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The evolution of plants

A concise report of the development of the flora

I. Silurian and Devonian

Plant evolutionII. Carboniferous and Early Permian
III. Permian - Early Cretaceous

IV. Cretaceous - present

Cooksonia from Wales

The oldest fossils of land plants visible with the naked eye are about 425 million years old. They are miniscule plants from the Mid-Silurian of Ireland en are called Cooksonia. Cooksonia plants had dichotomously divided little stems with small knobs at the end. These knobs were sporangia and they were filled with spores. During about 20 or 30 millions of years these were the most common plants. They became extinct at the end of the Early Devonian. In the photo on the left my finest specimen of Cooksonia. It comes from South Wales and measures 3.5 cm. Click on the photos for enlargement. Click here for more information on Cooksonia.
Recently microscopic remains have been found in Oman indicating that land plants existed already 475 million years ago. Probably land plants have developed from green algae living in the sea.

Reconstruction of Cooksonia

Sawdonia ornata During the Early Devonian, from about 410 million years ago, more variation came into the flora. New groups of plants came into existence, but still the plants were small (up to 50 cm). Flowers developed only 250 millions of years later. Leaves didn't exist either, except for the scalelike leaves, like those of the clubmosses (click the photo on the right).
Several plants had spines (
click photo on the left). These were not for defending the plant, for animal life on the land was still minimal and the land animals were very small (millipedes, collembolans, trigonotarbids ('spiders'), very small crustaceans, etc.). Probably the function of the spines was to enlarge the green surface  and so to enhance the assimilation. Furthermore the spines could have given more hold to the plants in forming small bushes of plants of the same species, hanging more or less on each other.  
Thursophyton milleri

All plants of that period were cryptogams (plants with spores), like at present the mosses, the ferns, the clubmosses and the horsetails. But they are so different from the living plants, that in nearly all cases it is impossible to identify to which group of recent plants they belong. All those ancient plants have become extinct.
Relatively little is known about these primitive plants, because only a small number of complete and well preserved plants have been found. In most cases the plants have been transported by rivers before they ended up in the mud and could fossilize. Therefore the plants were mostly cut to small pieces before getting fossil. There are only a limited number of places where plants from the Silurian, the Lower and Middle Devonian have been well preserved.

Rhynie in earlier times A huge source of information on the oldest land plants is the so called Rhynie chert.
About 400 million years ago there was a kind of Yellowstone Park near the place where Rhynie is now situated (
click on the photo on the left). There were erupting vulcanos and geysers belching boiling water, saturated with silicium. That is how a complete silicified marsh came into existence with exceptionally well preserved plants, even showing their cell structure. Many animals have also been found in the chert. The photo on the right shows stems of the plant Rhynia (1 mm in diameter). Click here for a more extensive description of the Rhynie chert.
Transverse sections of stems of Rhynia

Archaeopteris

During the Middle Devonian the plants became taller and little trees arose, sized up to a couple of meters. Recently a trunk with attached crown has been discovered in the State of New York. This tree is called Eospermatopteris and it could be sized up to 8 m. It lived in the late Middle Devonian. Read more about this subject.
From the Late Devonian, Archaeopteris (don't confuse it with the ancient bird Archaeopterix)
is the most well-known tree. It was possibly a predecessor of the conifers and this tree could reach a height of 18 m. The drawing on the left is a reconstruction from the fine website Devonian Times.
Especially in Belgium quite a number of plants from the Upper Devonian are found. An example is Rhacophyton, occurring in the region of Liège (
click photo on the right), which could very well be a precursor of the ferns. It lacks still leaves, but it has small hooks at the end of the branches.

Rhacophyton condrusorum

Moresnetia At the end of the Devonian the first plants with seeds came into being. The oldest seed plant of Europe has been found in Belgium and it is has got its name after the village of Moresnet: Moresnetia (click photo on the left). This plant has primitive seeds: they are not yet entirely enveloped, like modern seeds. Recently a precursor of this plant has been discovered in Belgium with still more primitive 'seeds'. Click here for the original paper.
The seed plants have become more ande more important in the course of the hundreds of million years. It appeared to be a successful formula because nowadays by far most plants are seed plants. The free-sporing plants have been pushed into the background. Click here for a more extensive description of the early land plants.

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