The earliest Cooksonia date from the middle of the Silurian (the Wenlock epoch);[1] the group continued to be an important component of the flora until the end of the Early Devonian, a total time span of 433 to 393 million years ago.

[4] 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.

[3] 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.

[2] 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.

[7] While reconstructions traditionally depict Cooksonia as a green and red, photosynthesising, self-sufficient stem, it is likely that at least some fossils are of a sporophyte generation that was dependent on a gametophyte for its nutrition – a relationship that occurs in modern mosses and liverworts.

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.

[4] The genus was defined as having narrow leafless stems (axes), which branched dichotomously, with terminal sporangia that were "short and wide".

A review in 2010 concluded that the delineation of the genus was inaccurate and that some species needed to be removed; in particular those in which sporangia were not more-or-less trumpet-shaped.

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.

This was based on data from an earlier study (by Kenrick and Crane[10]), supplemented by further information on Cooksonia species resulting from the authors' own research.

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.

[11] Boyce restricted the group to forms with axes usually less than 1 mm in diameter, and hence possibly not capable of independent growth.