex microbial communities were present even 3,500 million years ago, however, whether any of these comprised physiological symbioses is uncertain. Thus Winfrenatia reticulata, from the Rhynie chert is considered the oldest known lichen. The morphology of Winfrenatia is outlined below.

Morphology

The overall morphology of the fossil thallus of Winfrenatia is relatively simple. It comprises thin layers between 1 and 2mm thick that contain tightly packed aggregates of fungal hyphae (the mycobiont) (see inset right).

Winfrenatia thallus

Above: Thin section showing a longitudinal view of the thallus of Winfrenatia reticulata. Tightly aggregated fungal hyphae (F) can be seen (the mycobiont). The surface of the thallus showing a series of pockets (P) (Copyright owned by University Münster).

The surface of the lichen exhibits a series of small pockets that contain an open meshwork or 'net' of 1-4µm diameter fungal hyphae (see insets above right and right). Each 'net' is about 25µm in diameter and contains a single cell or cluster of daughter cells that represent the photobiont (see inset below right). The size of cells and number of daughter cells in the hyphal nets generally increases towards the top of the thallus. Each cell is coated with a relatively thick mucilaginous envelope.

Right: Close-up of 'net' showing fungal hyphae (H) enclosing cells of the photobiont (C) (Copyright owned by University Münster).

Above: Close-up of thallus showing tightly aggregated hyphae (F) and one of the surface 'pockets' containing a fine net of hyphae enclosing cells of the photobiont (C) (Copyright owned by University Münster).

Winfrenatia thallus

Palaeoecology

As with modern cyanolichen equivalents, during the early Devonian at Rhynie Winfrenatia was most likely an early coloniser of hard substrates. Degrading sinter surfaces could have provided a suitable substrate. Similarly it may have been able to weather the rock surfaces it was colonising and thus contributing to soil formation.

The presence of a cyanolichen in the early Devonian is thus important in understanding the ecology of early terrestrial communities as well as the evolution of land plants.