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Cooksonia

- Cooksonia was one of the earliest land plants, dating from the mid-Silurian to late Devonian periods. It had a simple green stem that branched dichotomously and bore terminal sporangia, representing a transition between non-vascular bryophytes and vascular plants. - Cooksonia likely had both a photosynthesizing sporophyte generation and a dependent gametophyte generation, though only fossils of the sporophyte have been found. Species of Cooksonia varied in stem width and sporangia shape. - Taxonomic reviews have refined the definition of Cooksonia and excluded some doubtful species, recognizing six possible valid species based on differences in spor

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Diversity of Vascular plants Haider Ali Malik COOKSONIA University of Sargodha
Cooksonia • Cooksonia is extinct grouping of primitive land plants. • First fossils discovered in 1937 • They date back to mid of Silurian till end of Devonian. • Stem with vascular tissues is present, thus are transition between primitive non-vascular bryophytes and vascular plants.
Cooksonia, Zosterophyllum, Aglaophyton [Ordovician? to Silurian colonizers] Psilophyton Drepanophycus Protolepidodendron On the right are later (middle Devonian) plants: one trimero- phyte (rear) and two Lyco- phytes.
Physiology • Cooksonia is green and red photosynthesizing self sufficient stem. • It is likely that some fossils preserve a sporophyte generation which was dependent on gamatophyte,. • However, no fossil record of gamatophyte has been discovered to date. • The width of Cooksonia fossils span an order of magnitude. • In 2018, the sporophyte of a new specie C. barrandae was described from 432 MYA
Taxonomy • Cooksonia species were described by William Henry Lang in 1937 and named in honour of Isabel Cookson. • The genus was defined by narrow leafless stem which branched dichotomously with terminal sporangia that were short and wide. • A central vascular cylinder consisting of annular tracheids were present.
• Six other species were added to genus: C. Crassiparietilies, C. Caledonica, C. Cambriansis, C. bohemica, C. Parenesis, C. banksii, C. pertoni, C. hemispherica. • A review in 2010 concluded that the delineation of genus was inaccurate and some species are needed to be removed in particular to those in which sporangia is not more or less trumpet shaped. • Five further species doubtful: C. acuminate, C. bohemica, C. cambransis, C. downtonesis, and C. rusanovii • Four species are excluded; C. hemispherica, C. banksii, C. caledonica, C. barrandei
Phylogeny • For some years, it was suspected that Cooksonia and its species were poorly characterized. • Thus four different kinds of spores , probably representing four different species were found in sporangia. • Originally identified as C. Pertoni
Other descriptions • Only sporophyte phase is currently known. • Individuals were small, a few cm tall and simple in structure. • They lacked leaves, flowers and roots. Although it has been speculated to form from rhizome • They had a simple stalk that branched dichotomously a few times, ending in sporangia. • Lang described sporangia as flattened with terminal sporangia that are short and wide.
• A 2010 view of genus by Gonez and Gerrienne produce a tighter definition: “which require to be sporangia more or less trumpet shaped with a lid or operculum which disintegrates to release the spores”. • Specimens of one species have dark stripe in the centre of their stalks which has been interpreted as the earliest remains of water carrying tissues, while others lack this habit. • Its specimens occur in range of sizes and vary in stem in width from 0.03 to 0.3mm • Specimens of different sizes were different species, not fragments of larger organism.
• The organisms probably exhibited determined growth. Stem did not grow further after producing sporangia. • Some species bore stomata which had role in gaseous exchange. • This clustering of stomata is reminiscent of some mosses. • According to Gonez and Gerriene, there are six possible species. C. pertonii, C. paranesis, C. banksi are all relatively similar with flat topped, trumpet- shaped. • Sporangia stems are narrow in C. Paransis and C. Pertoni.
• C. bohemica has stouter more branched stem. The original shape of sporangia is unclear because of poor preservation. • C. hemispherica described from same locality as C. pertoni but differs in having hemispherical tops rather than flat. • C. cambriansis also has spherical sporangia but without gradual widening at base characters of other species. Preservation is again poor • C. barrandei was described bin 2018.
Cooksonia