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

The evolution of the first land plants was a major event in the history of Earth. It cleared the way for the irresistible development of animal life on the land. And it was the land plants which changed the biosphere thoroughly, e.g. the oxygen rate, the carbonic acid rate, the soil structure and the character of the erosion. 
The most famous Silurian land plant is a small creature sized up to a couple of centimeters, named Cooksonia. It is not only the best-known plant, but up to now also the oldest one.
Yet a lot is unknown about it. For example the way in which the plant was attached to the soil.

Finding Cooksonia-fossils is difficult because only a few occurrences are known (marine, delta of river deposits from the Late Silurian and the earliest Devonian). The small size of the plant is also a disadvantage for finding. A rather complete plant is very rare.


W.H. Lang published the first species of Cooksonia in 1937: C. pertoni en C. hemisphaerica. He used Lower Devonian specimens from Wales for his description. They were a few centimeters high, one or more times dichotomously branched and they were bearing more or less globose sporangia at the end of the branches. Leaves and other appendices were absent.
He gave the genus name Cooksonia in honour of the Australian paleobotanist Isabel Cookson with whom he co-operated in the research of the likewise very old plant Baragwanathia from Australia.
Since then there are reports of Cooksonia-species from places all over the world: Wales, Scotland, England, Bohemia, Kazakhstan, Siberia, de state of New York, Canada, China, Bolivia and Brazil.

The best-known species are C. pertoni, C. hemisphaerica, C. cambrensis, C. caledonica, all described on the basis of British fossils. Furthermore recently the new species C. paranensis from Brazil has been described by P. Gerrienne et al (2001).
The species are charactarized by the shape of the sporangia and the subtending axis. Because of the variability of the form of the sporangia identification of the species is often difficult or even impossible. The angle with the layer in which the sporangium has been preserved is also of great influence on the shape of the fossil. Therefore it is important for identification to have several sporangia at one's disposal.

Cooksonia-sporangia did not have an special adaptation for dehiscence at maturity. Probably the sporangium simply tore open at the upper side. (Photo: with permission of Nature Publishing Group).


Species of Cooksonia
(drawing: Joep Hulst)

Cooksonia pertoni. The sporangia of this plant are much broader than high. They are often so flat that they look plate-shaped. The subtending stalk is strongly widening, whereby the bearing part is nearly as wide as the sporangium. The sporangium is not sunken into the axis.  
Cooksonia hemisphaerica. The shape of the sporangia is semiglobose, globose or elliptical. The stalk is widening below the sporangium and the sporangium is at most about three times as wide as the top of the stalk.
Sporangia close to the bifurcation are mostly nicely round, while sporangia at longer stalks are more irregularly shaped. The sporangia at short stalks are likely to be younger.
Cooksonia cambrensis. The plant bears globose or elliptical sporangia, sitting on stalks which are not or hardly thickening below the sporangium. The difference with C. hemisphaerica is that the axis of C. cambrensis is relatively much thinner immediately below the sporangium.
Aberlemnia (Cooksonia) caledonica. The sporangia are variably shaped but in most cases wider than high or even kidney-shaped. At the upper side a coaly border can be seen in some cases. The stalk is somewhat widening below the sporangium.
C. paranensis. This species has been described by P. Gerrienne et al in 2001 on the basis of hundreds of specimens from the Lower Devonian of Brazil. The plant has a plate-shaped or bowl-shaped sporangium, that is more or less sunken within the axis. Sporangium and stalk together are trumpet-shaped. The sporangium does not project like that of C. pertoni.

Cooksonia must be seen as an artificial genus. It probably consists of several genera, which cannot be distinguished at the moment.

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