Extinct Gymnosperm Phyla

Extinct Gymnosperm Phyla - characteristic of the seed...

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Extinct Gymnosperm Phyla The fossil record of ancient gymnosperms is surprisingly complete and provides good data from  which to construct phylogenies. Phylum Progymnospermophyta Many botanists believe the progymnosperms are the most likely ancestral group from which the seed  plants evolved. They have many features of the seed plants, but are still spore producers and are not  themselves seed plants. They were important members of the vegetation from Middle Devonian  through the Lower Carboniferous. Many were large trees with fern-like leaves and probably formed  forests in these early landscapes. One feature of evolutionary significance that advances and separates the group from the ferns and  trimerophytes is the bifacial cambium present in the progymnosperms; the group had a vascular  cambium that produced secondary xylem and secondary phloem. A bifacial cambiumis is a 
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Unformatted text preview: characteristic of the seed plants and it appears for the first time in this group. The fossil progymnosperm wood resembles that of more modern conifers with tracheids and bordered pits. Phylum Pteridospermophyta Another Late Devonian group is the seed ferns, with leaves so like ferns that if no seeds are attached the fossils often are cataloged as ferns. This is an unnatural mixed group (like the protista) with no taxonomic ranking and not enough specimens to determine phylogenies well. One hypothesis derives the seed ferns from the progymnosperms in a lineage with no modern descendants. Another places one of the seed fern lines, the Medullosans, in a clade with the cycads—with good arguments for and against the arrangement. The group remains a puzzle....
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