BSC 314 Lecture Notes - Lecture 60: Pteridospermatophyta, Progymnosperm, Vascular Cambium
28 Jun 2018
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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 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
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Document Summary
The fossil record of ancient gymnosperms is surprisingly complete and provides good data from which to construct phylogenies. 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 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.
