Fossil flora

A simple plant with a creeping rhizome and smooth, naked, upright axes. This plant grew to around 15cm in height and its axes were up to 6mm in diameter. The rhizomes occasionally had bulges bearing tufts of rhizoids for taking up water and nutrients. Branching was mainly dichotomous and fertile axes terminated with pairs of fusiform sporangia (see inset right). Aglaophyton was a sporophyte , the male gametophyte stage of the plant, called Lyonophyton rhyniensis, was much smaller, the upright axis ending in a cup-like structure that bore the antheridia .

The water-conducting cells in this plant did not have thickenings like true vascular plants, being more similar to those seen in some modern bryophytes (a group including mosses and liverworts). The simply branched naked stems are more comparable with the extinct rhyniophytes and as such its systematic position remains unknown.

Aglaophyton grew mainly on dry, litter covered, organic-rich substrates, on its own as monotypic stands or with other Rhynie plants, though it seems to have required wet conditions for germination.

One of the more advanced and complex Rhynie plants, Asteroxylon had an extensive subterranean system of branching rhizomes. The aerial upright axes of the plant grew to about 40cm in height with a maximum diameter of 12mm. Branching was dichotomous and monopodial . The aerial axes possessed scale like 'leaves' or enations (see inset right). Fertile axes bore stalked kidney-shaped sporangia attached between the 'leaves' and the stem.

In cross-section the water-conducting strand of this plant forms a characteristic star-shape pattern from which smaller strands radiate to meet the base of the 'leaves'.

Asteroxylon is a member of a group of plants called the lycophytes which includes modern club mosses.

Asteroxylon mainly grew in organic-rich soils as part of a diverse community together with other Rhynie plants and could probably tolerate quite dry habitats.

Horneophyton comprised naked upright aerial axes with a subterranean bulbous, corm-like rhizome bearing rhizoid tufts. The aerial axes were smooth and naked, growing up to 20cm in height with a maximum diameter of 2mm. Branching was mainly dichotomous and repeated. Fertile axes terminated in branched, tubular sporangia (see inset right) that internally possessed a central 'strand' or collumella. Horneophyton was a sporophyte , the female gametophyte stage of the plant, called Langiophyton mackiei, was much smaller, the upright axis ending in a cup-like structure that bore the archegonia .

The water-conducting cells in this plant, like true vascular plants, possessed thickenings. However, the presence of a collumella in the sporangia shows similarities with bryophytes (a group including mosses and liverworts). As such the systematic position of this plant remains uncertain.

Horneophyton preferred to grow on sandy and organic-rich substrates, often on its own as monotypic stands and probably flourished in damp to wet conditions.

Nothia possessed a laterally extensive, branched subterranean rhizome network, bearing a ventral ridge supporting tufts of rhizoids (see inset right). Locally branches turned upright to form the aerial axes. These axes were naked with a very irregular surface and showed repeated dichotomous branching, giving the plant a thicket-like appearance. Fertile terminal axes bore lateral kidney-shaped sporangia attached by stalks. Nothia was a sporophyte , the male gametophyte stage of the plant, called Kidstonophyton discoides, was much smaller, the upright axis ending in a cup-like structure with tubular outgrowths bearing the antheridia .

The water-conducting cells in this plant did not have thickenings like true vascular plants, being similar to those seen in some modern bryophytes (a group including mosses and liverworts). The stalked, lateral kidney-shaped sporangia are comparable with zosterophylls and the simply branched naked axes with rhyniophytes and as such its systematic position remains unknown.

Nothia preferred to grow in sandy soils and plant litter, on its own or with other plants.

Rhynia was one of the most common plants in the Rhynie ecosystem. Like Aglaophyton it was completely subaerial with creeping, branched rhizomal axes and smooth, naked upright axes (see inset right). The plant grew to a height of about 20cm, individual axes being up to 3mm in diameter. Branching was dichotomous and adventitious or monopodial . The plant possessed curious hemispherical projections on its axes, those on the rhizomes bearing tufts of rhizoids. Fertile axes bore terminal fusiform sporangia .

Rhynia is a member of an extinct group of primitive plants called the rhyniophytes, characterised by their simple branching and naked stems.

Rhynia commonly grew in thickets, typically on its own as monotypic stands and was often an early coloniser of well-drained sinter and sandy substrates. It also grew with other plants and was tolerant of a wide range of habitats.

Trichopherophyton was quite a rare plant in the Rhynie ecosystem. The height of this plant is uncertain but the aerial axes had a maximum diameter of 2.5mm. Branching was mainly dichotomous . Subterranean rhizomal axes were smooth with small blunt-tipped emergences which probably acted as rhizoids. Aerial axes bore curious spiny projections (see inset right) and the tips of the axes displayed a feature called circinate vernation . Fertile terminal axes bore lateral, stalked, kidney-shaped sporangia , also bearing spiny projections.

The water-conducting cells in this plant, like true vascular plants, possessed thickenings. This together with the shape and position of the sporangia suggest Trichopherophyton belongs to a group of plants called the zosterophylls.

Trichopherophyton was a late coloniser of organic-rich substrates, always growing with other Rhynie plants as part of a diverse flora.

Ventarura is the most recently discovered higher land plant from the Windyfield chert. The height of this plant is uncertain but was at least 12cm and the aerial axes had a maximum diameter of 7.2mm. Branching was mainly dichotomous and repeated (see inset right). Subterranean rhizomal axes were smooth with small blunt-tipped emergences which probably acted as rhizoids. Aerial axes bore curious peg-like projections. Characteristically, internally the axes had a lignified middle layer to the cortex called the sclerenchyma . Fertile terminal axes bore lateral, stalked, kidney-shaped sporangia .

The water-conducting cells in this plant, like true vascular plants, possessed thickenings. This together with the shape and position of the sporangia suggest Ventarura belongs to a group of plants called the zosterophylls.

The palaeoecology of Ventarura is not fully known, but it probably grew in localised patches, at least in the vicinity of freshwater ponds and probably in sandy and organic-rich substrates.

Introduction

Aglaophyton was originally described as Rhynia gwynne-vaughanii by Kidston and Lang in 1917 . In 1920 , however, they split the genus into two species and assigned this plant to the species Rhynia major. D. S. Edwards (1986) re-interpreted the plant, based primarily on the lack of thickenings on the xylem cells (a feature originally attributed to poor preservation by Kidston and Lang (1920a) ) and renamed the plant Aglaophyton major. Both the male and female gametophytes of Aglaophyton have been identified though, to date, only the male gametophyte, Lyonophyton rhyniensis, has been formally described and named ( Remy & Remy 1980b ). The overall morphology and palaeoecology of Aglaophyton is outlined below.

Morphology

'Aerial' Axes

The axes of this plant are superficially similar to Rhynia gwynne-vaughanii though there are a number of distinct differences. They exhibit a maximum diameter of 6mm and the plant probably attained a height of around 15cm. Branching in Aglaophyton is predominantly dichotomous (the angle of dichotomy being between 60 and 90⁰) with minor adventitious branching.

The epidermis of Aglaophyton is smooth and the cuticle appears deeply flanged. The stomata are flanked by two reniform or kidney-shaped guard cells. Most of the axis comprises the cortex (see inset right), this is divided into two, the division occasionally marked by a distinct brown layer.

The outer cortex comprises closely packed elongate cells of more or less uniform size. The inner cortex comprises more loosely packed cells with a well-developed inter-cellular air space network. The dark layer is formed by the presence of vesicular arbuscular mycorrhizae within intracellular air spaces. Partially decayed Aglaophyton axes exhibit a characteristic cellular decay pattern in the cortex (see inset right).

