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Cooksonia, a very old land plant (II)  (I)

The presence of xylem vessels with annular or spiral-shaped thickenings at the walls counts as characteristic for higher plants. Algae and mosses do not have such tracheids.
Several stems of Cooksonia show a central black line or thickening, which is probably the remainder of a xylem strand.

D. Edwards (1992) was the first one to publish photos of xylem vessels  of Cooksonia (Photo with permission of Nature Publishing Group). Edwards found the tracheids in axes with attached sporangia of the species C. hemisphaerica en C. pertoni from Lower Devonian river sediments in Shropshire.

Yet there are several snags in the assumption that every plant of Cooksonia had tracheids. In the first place no tracheids have been found in Silurian Cooksonia fossils. So it is possible that the tracheids developed only in the course of time.
Secondly it has been discovered with the use of electron microscopy that in different species of Lower Devonian plants two kinds of strengthened 'xylem' strands occurred. One of those appears to be related to the central strands of mosses ...

Tentatively it is assumed that the oldest Cooksonia's were also in the possession of tracheids.

Spore of C. pertoniIn most cases no spores are present in the sporangia. Sometimes they have been preserved, but as a heavily compressed mass in which the individual spores can hardly be distinguised. Only very seldom have they been preserved in a rather good, threedimensional form. On the right hand side a drawing after a SEM-photo of a spore of C. pertoni (diameter 30µms; drawing J. Hulst).

Research on spores of C. pertoni from different places in Great Britain has revealed an evolutionary trend. The existence of four different types of spores in C. pertoni has been proved: two are common and two rare. There appears to be a tendency from smooth spores in the Silurian to more ornamented ones in the Early Devonian. Of course the advantage of ornamented spores for the plant remains uncertain, but it could possibly be an adaptation to drought.
Comparable smooth spores have been found in the Lower Silurian. Though the oldest known Cooksonia-fossils are of Mid-Silurian age, this could point to an even earlier occurrence of Cooksonia or other plants.

The oldest Cooksonia-fossils have been found in Ireland. They date from the Late-Wenlock (425 million years). The preservation is not good enough to identify them at species name, but they are clearly Cooksonia-like plants with sporangia. Axes without sporangia are also found, some of them with up to three branchings.

Somewhat younger Cooksonia-fossils have been found in North-Wales, at a locality in the Lower Ludlow (420 million years). These fossils are diminutive. My biggest fossil from this place is a branchlet with two bifurcations measuring 15 mms. Axes like these are sometimes placed in the artificial genus Hostinella. Some of the axes show a black line, probably pointing at the presence of xylem vessels.
Silurian Cooksonia-fossils have also been found at some other places in Wales and Shropshire. Edwards even found all four from Great Britain described species at one locality on the South coast of Wales.

Outside Great Britain Silurian Cooksonia-fossils have been found in Canada (not identifiable at species name) , Bolivia (resembles C. caledonica), Czech Republic (C. hemisphaerica), Kazakhstan (not identifiable), China (the same), Siberia (C. pertoni, C. hemisphaerica), the state of New York (not yet identified) and perhaps in Libya.
C. paranenis from Brazil is dated as uppermost Silurian or lowermost Devonian. This last find demonstrates the occurrence of Cooksonia at a considerable shale in parts of the large southern continent Gondwana.

Cooksonia also occurs in younger deposits. C. caledonica has been described on the basis of Lower Devonian fossils from the area around Forfar (Scotland).
River deposits from the Lower Devonian of England have yielded fragmentary, but extremely well preserved fossils of all British species of Cooksonia. Detailed studies could be made of the structure of the sporangia and the spores of these fossils.
Recently a new species, C. banksii, has been described from this locality, which resembles very much the Brasilian Cooksonia (Habgood et al, 2002).

Way of growing
It is striking that most of the sediments containing Cooksonia are marine ones. Probably the plant grew at plains along the rivers which became submerged from time to time. Stems of the plants were then broken off, transported and buried in the sediments on the bottom of the sea. This could explain why roots or horizontally growing stems (rhizomes) have never been found until now. Cooksonia was likely to have rhizomes, for another early plant, Rhynia, related to Cooksonia, was attached to the soil in that way. The horizontally growing stems of this species developed root hairs at places where the stems touched the ground. Perhaps the strange branching at the right hand side could be a rhizome. That would be a major find!

Salt marshes have also been mentioned as possible growing places for Cooksonia.
It is furthermore likely that the plants were growing in vegetations existing of only one species. Most of the Early Devonian plants were also living in such monospecific vegetations.
The Brazilian C. paranensis grew in an area not far from the South Pole at that time under probably rather extreme circumstances. Though the flora indicates an ice-free environment, the sunken sporangia appear to be an adaptation to low temperatures.  

Cooksonia probably grew as a small shrub.