Rhynie Chert Flora:

Basic Information on the Remaining Plant Groups in the Rhynie Chert

 

Introduction

A number of other flora have been described from the Rhynie chert, including the enigmatic nematophytes, cyanobacteria, various types of algae (including simple filamentous and unicellular chlorophytes, and stoneworts or charophytes). Various types of fungi are also present, including terrestrial and aquatic types; the earliest recorded lichen has also been described.

Nematophytes

An extinct, enigmatic group of plants comprising primarily an inner loose mesh-like plexus of spirally coiled tubes. The tubes may be smooth-walled or show spiral-thickenings; branching of these takes place in small knots. The tubes become closely packed, and may be orientated perpendicular towards the edge of the plexus where they meet an outer cuticular envelope.

Two nematophytes are known from the Rhynie chert: Nematophyton taiti (Kidston & Lang 1921b) (see inset right) and Nematoplexus rhyniensis (Lyon 1962). Both are generally fragmentary and poorly preserved.

The gross morphology of nematophytes is not fully known, though some Carboniferous types are believed to have resembled prostrate logs, perhaps with fronds. Their internal structure show similarities with certain algae, and the spirally thickened tubes resemble the tracheids in vascular plants.

The habitat of nematophytes is also not fully known, but they may have been semi-aquatic plants with emergent fronds.

 

Nematophyton taiti

Above: Nematophyton taiti, showing loose plexus of coiled tubes (centre and bottom) with branch knots (b), passing upward into more tightly packed orientated tubes (top) with a probable external cuticle layer (c) (scale bar = 1mm).

 

Cyanophytes

These are simple photosynthetic bacteria, commonly termed cyanobacteria. These may be unicellular organisms or comprise filamentous chains of cells. Being prokaryotic organisms, the cells do not contain nuclei.

A number of probable cyanobacteria are found in the Rhynie chert, some contributing to distinct stromatolitic laminae in the laminated cherts, possibly having originally grown as cyanobacterial mats on sinter surfaces. Other types are found within 'clotted' chert textures, being deposited in more aquatic settings, and still others within decaying plants (see inset right).

Some of the Rhynie cyanobacteria, such as Archaeothrix (inset right), possessed heterocysts and therefore probably played a significant roll in fixing atmospheric nitrogen into the soil.

 

Archaeothrix oscillatoriformi

Above: Filaments of the cyanobacterium Archaeothrix oscillatoriformi within a stem of Rhynia gwynne-vaughanii (scale bar = 50µm).

 

Chlorophytes

Chlorophytes or green algae are photosynthetic eukaryotic organisms. Being eukaryotes their cells contain nuclei. They may be unicellular or form filamentous chains of cells, or may form more complex structures such as stoneworts or charophytes (see below). Most are found in freshwater settings.

A number of filamentous and unicellular chlorophytes are known from the Rhynie chert, particularly in chert beds deposited in aquatic environments, though very often the poor preservation of cell contents makes their identification very difficult (see inset right).

 

 

Unicells in Rhynie Chert

Above: A number of unicells in Rhynie chert. Some of these displaying cell contents may represent unicellular eukaryotic algae (scale bar = 50µm).

Charophytes

Charophytes are large structurally complex green algae. These plants comprise a series of multi-cellular nodes and long single cells or internodes. Branching occurs at the nodal cells and may be repeated. Charophytes also exhibit structurally complex gametangia. These organisms are aquatic, living in fresh to brackish water.

One probable charophyte has been described from the Rhynie chert, Palaeonitella cranii (Kidston & Lang 1921b). The reproductive structures of Palaeonitella have not been discovered and therefore its status has not been fully resolved.

Palaeonitella was an aquatic plant, being commonly found within 'clotted' chert textures, along with the crustacean Lepidocaris.

 

Right: Palaeonitella cranii showing clusters of nodal cells (n) interspersed with long internodal cells (p). Lateral branches (l) are emerging from the nodes (scale bar = 150µm).

 

 

Palaeonitella cranii
Fungi

Fungi are multi-cellular, non-photosynthetic eukaryotic organisms, generally being saprophytic (feeding on dead organic matter) or parasitic in their lifestyle; though some types form symbioses with plants (endotrophic mycorrhizae), or with an alga or cyanobacterium to form lichens.

Numerous fungi are recorded from the Rhynie chert (Taylor et al. in press), including the earliest best preserved examples of endotrophic mycorrhizae in plant tissue (Taylor et al. 1995b) (see inset right). The earliest ascomycetes (true fungi that produce their spores within a sack-like structure called the ascus) are also present (Taylor et al. 1999, in press); together with various tiny aquatic or soil living chytridiomycetes or chytrids.  A number of the latter have shown evidence of parasitism on other Rhynie plants and even on other fungi (an interaction called mycoparasitism).

 

Endotrophic mycorrhizae

Above: Endotrophic mycorrhizae (f) occupying intercellular spaces within the cortex of Aglaophyton major (scale bar = 30µm) (Copyright owned by University Münster).

Lichens

Lichens are non-vascular plants formed by the symbiotic relationship between a fungus and an alga or a cyanobacterium. The lichen thallus comprises distinct layers of fungal hyphae (the mycobiont) and the alga/cyanobacterium (the photobiont).

The earliest lichen is recorded from the Rhynie chert, Winfrenatia reticulata (Taylor et al. 1995, 1997) (see inset right). Winfrenatia most likely  colonised hard substrates. Degrading sinter surfaces could have provided a suitable substrate. It may have been able to weather the rock surfaces it was colonising, thus contributing to soil formation.

 

 

Winfrenatia reticulata

Above: 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) with nets of fine hyphae containing the cyanobacterium (the photobiont) (Copyright owned by University Münster).