The ' vascular strand ' of Aglaophyton comprises a zone of ' phloem ' surrounding a central xylem strand. The phloem is of uniform thickness, the individual cells showing acute apices. The xylem is terete and displays an endarch maturation pattern (see inset right). A significant difference between the xylem in this plant and that in Rhynia is the fact that the xylem cells in Aglaophyton exhibit no thickenings.

Rhizomal Axes

Aglaophyton exhibits creeping rhizomes that in life were subaerial, laying directly on the substrate surface. These branch repeatedly locally turning upwards and passing into the 'aerial' axes described above. The rhizomal axes are cylindrical, naked and generally exhibit a similar morphology and internal anatomy to the upright aerial axes, also bearing stomata. Unicellular rhizoids are present as tufts on 'bulges' on the ventral side of the rhizomes, the bulges formed by elongate cortical cells ( D.S. Edwards 1986 ).

Sporangium

The sporangia of Aglaophyton are elongate and fusiform in shape and relatively large with a maximum size of 12mm by 4mm (see inset right). The dehiscence mechanism was determined by Remy (1978) ; it would split obliquely along its length.

The disposition of the sporangia is terminal and they usually occur in pairs above the last point of dichotomy (see reconstruction below).

The spores of Aglaophyton are retusoid and have smooth walls with a trilete mark located in a thinning of the exine. They are relative large, ranging in diameter from 64μm to 85μm. These spores are comparable to species of the spore genus Retusotriletes. The spores of Aglaophyton have often been found preserved at various stages of germination (Lyon 1957) (see the section on spores for images of some of these).

Gametophytes

Both male and female gametophytes of this plant have been identified though to date only the male form has been formally described. The male gametophyte has been assigned the name Lyonophyton rhyniensis ( Remy and Hass 1980b ).

This free-living gametophyte of Aglaophyton consists of an aerial axis that widens and terminates in a conspicuous cup-like structure which bears the antheridia (see inset right). Although smaller in size, the axis of the gametophyte is very similar to the sporophyte in its anatomy.

Reconstruction

Relationships

The systematic position of Aglaophyton, like a number of other Rhynie plants, remains unresolved because it shows a mixture of anatomical and morphological features that are not typical of any one group of plants. It shows many features characteristic of the rhyniophytes, a group of primitive plants, known only from fossils, showing simply branched naked stems of which Rhynia is an example. The water-conducting cells of the xylem strand do not show thickenings and as such are more reminiscent of the hydroids of some bryophytes (a group of plants including mosses and liverworts). Since the xylem of Aglaophyton does not possess true tracheids, this suggests it is not a true vascular plant.

Palaeoecology

Aglaophyton was a common plant and significant component of the Rhynie ecosystem during the Early Devonian. It appears to have preferred growing on litter-covered, organic-rich surfaces and was never a primary coloniser of sinter substrates, though the creeping rhizomal axes of Aglaophyton probably allowed it to occupy large surfaces of substrate. It occasionally grew as monotypic stands but is often seen associated with other plants, particularly Nothia, Asteroxylon, Horneophyton and occasionally Rhynia.

Since Aglaophyton exhibits stomata on the rhizomal axes as well as the upright aerial axes, it seems likely that the plant colonised mainly dry substrates. The fact that the cuticle and stomata of Aglaophyton display adaptations to prevent water loss suggests it could also tolerate periods of drought ( Powell 2000b ).

However, the matrix of chert beds containing Aglaophyton in growth position occasionally contain the charophyte Palaeonitella, in association with the freshwater crustacean Lepidocaris and the 'aerial' axes of the plant may display infestation by chytrids (chytridomycetes) which are a type of tiny, simple fungi that thrive in damp and especially aquatic conditions. Another feature noted by Remy and Hass (1996) is the association of germinating Aglaophyton spores with these aquatic elements. Although the plant could probably also tolerate humid or damp conditions, this does not necessarily mean that Aglaophyton lived in standing water, perhaps these particular beds represent instances where areas colonised by the plant were occasionally flooded. It does seem likely, however, that wet conditions may have been necessary for spore germination.

Introduction

Asteroxylon, one of the better known Rhynie plants, was originally described by Kidston and Lang (1920b) and assigned the name Asteroxylon mackiei. They noted fertile elements probably belonging to Asteroxylon within their material; however, the fertile elements of the sporophyte were not finally resolved until Lyon (1964) discovered sporangia in organic connection with the plant and he concluded that the fertile axes observed by Kidston and Lang were in fact of another new plant, Nothia aphylla. To date the gametophytes of Asteroxylon remain unknown. The overall morphology and palaeoecology of Asteroxylon is outlined below.

Morphology

Aerial Axes

The aerial axes of Asteroxylon exhibit a maximum diameter of 12mm and possess characteristic 5mm long scale-like 'leaves' or enations surrounding each axis (see inset below right). Asteroxylon is perhaps the largest of the known plants from the chert, in life probably attaining a height above ground of about 40cm (it's rhizomes penetrating the substrate to a depth of up to 20cm). Branching is dichotomous and monopodial .

One of the characteristic features of Asteroxylon are the scale like enations that emerge from the epidermis in a spiral arrangement around the axes. These are not true leaves since they do not possess a vascular strand (see inset right). The cuticle on both axes and enations bear abundant stomata with distinctive dark-coloured guard cells. The surface of the epidermis of the enations varies from smooth to papillate whereas that of the axes is smooth.

The cortex may be divided into a narrow outer zone of closely packed cells and a broader inner cortex that can be further subdivided into three zones: an outer and inner layer of compact cells with a trabecular middle layer of elongate cells with a well-developed inter-cellular air space network. Occasionally the inner zones of the cortex display fungal infestation (see insert below right).

The vascular strand is quite distinctive. Asteroxylon possess an actinostele, in other words the vascular strand appears star shaped or stellate in transverse cross-section (see insert below left and heading photograph). The xylem is exarch to locally mesarch where the protoxylem occurs at the ends of the 'lobes' of the actinostele, and displays spiral thickenings (see insert below right). Phloem tissue is best developed between the 'lobes' of the xylem strand. 'Leaf traces' are often seen where vascular tissue splits from the central stele. These traces end at the bases of the enations.

Rhizomal Axes

The rhizomes of Asteroxylon display a diameter of up to 4.2mm, they are naked, lack rhizoids and branch repeatedly. The epidermis of the rhizomes is papillate. The cortex of Asteroxylon is divided into three zones; an outer zone of closely packed cells, the inner zones characteristically infected by fungi. The rhizomes exhibit an elliptical exarch xylem strand with spiral thickenings (see inset right).

Sporangium

The sporangia of Asteroxylon were first described by Lyon (1964). Though many anatomical details remain unresolved, the sporangia are reniform (kidney-shaped), rather flattened medially, with a marginal dehiscence mechanism (see inset right). The maximum dimensions for a single sporangium are 7mm by 2.5mm. Their disposition on the aerial axis is rather scattered, each sporangium being laterally attached via a stalk emerging between the angle of an enation and the main stem.

Reconstruction

Relationships

Asteroxylon is rather more anatomically complex than the other known Rhynie chert plants. The plant is considered a true lycophyte (a group of plants which includes the 'club mosses') based on the structure of its apical meristem ( Hueber 1992 ) together with the characteristic exarch actinostele and the lateral disposition of the sporangia. The spirally thickened and reticulate cell walls of the xylem cells are also typical of lycophytes ( Kenrick & Crane 1991 ). Modern lycophytes are also characterised by their enations or microphylls which possess a single vascular strand. In Asteroxylon the vascular trace stops at the base of the enations which may suggest the plant represents an intermediate stage in the evolution of lycopsid leaves.

