Kingdom : Plantae Subkingdom : Embryophyta Clade : Polysporangiophyta Division : Tracheophyta Genus : Cooksonia Species : C. pertoni, C. hemisphaerica, C. cambrensis, C. bohemica, C. paranensis, C. banksii
Only the sporophyte phase of Cooksonia is currently known (i.e. the phase which produces spores rather than gametes). 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 an unpreserved rhizome. 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 of the genus, Lang described the sporangia as flattened, “with terminal sporangia that are short and wide”, and in the species Cooksonia pertoni “considerably wider than high”. A 2010 review of the genus by Gonez and Gerrienne produced a tighter definition, which requires the sporangia to be more-or-less trumpet-shaped (as in the illustration), with a ‘lid’ or operculum which disintegrates to release the spores.
Specimens of one species of Cooksonia have a dark stripe in the centre of their stalks, which has been interpreted as the earliest remains of water carrying tissue. Other Cooksonia species lacked such conducting tissue.
Cooksonia specimens occur in a range of sizes, and vary in stem width from about 0.03 mm to 3 mm. Specimens of different sizes were probably different species, not fragments of larger organisms: fossils occur in consistent size groupings, and sporangia and spore details are different in organisms of different sizes. The organisms probably exhibited determinate growth (i.e. stems did not grow further after producing sporangia).
Some Cooksonia species bore stomata, which had a role in gas exchange; this was probably to assist in transpiration-driven transport of dissolved materials in the xylem, rather than primarily in photosynthesis, as suggested by their concentration at the tips of the axes. These clusterings of stomata are typically associated with a bulging in the axis at the neck of the sporangium, which may have contained photosynthetic tissue, reminiscent of some mosses.
As the genus is circumscribed by Gonez and Gerrienne, there are six possible species. C. pertoni,C. paranensis and C. banksii are all relatively similar with flat-topped, trumpet-shaped sporangia; stems are somewhat narrower in C. paranensis than in C. pertoni. Only one specimen of C. bohemica is known. It has stouter, more branched stems; the original shape of the sporangia is unclear because of poor preservation. C. hemisphaerica, described from the same locality as C. pertoni, differs in having sporangia of which the tops, at least as preserved, are hemispherical rather than flat. C. cambrensis also has spherical sporangia, but without the gradual widening at the base characteristic of the other species. Preservation of the sporangia is again poor.
The geyser hisses, its mists pattering into the warm pools. When it stops, the world is silent. The oceans of the Silurian churn with threshing ammonites, writhing crinoids, and scuttling trilobites. The seas are violent; if they weren’t, there would be no shells. But here, around these shallow ponds, life proceeds through stillness. Competition moves slowly, and in absolute silence. Nothing sings or buzzes or speaks. Life is noiseless—hymns are physical, not aural—reaching, branching, longing for light. This is the dynasty of plants when nature has no voices.
For plants, the journey onto land did not happen over night. It began some 485.4–443.4 million years ago during the Ordovician. The best evidence we have for this comes in the form of fossilized spores. These spores resemble those of modern day liverworts. Under high powered microscopes, one can easily see that they were indeed adapted for life on land. These early plants were a lot like the liverworts and mosses we see today in having no vascular tissues for transporting water, an adaptation that would not come along for a few million years.
Without vascular tissues, plants like liverworts and mosses cannot transport water very far. They instead rely on adhesion between water molecules to get it to where it needs to be. This severely limits the size of these types of plants to only a few centimeters. This growth pattern carried on well into the Silurian. Until then, the greening of our planet happened in miniature.
Around 415 million years ago, however, plants became vascularized. This changed everything. It set the stage for the botanical world we know and love today. Paleobotanists place the fossil remains of these newly evolved vascular plants in the genus Cooksonia. Based on what we would call a plant today, Cooksonia probably pushes the limits. However, in some species the branching structure is full of dark stripes, which have been interpreted as vascular tissues. It still wasn’t a very tall plant with the tallest specimen standing only a few centimeters but it was a major step towards a much taller green world.
Cooksonia did not have any leaves that we are aware of but some species certainly had stomata (another major innovation for water regulation in plants). Each branched tip ended in a sporangium or spore-bearing capsule. It has been suggested that Cooksonia may not represent an individual, photosynthetic plant but rather a highly adapted sporophyte that may have relied on a gametophyte for photosynthesis. This hypothesis is supported by the diminutive size of many Cooksonia fossils. They simply do not have enough room within their tissues to support photosynthetic machinery. Because of this, some botanists believe that vascularization sprang from a dependent sporophyte that gradually became more and more independent from its gametophyte over time. Until an associated gametophyte fossil is found, we simply don’t know.
Anybody up for a little plant history lesson? Meet Cooksonia!
Cooksonia is thought to be one of the first groups of land plants and was around between about 428-398 million years ago, from the middle of the Silurian Period to the beginning of the Early Devonian Period. These little guys were only a few centimeters tall!