Cooksonia encyclopedia topics

Morphology

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 an unpreserved rhizome. They had a simple stalk, that branched dichotomously a few times. Each branch ended in a sporangium, a rounded, spore-bearing structure. 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 varied in width from ~0.03 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.

Some Cooksonia species can be shown to bear sporangia, which had a role in gas exchange; this was probably to assist in transpiration-driven transport of solutes 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 nexk of the sporangium, which may have contained photosynthetic tissue, reminiscent of some mosses.

Physiology

While reconstructions traditionally depict Cooksonia as a green, photosynthesising, self-sufficient stem, it is likely that the fossils instead preserve a sporophyte generation which was parasitic on a gametophyte - a relationship employed in modern mosses. 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. Further, the axis thickness is what would be expected if their sole role was to support the sporangium on their ends. It appears that, originally at least, the axes' role was solely to ensure continued spore dispersal, even if the axis desiccated.

The widths of Cooksonia fossils span an order of magnitude; while the smaller ones could not possibly be self-sufficient, the larger axes could; this provides a possible illustration for the evolution of an independent sporophyte generation.

Taxonomy

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 a evolutionary grade or form genus.

Specimens

The first Cooksonia species were described by William Henry Lang in 1937 and named in honor of Isabel Cookson, with whom he had collaborated, and who collected the type specimens of Cooksonia pertoni in Perton Quarry in 1934.

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