Palaeoecology

Asteroxylon is quite commonly encountered in a number of chert beds and apparently formed a significant component of the Rhynie flora during the Early Devonian. Its rhizomes are usually found traversing plant litter and the plant is primarily found to occur in situ with two or more other genera, commonly Nothia, Rhynia and Aglaophyton and occasionally Ventarura. It seems therefore that Asteroxylon primarily lived as part of a diverse plant community rather than as monotypic stands ( Powell et al. 2000b ).

The fact that Asteroxylon possesses an extensive, substrate-penetrating 'root' system suggests it was capable of exploiting larger volumes of water and nutrients than the other Rhynie plants. Also the presence of microphylls increase the surface area of the aerial axes. This would have created a larger photosynthetic surface and with the greater density of stomata the plant possesses would also have meant greater efficiency in gaseous exchange and transpiration. It is likely that Asteroxylon could tolerate quite dry habitats compared with most of the other Rhynie flora.

Introduction

Horneophyton was originally described by Kidston and Lang (1920a) and assigned the name Hornea lignieri, however, the generic name was already occupied and it was therefore renamed Horneophyton lignieri by Barghoorn and Darrah in 1938 . Although one of the better known Rhynie plants, it's systematic position is still very much debated. As well as the sporophyte, the female gametophyte of this plant, Langiophyton mackiei, has also been described ( Remy and Hass 1991c ). The overall morphology and palaeoecology of Horneophyton is outlined below.

Morphology

Aerial Axes

The aerial axes of Horneophyton display a maximum diameter of 2mm and are cylindrical and naked. The plant probably reached a maximum height of 20cm. The branching of Horneophyton is dichotomous and repeated.

The cuticle of the epidermis displays a regular arrangement of elongate cells. Stomata are relatively rare and are enclosed by distinctly modified cells ( Hass 1991 ) (see inset right). The cortex is generally poorly preserved and where present appears to be undivided.

The vascular tissue comprises an endarch xylem strand with irregular spiral and reticulate thickenings. This is surrounded by a zone of thin-walled cells interpreted as phloem . The vascular strand becomes less distinct towards the base of the aerial axis (see inset below right).

Rhizomal Axes

The 'rhizomal axes' of Horneophyton distinguish it easily from all other Rhynie plants. They comprise a corm-like lobe at the base of the aerial axis with numerous unicellular rhizoids emerging from the epidermis (see insets right and below right). A vascular strand is not present in the rhizome, emerging from parenchymatous tissue upwards from the base of the aerial axis.

Occasionally, fungal activity is evident in the cortex of the rhizome (see inset below left).

Sporangium

The sporangia of Horneophyton are very distinctive. Their disposition on the aerial axes is terminal. They are cylindrical to globate in shape and may be branched displaying a number of connected lobes (see heading photograph and reconstruction below); the maximum dimensions of a single sporangium being 7.5mm by 5mm. The dehiscence mechanism is apical and particularly well-developed ( Eggert 1974 ; El-Saadawy & Lacey 1979a ).

One of the more curious features of the sporangia of Horneophyton is the presence of a central columella in the sporangial cavity (see inset right).

The in situ spores of Horneophyton are relatively well known ( Bhutta 1973a ) and may be assigned to the spore species Emphanisporites decoratus. These vary between 42 and 54μm in size, the proximal face bearing a distinct trilete mark. The proximal face of the spore is also ornamented with radial ribs; the distal face characteristically showing an apiculate ornament, typically of closely spaced micron-sized spines.

Gametophytes

To date the female gametophyte of this plant has been described and has been assigned the name Langiophyton mackiei ( Remy and Hass 1991c ). This free-living gametophyte of Horneophyton probably grew to about 6cm in height, the aerial axis terminating in a conspicuous cup-like structure with numerous tubular outgrowths bearing the archegonia (see inset right).

Reconstruction

Relationships

The presence of a sterile central columella in the sporangia of Horneophyton is a feature in extant plants seen only in some bryophytes (e.g. mosses) suggesting the plant may have some affinity with the latter, however, the fact that the aerial axes of the plant exhibit a well-developed vascular strand with tracheids would suggest it is not part of the bryophyte lineage. This mixture of features seen in Horneophyton, as with other Rhynie plants, has led to much debate on the systematic position of the plant which still remains in some doubt.

Palaeoecology

Horneophyton appears to have been one of the more common plants in the Early Devonian ecosystem at Rhynie. Where found in situ and in life position, Horneophyton often occurs at the base of composite chert beds, it's subterranean corm-like rhizomes cutting through pre-existing plant litter and it's rhizoids probably helped to anchor the plant. It therefore appears that Horneophyton preferred sandy and organic-rich substrates. The plant is commonly present as monotypic stands, perhaps indicating it was also an early coloniser of sinter surfaces being able to tolerate environmental conditions unfavourable to many other Rhynie plants ( Powell, et al. 2000b ). For example the plant is seldom associated with Rhynia gwynne-vaughanii in growth position suggesting the two plant taxa required different optimum conditions for growth.

It is also likely that Horneophyton flourished in damp to wet conditions ( Powell, et al. 2000b ; Remy & Hass 1991c ). This seems likely for two main reasons. Firstly, looking at modern sinters, their hygroscopic nature generally means they easily retain water and in some cases the sinter surface where 'early colonising' plants are growing is covered by an intermittent film of water (see also the section on The Ancient Environment & Modern Analogues ). Secondly, Remy and Hass ( 1991c ) noted that Horneophyton was often associated with chytrids (chytridiomycetes) which are a type of tiny, simple fungi that thrive in damp and especially aquatic conditions (see also the sections on Fungi under Other Non-Vascular Plants and Evidence for Plant/Animal Interactions ).

Introduction

Nothia was originally described as the probable fertile region of another Rhynie plant, Asteroxylon mackiei by Kidston and Lang (1920b) . This was assumed to be the case until Geoffrey Lyon discovered the unequivocal fertile leafy shoots belonging to Asteroxylon ( Lyon, 1964 ). In the same paper Lyon gave a preliminary description of this enigmatic new vascular plant and assigned it the name Nothia aphylla though no diagnosis was given. El-Saadawy and Lacey (1979b) wrote a more detailed account of Nothia and were the first to provide a diagnosis. The most recent work, concentrating on the rhizomal anatomy, has been written by Kerp et al. (2001) .

The systematic position position of Nothia remains unresolved. As well as the sporophyte, the male gametophyte of this plant, Kidstonophyton discoides, has also been described ( Remy & Hass 1991b ). The overall morphology and palaeoecology of Nothia is outlined below.

Morphology

Aerial Axes

The aerial axes of Nothia are quite distinctive. They are naked and display a conspicuous, very irregular surface (see heading photograph). The maximum diameter of the axes is 2.5mm and the axes exhibit commonly repeated dichotomous branching giving the plant a thicket-like appearance. The plant probably attained a height of about 15cm.

The epidermis of Nothia shows a characteristic pattern of longitudinally orientated files of short cells alternating with 'giant cells' ( Kerp et al. 2001 ) (see inset right). It displays elliptical to lenticular stomata-bearing emergences up to 350μm comprising longitudinal files of short cells with up to four intervening 'giant cells'; the files of short cells bearing the stomata . The pores of the stomata are extremely narrow and their two guard cells are usually wider than long. The cortex is generally poorly preserved (see inset below right) though an inner and outer cortex can occasionally be differentiated based on cell shape (Edwards et al., in press), being short in the outer cortex and elongated in the inner cortex. Below the epidermal emergences the intercellular spaces of the outer cortex are large and within the emergences the outer cortex appears spongy ( Kerp et al. 2001 ).

