Cooksonia

From Freepedia

Cooksonia are an extinct genus of primative land plants. They may represent the earlist vascular plants. The earlist cooksonia date from the middle of the Silurian period, about 425 million years ago. They have been found in an area stretching from Siberia to the Eastern USA, and in Brazil. They are found mostly in the area of Euramerica, and most of the type specimens are from Britain.

Cooksonia were small, a few centimetres tall, and had a simple structure: They didn't have leaves or flowers. They had a simple stalk, that branched a few times. Each branch ended in a sporangium, a rounded structure that contained the spores. No specimen has been found attached to roots. Either it connected to the ground with very fine root hairs, or the fossils are of fragments. Some specimens have a dark stripe in the centre of their stalks which is interpreted as being the remains of water carrying tissue. Not all specimens have this stripe, either some Cooksonia lacked vasular tissue, or it was destroyed in the fossilization process.

The relationships between the known species of Cooksonia and modern plants remain unclear. They appear to represent plants that are near to the branching between Rhyniophyta and to the club mosses. It is considered likely that Cooksonia is not a clade but rather represents an evolutionary grade.

Five species of Cooksonia have been clearly identified. C. pertoni, C. hemisphaerica, C. cambrensis, C. caledonica and C. paranensis. They are distiguished primarily by the shape of the sporangia.

The Cooksonia were discovered by W.H. Lang in 1937 and named in honour of Isabel Cookson, with whom he had collaborated.

References

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