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Cooksonia is an extinct group of primitive , treated as a genus, although probably not Monophyly. 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, a total time span of . 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 and the .
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 Tracheid. Other Cooksonia species lacked such conducting tissue.
Cooksonia specimens occur in a range of sizes, and vary in stem width from about 0.3 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 , 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,Two spellings are in use: the spelling used by the original author of the name, C. pertoni, and the spelling C. pertonii (e.g. in ). The second is appropriate in botanical Latin when an epithet relates to a person, according to Article 60.11 of the International Code of Nomenclature for algae, fungi, and plants. 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. C. barrandei was described in 2018.
The widths of Cooksonia fossils span an order of magnitude. Study of smaller Cooksonia fossils showed that once the tissue required to support the axes, protect them from desiccation, and transport water had been accounted for, no room remained for photosynthetic tissue, and the sporophyte may therefore have been dependent on the gametophyte. Further, the axis thickness is what would be expected if its sole role was to support a sporangium. It appears that, originally at least, the role of the axes in smaller species was solely to ensure continued spore dispersal, even if the axis desiccated. The potentially self-sufficient larger axes may represent the evolution of an independent sporophyte generation.
In 2018, the sporophyte of a new species, 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, unlike modern mosses and liverworts, hornwort sporophytes do have a degree of nutritional independence through photosynthesis, C. barrandei suggests that independent gametophyte and sporophyte generations could have been ancestral in land plants, rather than evolving later.
Seven further species are considered doubtful because of the poor preservation of the specimens, but are left in the genus:
Four species are excluded from the genus by Gonez and Gerrienne. Species that have been transferred or removed are:
C. caledonica and the less well-preserved C. crassiparietilis have sporangia which are composed of two 'valves', splitting to release their spores along a line opposite to where they are attached to the stem (i.e. distally).
A 2010 study of the genus produced the consensus cladogram shown below (some branches have been collapsed to reduce the size of the diagram). This was based on data from an earlier study (by Kenrick and Crane), supplemented by further information on Cooksonia species resulting from the authors' own research.
A more recent phylogeny by Hao and Xue from 2013:
This confirms that the genus Cooksonia sensu Lang (1937) is Polyphyly. A core group of five species are placed together, unresolved between the and the . The poorly preserved C. hemisphaerica is placed as the most basal tracheophyte. Two other species, C. crassiparietilis and C. caledonica, are placed in the stem group of the lycophytes. These two species have been removed from Cooksonia sensu Gonez & Gerrienne ( C. caledonica has since been placed in a new genus Aberlemnia). Both have sporangia which, although borne terminally rather than laterally, have a mechanism for releasing spores similar to those of the .
A second cladistics analysis was carried out using only the three best preserved and thus best known species, C. pertoni, C. paranensis, and C. caledonica. The position of C. caledonica was confirmed, but C. pertoni and C. paranensis now formed a single clade more clearly related to the lycophytes than the euphyllophytes.
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