In many cross-sections the aerial axes of Nothia appear to show a double ' vascular strand ', reflecting the repeated dichotomy of the plant. In transverse section the xylem appears elliptical or crescentic in shape (see inset right). It is sub-terete and endarch with small central cells being surrounded by larger cells. Unlike most of the other Rhynie plants the xylem does not exhibit thickenings ( El-Saadawy & Lacey 1979b ) and therefore appears fibrous. Thus Nothia does not possess a vascular strand in the strictest sense, a feature it shares with Aglaophyton.

Rhizomal Axes

The rhizomes of Nothia have only recently been adequately described ( Kerp et al. 2001 ). Though they have never been discovered in organic connection with the aerial axes, they are assigned to Nothia on the basis of their similarity in epidermal and 'vascular' morphology and anatomy. They have primarily been found in situ in a number of cherty sandstone beds, exhibiting repeated branching and characteristically show a ridge on the ventral surface that bears the rhizoids (see inset right). These smooth primary axes locally turn upright and are believed to pass upwards into the aerial axes.

The morphology of epidermal and 'vascular' cells of the rhizome are very similar to that of the aerial axes. The epidermis again consists of alternations of files of short cells and 'giant' cells and the xylem strand exhibits small cells surrounded by larger water-conducting cells. Again the xylem cells do not show partial thickenings and thus have a fibrous appearance. The main differences between the rhizomal and aerial axes is the lack of 'emergences' (the axes consequently appearing 'smooth'), lack of stomata and the thickening of walls in the short cells of the epidermis.

Sporangium

The sporangia of Nothia have a lateral disposition, each sporangium being attached to the aerial axes by a sporangial stalk (see inset right). The sporangia occur in numerous arrangements, being paired, whorled, as terminal clusters or random ( El-Saadawy & Lacey 1979b ). The shape of the sporangia ranges from typically reniform (kidney-shaped) to pear-shaped and they display a well-developed marginal dehiscence mechanism; the maximum dimensions of a single sporangium being 3.1mm by 1.8mm by 1.4mm. Characteristically the sporangial epidermis is very similar to that of the aerial and rhizomal axes showing the alternation of 'giant' cells and files of short cells ( Kerp et al. 2001 ).

Gametophytes

To date the male gametophyte of this plant has been described and has been assigned the name Kidstonophyton discoides ( Remy & Hass 1991b ). The aerial axis of the free-living gametophyte show the same general morphology and anatomy as that of the sporophyte, however, the distal end terminates in a distinct cup or disc with tubular projections that bear the antheridia (see inset right).

Reconstruction

Relationships

Nothia aphylla is yet another Rhynie chert plant whose taxonomic relationship remains unclear, exhibiting morphological and anatomical features characteristic of a number of plant groups. Firstly it shows features characteristic of the bryophytes, namely the unthickened water-conducting cells (a feature also seen in the ' hydroids ' of Aglaophyton major), a feature which suggests it is not a true vascular plant. Secondly, Nothia also shares features with the primitive rhyniophytes since its axes are naked and show similar simple branching. Also the sporangia of this plant show similarities with zosterophylls, being lateral in their disposition on the axes, reniform in shape with a well-developed marginal dehiscence mechanism.

Palaeoecology

Nothia is a relatively common plant in a number of chert beds occurring in allochthonous and autochthonous plant litter, though, with the exception of rhizomal axes, not commonly preserved in growth position. In a few horizons rhizomes and aerial axes of Nothia occur as monotypic assemblages. In most cases the rhizomal axes of Nothia appear to be preserved in cherty sandstone beds along with plant litter. The presence of rhizoids and lack of stomata on the rhizomal axes suggest the rhizomes of Nothia were subterranean, apparently preferring sandy soils.

In some beds Nothia rhizomes appear to have penetrated earlier Nothia rhizomes or the rhizomes of other plants (e.g.: Asteroxylon) which also suggests they grew within plant litter. The rhizomes or primary axes are always far better preserved and less decayed than the aerial axes, even in the few instances where the latter are found in situ in vuggy and massive cherts that usually contain very well preserved axes of other Rhynie plants. These observations tend to suggest a much longer lifespan for the rhizomal (subterranean) axes than the aerial axes the growth of which was perhaps determined by seasonal changes ( Kerp et al. 2001 ).

Introduction

One of the first Rhynie chert plants to be described and perhaps the most abundant is the form Rhynia. The plant was originally described and classified by Kidston and Lang in 1917, 1920a and assigned the species name Rhynia gwynne-vaughanii. Another plant now known as Aglaophyton, possessing a slightly similar anatomy, was originally described by Kidston and Lang in their 1917 paper as also belonging to Rhynia gwynne-vaughanii, but differs significantly in its vascular anatomy and was subsequently reassigned (see also Aglaophyton in the top panel). Unequivocal gametophytes of Rhynia have recently been discovered ( Kerp et al. in press ) and will be illustrated here once published. The overall morphology and palaeoecology of Rhynia is outlined below.

Morphology

'Aerial' Axes

The axes of Rhynia exhibit a maximum diameter of 3mm and the plant probably attained a height of up to 20cm. The aerial or rather the 'upright' axes are cylindrical, naked and upwardly tapering. The branching of Rhynia is both dichotomous and adventitious or monopodial , with dichotomy occurring at an angle between 17 and 35⁰ ( D. S. Edwards 1980 ).

The surface of the axis bear numerous conspicuous emergences or hemispherical projections (see insert below right) from the epidermis which are occasionally located beneath stomata and at the base of adventitious branches and in other instances internally display fungal activity and dark necrotic tissue.

The stomata typically appear circular on the cuticle surface and are flanked by two guard cells (see inset above right). The cells of the cuticle often exhibit a median ridge giving the cuticle a flanged appearance.

The cortex is divided into two distinct zones separated by a brown line of amorphous material. The outer cortex comprises closely packed uniform cells, becoming noticeably elongate below the hemispherical projections. The inner cortex comprises uniform cells with a well-developed inter-cellular air space network and commonly exhibits vesicular arbuscular mycorrhizae. The vascular tissue or stele comprises a zone of ' phloem ' of uniform thickness surrounding a central xylem strand. The phloem cell walls exhibit what appear to be pores ( Satterthwait and Schopf 1972 , Kenrick and Crane 1991 ). The xylem strand is terete , exhibiting endarch cell development. The xylem cells also exhibit annular and rare spiral thickenings.

Rhizomal Axes

Like Aglaophyton, Rhynia laid directly on the ground surface. Rhynia possesses a creeping rhizome displaying repeated dichotomous and adventitious branching, locally turning upright, passing upwards into the 'aerial' axes. The rhizomal axes are cylindrical and naked and generally exhibit a similar morphology and internal anatomy to the aerial axes though they lack stomata. The other main difference is exhibited by the hemispherical projections, which commonly support tufts of unicellular rhizoids (see inset right). However, occasionally hemispherical projections on the upright 'aerial' axes may also bear rhizoids.

Sporangium

The sporangia of Rhynia are not particularly common. They are fusiform, displaying a maximum size of 3.6mm by 2.4mm. The disposition of the sporangia is terminal, being located on the adventitious branches of fertile aerial axes. No dehiscence mechanism has been observed though a dark cellular layer or 'sterile pad' at the base of the sporangium has been interpreted as a site of abscission by D. S. Edwards (1980) (see inset right). The sporangial wall comprises three layers: an outer epidermis, a poorly preserved parenchymatous layer and an inner tapetal layer .

