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The term Pteridospermophyta (also called seed ferns or pteridosperms) is used as a collective name for several groups of extinct gymnosperms on with leaves resembling those of ferns. It is a somewhat misleading name, because the plants from these groups have little to do with ferns. Moreover Pteridosperms are not a monophyletic taxonomic unit. It is a fairly informal group, which is used to indicate seed plants which do not belong to the flowering plants, conifers, cordaites , ginkgos, and Cycas- like species. Some of the oldest seed plants belong to the pteridosperms. During the Carboniferous and Permian, seed ferns were an important component of the flora. During the Mesozoic, however, their numbers declined and by the end of the Cretaceous most pteridosperms were extinct. Some exceptions like Komlopteris cenozoicus from the Eocene of Tasmania, could survive into the Paleogene. Below we discuss briefly some orders of seed plants that are usually classified under the Pteridospermophyta.
The Lyginopteridales are among the oldest pteridosperms. This order consists of two families, the Elkinsiaceae and Lyginopteridaceae. Elkinsia from the Devonian of North America 's is the most famous Elkinsiaceae. It was a plant of less than a meter tall , with spirally arranged, compound leaves with basal dichotomy. Elkinsia and similar plants play an important role in research into how the use of seeds have developed in plants. During the Carboniferous the Elkinsiaceae were gradually reoplaced by a second Lyginopteridales family, Lyginopteridaceae. Compared to other plants of the Carboniferous the Lyginopteridaceae were relatively small. Often these are climbing, or vine-like plants. Mariopteris, Eusphenopteris and Lyginopteris belong to this family.
The Medullosales are among the largest pteridospermen. This order emerged at the end of the Lower Carboniferous, and became extinct probably early in the Lower Permian period. The stems and stalks of Medullosales have a special structure of far apart vascular bundles and parenchyma. This structure is what distinguishes Medullosales from other pteridosperms. The structure gave the plant sufficient strength without the need of much secondary wood. The Medullosales formed a major component of the vegetation in the Upper Carboniferous. Their fern-like leaves, such as Neuropteris, Alethopteris, Lonchopteris and Odontopteris, are frequently found in rocks of this age. Some Medullosales were probably big plants, because complete Alethopteris leaves can grow seven meters tall.
The Umkomasiales form a relatively small, but still quite extensively studied group of Mesozoic pteridosperms. The name is derived from Umkomasis, the female reproductive organ of those plants. The (male) pollen organs are known by names like Pteruchus and Pteroma. The Umkomasiales usually had leaves of the Dicroidium type. They formed an important component of Gondwana flora during the Mesozoic. The oldest known Umkomasiales come from the Permian of the Middle East. During the Triassic, the group developed rapidly and there was a great variety of shapes.
The Glossopteridales had no fern-like leaves, but compound leaves, with smooth edges. However, some researchers classify these plants with the pteridosperms. In addition to the groups described above, the Caytoniales, Leptostrobales, Matatiellales, Petriellales, Hamshwviales and Callistophytales are often classified among the pteridosperms.
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