Cooksonia
An extinct genus of early land plants, transitional between bryophytes and vascular plants.
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Cooksonia is an extinct group of primitive land plants, treated as a genus, although probably not monophyletic. The earliest Cooksonia date from the middle of the Silurian (the Wenlock epoch); the group continued to be an important component of the flora until the end of the Early Devonian. While Cooksonia fossils are distributed globally, most type specimens come from Britain, where they were first discovered in 1937. Cooksonia includes the oldest known plant to have a stem with vascular tissue and is thus a transitional form between the primitive non-vascular bryophytes and the vascular plants.
Description
Only the spore-producing sporophyte phase of Cooksonia is known; the gametophyte generation has not been found. These plants were small, reaching only a few centimetres in height, with a simple structure lacking leaves, flowers, and roots, though they may have grown from an unpreserved rhizome. Their stalks branched dichotomously a few times, each branch ending in a sporangium.
The sporangia were typically short, wide, and trumpet-shaped, with a lid-like operculum that disintegrated to release spores. Some species show a dark central stripe in the stalk, interpreted as the earliest known water-conducting tissue, while others lacked this feature. Stems varied in width from about 0.3 to 3 millimetres, and consistent size groupings suggest different species rather than fragments of larger organisms. Growth was determinate, ceasing after sporangia formed.
Physiology
Certain species bore stomata, concentrated near the sporangium neck, likely aiding transpiration-driven transport rather than photosynthesis. The genus includes several species, such as C. pertoni, C. hemisphaerica, C. cambrensis, C. bohemica, C. paranensis, C. banksii, and the more recently described C. barrandei, which differ in stem width, branching, and sporangium shape. Smaller specimens appear to have lacked room for photosynthetic tissue, suggesting the sporophyte may have been dependent on the gametophyte for nutrition, while larger, potentially self-sufficient axes may represent an evolutionary step toward an independent sporophyte generation.
Scientific classification
- CladeTracheophytes
- Stem group†Rhyniophytes
- Genus†Cooksonia
Named by Lang 1937 emend. Gonez & Gerrienne 2010 non Druce 1905
Sources: Wikipedia taxobox: Cooksonia (CC BY-SA 4.0)
Quick Facts
- Type species
- Cooksonia pertoni
Facts from the source article.
Lore & Background
Only the sporophyte phase of Cooksonia is currently known. Individuals were small, a few centimetres tall, and had a simple structure. They lacked leaves, flowers and roots—although it has been speculated that they grew from a rhizome that has not been preserved. They had a simple stalk that branched dichotomously a few times.
Each branch ended in a sporangium or spore-bearing capsule. In his original description, Lang described the sporangia as flattened, 'with terminal sporangia that are short and wide', and in Cooksonia pertoni 'considerably wider than high'. A 2010 review by Gonez and Gerrienne produced a tighter definition, which requires the sporangia to be more-or-less trumpet-shaped, with a 'lid' or operculum which disintegrates to release the spores. Specimens of one species have a dark stripe in the centre of their stalks, interpreted as the earliest remains of water-carrying tissue; other species lacked such conducting tissue.
Cooksonia specimens occur in a range of sizes, stem width from about 0.3 mm to 3 mm. Some species bore stomata, concentrated at the tips of the axes, typically associated with a bulging at the neck of the sporangium, which may have contained photosynthetic tissue. As circumscribed by Gonez and Gerrienne, there are six possible species: C. pertoni, C. paranensis, C. banksii, C. bohemica, C. hemisphaerica, and C. cambrensis. C. barrandei was described in 2018.
Reader's Guide
While reconstructions traditionally depict Cooksonia as photosynthesising and self-sufficient, it is likely that at least some fossils are of a sporophyte generation that was dependent on a gametophyte for nutrition. No fossil evidence of a gametophyte has been discovered. Study of smaller fossils showed no room for photosynthetic tissue, and the sporophyte may therefore have been dependent on the gametophyte. The potentially self-sufficient larger axes may represent the evolution of an independent sporophyte generation.
In 2018, the sporophyte of Cooksonia barrandei was described, from about 432 million years ago; it is the oldest-known megafossil of land plants. It was sufficiently robust to pass Boyce's test for possible self-sufficiency. Together with evidence that hornwort sporophytes have a degree of nutritional independence, C. barrandei suggests that independent gametophyte and sporophyte generations could have been ancestral in land plants.
Did You Know?
- The genus was named in honor of Isabel Cookson, who collected specimens of Cooksonia pertoni in 1934.
- The genus is probably not monophyletic, and several species have been moved to other genera.
More in Plant Families & Taxonomy
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Cooksonia (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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