Reconstruction

Relationships

Rhynia gwynne-vaughanii being a naked, simply branched sporophyte has been assigned to a primitive group of vascular plants known only from fossils, and called the rhyniophytes.

The presence of hemispherical projections on the axes remains a point of speculation and their like is not seen in any of the other Rhynie plants. Various interpretations have been proposed:

• Damage from arthropods sucking sap ( Kevan et al. 1975 ).
• Wounding by nematodes, mites, parasites or fungi ( Edwards and Selden 1993 ).
• Damage from splashes of hot water from geysers, or from volcanic ash ( Kidston and Lang 1921a ); an unlikely explanation considering the small size and disposition of the projections on the plant axes, and also since they have not been observed on any of the other Rhynie plants.
• Pant (1962) and Lemoigne (1968a, b) interpreted the hemispherical projections as sites of archegonia , though this interpretation has never been generally accepted.

Palaeoecology

Rhynia was the most common vascular plant in the Early Devonian ecosystem at Rhynie, at least in the areas of sinter deposition, both numerically and in terms of ground cover ( Powell et al. 2000b ). The plant appears to have been entirely subaerial with its naked, branching 'rhizomes' creeping across the ground surface with the upright portions of the plant growing to give a thicket-like appearance. Although the hemispherical projections on the 'rhizomes' bear the rhizoids for taking up water from the ground surface, the fact that those on the upright stems occasionally exhibit rhizoid tufts suggest Rhynia was also capable of taking up atmospheric water.

Where found in growth position in the chert beds it is typically, though not exclusively the only in situ vascular plant present, and is commonly found above sandy chert layers and allochthonous litter horizons. This suggests Rhynia commonly grew as monotypic stands, an early colonizer of well-drained sinter and sandy substrates.

However, the plant is also found associated with all other Rhynie plants, though only very rarely with Horneophyton. This suggests Rhynia was tolerant of a wide range of habitats and could also withstand interspecies competition within the Early Devonian ecosystem at Rhynie ( Powell et al. 2000b ).

Introduction

Trichopherophyton is one of the more recent additions to the Rhynie chert list of vascular plants. It was formally described by Lyon & Edwards (1991) and assigned the species name Trichopherophyton teuchansii, and was the first true zosterophyll recorded from the cherts. This is the scarcest of the Rhynie plants having only been found in a few beds of chert.

A reconstruction of the whole plant has not yet been attempted and the gametophyte stage of the plant remains unknown. The overall morphology and palaeoecology of Trichopherophyton is outlined below.

Morphology

Aerial Axes

The aerial axes of the plant exhibit a maximum diameter of 2.5mm. The branching of Trichopherophyton is both dichotomous and pseudomonopodial and it is the only plant known in the Rhynie flora to display circinate vernation .

Generally the cuticle of Trichopherophyton is poorly preserved and as a result stomata have not been observed, the rest of the epidermis , however, is usually well preserved and very often exhibits unicellular spinose projections emerging at 90⁰ to the axis of the plant (see inset right). These spiny outgrowths are one of the most characteristic features of Trichopherophyton.

The cortex can be divided into a distinct outer cortex comprising closely packed cells and an inner cortex or more loosely packed cells with an inter-cellular air-space network. Many cells of the inner cortex contain dark coloured residues (see inset right).

The vascular tissue comprises a very distinct xylem strand (see inset above right). It is sub-terete , exarch and displays both annular and spiral thickening. A narrow zone, uniform in thickness, of thin-walled cells surrounds the xylem strand and probably represents phloem (see inset above right).

Rhizomal Axes

Observed within many of the chert specimens containing the spinose aerial axes of Trichopherophyton are other generally more common axes of a slightly differing but distinctive morphology that are believed to represent the rhizomes of the plant although there remains no unequivocal evidence for organic continuity between the two.

These axes are generally slightly smaller than the aerial axes, having a maximum diameter of 2.3mm.

They exhibit a generally smooth epidermis, lacking the spinose outgrowths though epidermal cells may be papillate and occasionally may bear blunt-tipped emergences that may represent rhizoids. The cortex is quite narrow and less distinctly divided compared with the aerial axes, though again cells of the inner cortex commonly show dark coloured residues (see inset right). A zone of thin-walled cells surrounding the xylem strand may represent phloem. As with the aerial axes the sub-terete xylem strand is very distinctive being exarch and showing narrow annular and spiral thickenings (see inset right).

Sporangium

The sporangia or fertile elements of Trichopherophyton have been observed in a few specimens but have not as yet been fully described. However, sporangia do appear to be reniform (kidney-shaped) with dimensions up to a maximum 3.7mm by 2.5mm. They are attached laterally to the aerial axes with a vascularised sporangial stalk though their spatial distribution is uncertain. The sporangia appear to be bi-valved with a marginal dehiscence mechanism. Characteristically the sporangia display the same unicellular spinose projections emerging from the epidermis as seen in the aerial axes.

The in situ spores (see panel below) of Trichopherophyton appear to vary between 55 and 80μm in size, are retusoid and smooth walled and display a triangular thickening associated with the trilete mark. These spores are comparable to species of the spore genus Retusotriletes.

Relationships

Trichopherophyton teuchansii is undoubtedly a zosterophyll, fitting into the classification of Banks (1975) , showing lateral, reniform sporangia with a well-developed marginal dehiscence mechanism as well as an exarch xylem strand with thickenings. However, there remain other features, although characteristic of this plant, that are rarely encountered in true zosterophylls; namely a sub-terete xylem strand and the presence of unicellular spiny epidermal projections on the aerial axes and sporangia.

The circinate vernation seen exclusively in Trichopherophyton is a feature that is also observed in most extant ferns. This suggests that the plant has a more advanced anatomy than that of the other Rhynie plants.

Palaeoecology

Generally Trichopherophyton appears to be relatively rare in comparison with the other vascular plants and has been found to occur in only a few beds of chert. This may partially reflect the scarcity of the plant within the original ecosystem or may simply reflect sampling bias. However, within the few beds within which it occurs, it is preserved both in autochthonous growth position and as litter, but always as part of a diverse flora including Nothia, Horneophyton and locally Rhynia. It seems likely therefore that Trichopherophyton was a late coloniser of humus-rich substrates ( Powell et al. 2000b ).

Introduction

Ventarura is the most recently identified element of the Rhynie flora. The plant was formally described by Powell et al. (2000a) and assigned the name Ventarura lyonii. The float blocks from which the plant were described all originated from the Windyfield chert of the Rhynie hot spring complex. Many thin sections made of this material have revealed details of the subaerial and probable rhizomal axes, the pattern of branching, and the morphology of the sporangia of the plant.

A preliminary reconstruction of the sporophyte stage of the plant has recently been attempted (see below), though the gametophyte stage of the plant remains unknown. The overall morphology and palaeoecology of Ventarura is outlined below.

Morphology

Aerial Axes

The aerial axes of Ventarura (see inset right) have a maximum diameter of 7.2mm and exhibit primarily dichotomous branching. The angle of dichotomy usually being at 25⁰. It is not known what height the plant attained, but it was in excess of 12cm. The axes are smooth though the epidermis exhibits unicellular peg-like projections. The details of the cuticle are not fully known and the morphology of the stomata has not been resolved. Occasionally, enigmatic elliptical features are seen on the surface of the cuticle. It is not known exactly what these are, but they may represent the sites of projections that have been lost.

