Cycad Anatomy and Fossil Occurrences in Texas
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چکیده
ycadophyta presently consists of the living order Cycadales, which is comprised of three families worldwide. In the Mesozoic, cycads were represented by the Cycadales, plus another large order, Cycadeoidales (also referred to as Bennettitales). Cycadophyta is thought to have evolved from the Paleozoic seed ferns (pteridosperms), as did conifers, and eventually, angiosperms. However, Cycadales and Cycadeoidales likely arose from two distinct forms of pteridosperms. Their similarity is thought to be the result of convergent and/or parallel evolution (Norstag & Nicholls, 1997). Cycadeoid remains are much more prevalent in the fossil record, and thus it is thought they were a common constituent of the plant assemblage. (Conversely, Cycadales are relatively rare in the fossil record). Ultimately, Cycadeoids became extinct by the end of the Cretaceous. One theory postulates that they were dependent on specific insects to spread their pollen from plant to plant because the female ovule was completely encapsulated within the cone, and that for some reason they lost their insect-pollinators (Norstag & Nicholls, 1997). However, others feel that the Cycadales had evolved a slightly more advanced megaspore and ovule structure, thus allowing them to survive into the Tertiary whereas many other organisms, including dinosaurs, failed to adjust to the changes at the end of the Cretaceous (Tidwell, pers. comm.). C
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