The cortex of the aerial axes of Ventarura is quite distinctive. An inner and outer cortical layer is separated by a dark middle cortex zone of sclerenchymatous cells (see inset below left). The outer cortex comprises closely packed thin-walled cells. The inner cortex again comprises thin-walled cells but with a well-developed inter-cellular air space network.

The vascular strand of Ventarura comprises a generally poorly preserved outer zone of uniform width, tentatively interpreted as phloem tissue. The xylem strand is terete and exarch with annular and spiral thickenings (see inset below right).

Rhizomal Axes

Observed within many of the chert blocks containing the aerial axes of Ventarura are other smaller axes of a slightly differing but distinctive morphology that are believed to represent the rhizomes of the plant although there remains no unequivocal evidence for organic continuity between the two.

These axes branch repeatedly and tend to exhibit unicellular peg-like outgrowths from the epidermis (interpreted as representing rhizoids) and characteristically lack the sclerenchymatous middle cortex seen in the aerial axes.

Sporangium

The sporangia of Ventarura are reniform (kidney-shaped) to pear shaped with dimensions up to a maximum 5.2mm by 2mm. They are attached laterally to the aerial axes with a vascularised sporangial stalk.

Their spatial distribution is not fully known but at least appear to form in a vertical sequence along the fertile axes (see inset top right in section on 'aerial axes').

The sporangia are bi-valved with a well-developed marginal dehiscence mechanism (see inset right).

Preliminary reconstruction

Serial sections of a block of Windyfield chert have revealed more of the general anatomy of Ventarura than could be described by Powell et al. (2000a) . This reconstruction (see inset left) was displayed at the conference on the Rhynie chert held in Aberdeen in September 2003.

In the sketch the individual sections of the plant that could be traced in the block are darkly shaded, and reveal a variety of branching styles are present.

Numbered features on the reconstruction are:

1. Branched sections of aerial axes traced within a chert block.

2. Sporangia in ?2 regular en echelon rows on axes of 2-3 mm diameter. Sporangia up to 4 mm wide.

3. Branched horizontal rhizomes attached to aerial axis.

4. Rhizomes penetrating bacterial meshworks and decayed aerial axes in water. Growth of the plant probably extended into floating bacterial mats, resulting in vertical penetration of rhizomes into water underneath the mat.

5. Portions of aerial axes were preserved in an inverted position after floating in pools adjacent to growing areas (see heading photograph).

We are still seeking the connections between the fertile axes seen in thin sections and the main aerial axes, also further information on rhizome branching, sporangial arrangement, and the terminations of aerial axes and rhizomes. The search continues prior to formal publication!!

Relationships

Ventarura exhibits a number of features that assist in classification; particularly the morphology of the xylem tissue, and the morphology and disposition of the sporangia. The xylem strand is characteristically exarch with thickenings. The sporangia are reniform to pear-shaped with a well-developed dehiscence mechanism, and have a lateral disposition on the aerial axes to which they are attached by a vascularised sporangial stalk. These features indicate Ventarura is undoubtedly a tracheophyte allied with the zosterophylls and as such is the second zosterophyll known from the Rhynie chert (see also Trichopherophyton in the panel above).

Palaeoecology

To date Ventarura has only been found in a number of chert float blocks from pods of the Windyfield chert. Although locally very well preserved, much of the vegetative and fertile material appears to be at least partially transported, and deposited in freshwater pools evidenced by many of the branched aerial axes being inverted (see heading photograph). The chert texture is typical of silicified subaqueous deposits, containing the crustacean Lepidocaris, microcoprolites and bacterial coatings on plant debris.

As such it remains difficult to sufficiently interpret the palaeoecology of this new plant but with the presence of apparently in situ rhizomatic axes in the chert blocks it seems likely that the plant probably grew at the edges of or at least within the vicinity of these pools and may have preferred sandy and organic-rich substrates.

Fossilised spores from vascular plants are quite common in beds of the Rhynie chert and also within the finer grained interbedded sediments such as shales and mudstones. They are usually extracted from the sedimentary rocks by macerating rock samples in solutions of Hydrofluoric acid which dissolves the silica minerals in the sediment and leaves the resistant spores and other acid-insoluble organic remains for analysis ( Wellman & Axe 1999 ). Interestingly, upon extraction the spores often appear better preserved within the mudrocks than in the cherts, though this may be a reflection of the effects of silicification and replacement of the original organic material.

Many genera and species have been recognised and described, though there still remains a degree of uncertainty as to which of the known vascular plants of the Rhynie chert they can be assigned to. In situ spores have been described in the past for Horneophyton and Rhynia by Bhutta (1973a, b) . There may well be spores present that are representative of other plants that have not been so far discovered in the cherts. The abundance and variety of spores present in the sequence of Rhynie sediments has been useful in terms of biostratigraphy in determining the age of the rocks ( Wellman 2004 ).

The images above left and right display a few of the spore types or palynomorphs that have been found.

The study of the fossil spores or palynology of the Rhynie chert and sediments is ongoing at the University of Sheffield.

In the Rhynie chert many spores have also been found preserved during various stages of germination. This feature was first described by Lyon (1957) . The following four images below are some examples of stages in germination of Retusotriletes (see also insets above right), these are spores of Aglaophyton major.

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.

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.

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).

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.

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).

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.

Introduction

Nematophytes or the Nematophytales are an extinct group of enigmatic plants known only from Devonian and Carboniferous sedimentary rocks. Their systematic position is unresolved, showing certain affinities with fungi, algae and also, tentatively, with vascular plants. Their gross morphology and habitat (particularly as to whether they were aquatic, semi-aquatic or fully terrestrial plants) are also not fully known, but some appear to have been cylindrical organisms up to 1 metre in length ( see Stewart 1999 ). Nematophytes appear to generally comprise networks of intertwined spirally coiled tubular cells showing both smooth and spirally-thickened walls (the latter similar in appearance to tracheids in vascular plants), with branching localised in distinct 'knots'. The plants appear to have had a variably preserved cuticle-like layer on their outer surface. Certain Carboniferous nematophytes appear to have attained quite a large size, in life probably looking like prostrate logs.

At Rhynie, sandstone casts of probable nematophytes have been found in the past in the Quarry Hill Sandstone . Two incomplete nematophytes have been described from the Early Devonian Rhynie chert: Nematophyton taiti ( Kidston & Lang 1921b ) and Nematoplexus rhyniensis ( Lyon 1962 ). Fragmentary nematophyte remains also occur in the Windyfield chert ( Fayers & Trewin in press ). For the purposes of this resource, the morphology of one of the Rhynie nematophytes, Nematoplexus rhyniensis is outlined below:

Morphology

Nematoplexus rhyniensis

Nematoplexus rhyniensis was first described by Lyon (1962) , and is only known from incomplete, partially decayed, and generally fragmentary remains. As such, like other nematophytes, the gross morphology of the organism and its overall size is unknown. Conforming to the general internal nematophyte structure, Nematoplexus comprises an open meshwork or plexus of intertwined, spirally coiled tubular cells (see heading photograph). The tubes show no preferred orientation, but in the sample described by Lyon (1962) they appear more compacted towards the outer surface of the plexus where they abut onto a structureless clear zone which may represent a partially decayed natural surface layer or membrane.

Branching of the tubes occurs in what appear to be dark, amorphous spots, but in better preserved samples these, in fact, comprise very tightly coiled knots of tubes showing repeated and closely spaced branching (see insets right and below right). These branch-knots occur in two discrete sizes, the smaller ranging from 45µm to 76µm in size and the larger knots between 99µm and 270µm.

The tubes show two distinct morphologies. The most abundant form are smooth-walled, non-septate tubes ranging from 7µm to 10µm in diameter, typically appearing as lax intertwined coils (see inset below left). Branching of the smooth-walled tubes occurs within the smaller branch-knots.

The second type are thin-walled tubes displaying conspicuous spiral thickenings within the wall layer. These range from 2µm up to 28µm in diameter and are again non-septate (see inset below right). The spirally thickened tubes are generally found as short isolated fragments or as irregular clusters within the smooth-walled tube plexus. Branching of these tubes occurs within the larger branch-knots. Occasionally tubes of both types may originate from a single, large branch-knot, though no organic connectivity between the two types have been observed.

Palaeoecology

As stated in the introduction, the habitat of Nematoplexus and nematophytes in general remains unresolved.

A number of authorities believe that nematophytes were aquatic plants. In the Rhynie chert the two nematophytes Nematoplexus and Nematophyton are typically found in association with filamentous green algae (chlorophytes), charophytes, cyanobacteria, occasionally with the crustacean Lepidocaris and with coprolites in a 'clotted' chert matrix; indicative of silicification in a freshwater, aquatic environment. However, in these instances, it may be that the nematophyte remains had been transported, after death, into such aquatic settings or alternatively had been drowned in situ after a flooding event.

They may, however, have been terrestrial or at least semi-aquatic organisms, suggested by the resemblance of the spirally-thickened tubes seen in the Rhynie chert nematophytes to the thickened tracheids in the xylem of vascular plants. It may be that these organisms grew in shallow water with the spirally-thickened tubes differentiated in emergent fronds or 'leaves'. However, to date, no unequivocal evidence of such differentiation has been found in any fossil nematophytes.

Introduction

Cyanophytes are an enigmatic group of prokaryotic organisms that comprise what were formerly known as the 'blue-green algae'. Strictly speaking these are only termed 'algae' because they are organisms that can undergo photosynthesis. They are in fact a large and varied group of bacteria, the cyanobacteria, that possess a type of chlorophyll (chlorophyll a) and are able to photosynthesise in the presence of air and sunlight and produce oxygen. Cyanobacteria, unlike chlorophytes, do not possess chloroplasts. In fact the chloroplast in the cells of eukaryotic algae and plants is actually a symbiotic cyanobacterium. In cyanobacteria the chlorophyll is carried on specialised membranes within the cells called thylakoids (see inset below).

Many cyanophytes are important in the fixation of atmospheric nitrogen. This process can only occur in the absence of oxygen, therefore certain cyanobacteria possess specially thickened cells or heterocysts which contain an anaerobic microenvironment in which this can take place (see inset below). Cyanobacteria may be unicellular or filamentous and may or may not be colonial (see inset below).

Modern cyanobacteria live in many different environments ranging from marine to freshwater habitats and also in soil, on rocks and on plants. A few types of cyanobacteria have a symbiotic relationship with fungi forming lichens.

Certain cyanobacteria help to form rigid biogenic structures such as pisoliths and stromatolites. Both are formed by the organisms growing on a particular surface or substrate to which they gradually bind consecutive layers of very fine sediment and precipitated minerals, eventually forming (especially in stromatolites) a stacked sequence of smooth, wavy and crenulated laminae, often forming mounds. Pisoliths usually form by growth around a moveable object such as a sand grain or shell fragment which may be periodically turned over by current action thus creating an almost concentric layer of laminae.

Stromatolites and pisoliths are locally common in the geological record and in certain areas of the world are being formed today such as at Shark Bay in western Australia. Thermophyllic cyanobacteria also produce stromatolitic textures in hot spring areas where they bind detrital grains and precipitated opaline silica to the substrate (see inset below, and section on The Ancient Environment and Modern Analogues ).

Fossil Record

Cyanobacteria comprise the earliest forms of life, their fossils having been found in Archaean Precambrian rocks in western Australia dated at almost 3500 million years old. Some of the best preserved fossil cyanobacteria have been found in cherts (including the Early Devonian Rhynie chert) such as in the Late Proterozoic Bitter Springs Chert in Australia. In many cases it is the structures formed by these organisms, primarily pisoliths and stromatolites, that are often preserved in the fossil record, particularly in limestones and carbonate rocks. Both are common and locally well-developed, for example, in the Carboniferous limestones of Ireland and Derbyshire in England and also in the Upper Jurassic Purbeck Limestones of southern England.

The Early Devonian Rhynie chert yields a number of fossil micro-organisms and sedimentary textures that may be attributable to cyanobacteria. Most of the latter occur as well-developed stromatolitic structures that are highly comparable with those formed by the growth of cyanobacterial mats on modern sinter terraces and in hot springs (see heading photograph and sections on Chert Textures and The Ancient Environment and Modern Analogues ). However, because of the paucity of well-preserved diagnostic features, only a few probable cyanobacteria have been formally described and named by Kidston and Lang (1921b) , Croft and George (1959) and D.S. Edwards and Lyon (1983) ; Archaeothrix contexta, Archaeothrix oscillatoriformis, Kidstoniella fritschii, Langiella scourfeldii, *Rhyniella vermiformis and Rhyniococcus uniformis. Many other unicellular and multicellular forms of probable cyanobacterial origin are also present but for the lack of sufficient diagnostic features have not been formally described and named.

For the purposes of this resource we will concentrate on the genus described by Kidston and Lang (1921b) , Archaeothrix, the morphology of which is outlined below.

*Note: For the species of probable cyanobacteria described by Croft and George (1959) , Rhyniella vermiformis, the generic name was already occupied by the collembolan Rhyniella praecursor.

Morphology

Archaeothrix

Two species of Archaeothrix were originally described by Kidston and Lang (1921b) ; Archaeothrix contexta and Archaeothrix oscillatoriformi. Both consist of simple unbranched filaments comprising discoid cells and possessing probable heterocysts. The difference between the two species is mainly in the diameter of the cells. In A. oscillatoriformi the cells range from 3µm to 4µm in diameter whereas in A. contexta the cells are narrower being approximately 2µm in diameter.

Neither of these cyanobacteria are especially common; A. oscillatoriformi has been found within partially decayed stems of Rhynia (see inset right) and A. contexta has been occasionally found as large masses lying loose within the chert matrix.

Introduction

Chlorophytes are a particular division of eukaryotic algae comprising the green algae. They are characterised by containing two types of chlorophyll, a and b and starch is formed in chloroplasts (see inset below). Green algae may be unicellular but can also form complex multicellular structures such as that seen in the stoneworts or charophytes (see section on charophytes below). These non-vascular plants are typically found in freshwater environments.

Fossil Record

Chlorophytic algae are among the oldest known fossils, being recorded from Precambrian rocks primarily from the Ediacara fauna. These Ediacaran fossils are acritarchs and were probably formed by unicellular algae. In fact the discovery of undoubted chlorophytes in the 850 million year old Late Proterozoic Bitter Springs Chert in Australia was the first ever evidence of early eukaryotes. Multicellular chlorophytes are first seen in Cambrian strata where they helped to build algal reefs. Very well preserved chlorophytes are also known from the famous Middle Cambrian Burgess Shale in British Columbia, Canada.

Apart from 'charophytes', a number of other chlorophytes have been found in the Rhynie chert including unicellular and filamentous types. Of these, to date, two filamentous types have been formally described by D.S. Edwards and Lyon (1983) ; Mackiella rotunda and Rhynchertia punctata. The morphology of these simple green algae is outlined below.

Note: It must be remembered that the affinities of many fossil algae, including those found in the Rhynie chert remain uncertain. This is primarily because modern classifications of algae are based on biochemical and ultrastructural features which are rarely or indeed never preserved in the fossil record ( D.S. Edwards & Lyon 1983 ). Also, positive identification may be compounded by the variable preservation of cell contents.

Morphology

Mackiella rotunda

This alga comprises unbranched filaments up to 850µm in length forming an unattached thallus with no rhizoids. The filaments consist of up to 25 cylindrical cells, each cell typically being more-or-less equal in length and in width (23µm to 41µm and 28µm to 40µm respectively). The terminal cells of the thallus are rounded and slightly longer than the other cells (29µm to 49µm in length). The cell walls are thin and do not show a mucilaginous sheath. The cell contents comprise fine granular material and a dark spherical body, 3.5µm in diameter, interpreted by D.S. Edwards and Lyon (1983) as a pyrenoid or the chloroplast.

Mackiella was a eukaryotic alga. Because the alga shows unattached filaments that may fragment into short filaments and the cells possess a single pyrenoid, D.S. Edwards and Lyon (1983) tentatively assigned Mackiella to the extant order Ulotrichales.

Rhynchertia punctata

This alga consists of an unbranched, unattached thallus comprising multicellular filaments of cells 8µm to 17µm in width and commonly twice that in length. The terminal cells of the thallus are rounded and slightly shorter than the other cells. The cells are thin walled and do not show a mucilaginous sheath. The cell contents may be uniform, or with a single dark body (possibly a chloroplast) or may contain many small ovoid bodies. The latter have been interpreted to be reproductive elements, possible zoospores or gametes ( D.S. Edwards & Lyon 1983 ).

Rhynchertia filaments have been found in association with the nematophyte Nematoplexus, primarily within 'gelatinous' areas where the latter appears to have degraded ( Lyon 1962 ). Based on gross morphology, this alga has also been tentatively assigned to the Ulotrichales by D.S. Edwards and Lyon (1983) .

Introduction

Charophytes or stoneworts are one of the largest and most structurally complex of the green algae, and are thought to be part of the evolutionary lineage that lead to vascular plants ( Kenrick 1994 ). As green algae, charophytes are sometimes classed within the Chlorophyta but are often classed as a distinct group, the Charophyta. These plants are aquatic, and modern forms are found in many freshwater to brackish habitats such as ponds, lakes, lagoons and streams. The main axes of mature plants comprise a series of multicellular nodes interspersed by relatively long single cells or internodes . Branches and branchlets grow from individual nodal cells in a whorled arrangement (see inset below). The gametangia , or fertile elements of the plant are quite characteristic in charophytes. The female gametangium or oogonium is formed from a number of divided cells that grow out from one of the nodes; the outer most cells are elongate and spiral around the oogonium forming a protective sheath (see inset below). The male gametangium or antheridium also grows from a nodal cell and is a complex spherical structure usually comprising eight shield-cells, each one enclosing a stalked cell bearing tiny rounded head-cells which in turn bear filaments comprising the cells which produce the spermatozoids. The position of the gametangia on the plants is variable with different charophyte species and can be diagnostic.

As fossils, charophytes can be quite in sediments deposited in suitable environments. It should be noted that the oogonia may be allochthonous or easily transported, but in instances where the axes of the plant are also preserved it is likely these are more-or-less in situ or autochthonous .

Fossil Record

Charophytes are quite well represented in the fossil record from the Tertiary, Cretaceous and Jurassic where they are locally abundant, particularly in limestones and marls deposited in brackish or freshwater settings. In these rocks it is usually the fossilised oogonia that are found (commonly called gyrogonites ). This is because the oogonia are often calcified and are easily preserved as a mineral of calcium carbonate, typically calcite . In some instances, for example in the Upper Jurassic/Lower Cretaceous Purbeck Limestones of southern England, fossil charophyte oogonia are so common, diverse and widespread they can be used as zone fossils for biostratigraphy ( e.g. Feist et al. 1995 ). The preservation of whole plants is not common, but they can be quite exquisite in their detail ( e.g. Martín-Closas, C. & Diéguez 1998 ).

The earliest fossil charophytes have been recorded from the Upper Silurian, though whether or not these are true charophytes is still debated. Early Devonian charophytes have been found exquisitely preserved in the Rhynie chert and were first recorded and described by Kidston & Lang (1921b) . The Rhynie charophytes show many similarities with the extant charophyte group, the Nitelleae, and have been assigned to the species Palaeonitella cranii. The morphology of Palaeonitella is outlined below.

Morphology

The morphology of Palaeonitella is relatively simple, its structure being closely comparable to that of modern Nitelleae (see inset above). An entire reconstruction of the alga has not been published, though much of the plant is known, primarily the branch whorls, rhizoid nodes and possible bulbils. The status of Palaeonitella has been questioned by Tappan (1980) , but recent discoveries of fertile elements in the chert indicate unequivocally that the plant is a charophyte alga ( Kelman et al. in press ). The fertile elements will be figured here once published.

Axes

The overall size of the plant is not known, but it comprises upright primary axes that bear regularly spaced clusters of nodal cells with whorls of lateral branches. The spacing of nodal cells or rather the length of the internodes does, however, tend to decrease towards the distal end of the axes (see inset below left). These branches in turn may bear secondary branches. The number of branches or rays in a whorl can be variable. Whorls commonly bearing up to ten secondary rays have been observed on the lateral branches. The primary axes are slender generally being up to 200µm in diameter, lateral branches are conspicuously narrower being up to 100µm in diameter with very fine secondary branches being up to 50µm in diameter (see inset below right). Occasionally the nodal and internodal cells appear hypertrophic; mutated and swollen where they have been infested by aquatic fungi ( Taylor et al. 1992 ).

Rhizoids and Bulbils

At the bases of the primary axes occur tubular cells occasionally displaying oblique septa and nodes. These structures represent the rhizoids of Palaeonitella. The nodal cells associated with the septa commonly bear small rhizoidal branches (see inset right). These structures are closely comparable to those seen in the rhizoids of extant Nitelleae ( D.S. Edwards & Lyon 1983 ). Occasionally, conspicuous oval or spherical vesicular structures are seen in connection with these nodal cells. They vary in size, typically ranging between 200µm and 500µm in diameter and represent bulbils, another feature seen in some contemporary charophytes.

Palaeoecology

Palaeonitella was undoubtedly an aquatic plant. It is typically found in chert associated with other aquatic biota, particularly crustaceans, chlorophytic algae and chytrids (tiny aquatic fungi). The enclosing matrix commonly contains coprolites and exhibits a clotted or 'mulm-like' texture. It is likely that the alga was an early coloniser, primarily living in still, relatively shallow freshwater temporary ponds, apparently with rather soft silty and organic-rich substrates. If Palaeonitella had a similar ecology to that of most modern Nitelleae it is likely the waters it inhabited were generally quite alkaline. Most extant charophytes tolerate waters with a range of between pH 6 and pH 9. In a few charophyte-bearing chert horizons, many of the specimens preserved in situ exhibit an orientation with axes appearing to be swept or bent over in a particular direction. This probably represents current alignment ( Fayers and Trewin in press ); maybe Palaeonitella could also tolerate areas of slow moving water, perhaps colonising rather sheltered areas in the cool distal reaches of run-off channels from hot springs.

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).

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.

Ascomycetes

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.

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.

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.

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).

A review of the Rhynie chert fungi is currently awaiting publication ( Taylor et al. in press ).

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.

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).

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 right and below left). 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.

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.