Origin of Angiosperms
Background: The origin of Angiosperms is still as enigmatic as during the times of Darwin, who spoke of an "abominable mystery", when considering the rapid diversification of Angiosperms. Although the seed plants are usually considered as monophyletic (deriving from one common fern ancestor), there exist numerous transition forms of the fern plants that have gone extinct, and there exist very archaic gymnosperm groups such as Ginkgo, Welwitschia, the Gnetales, or the Cycadales that strongly differ from the dominating gymnosperm group, the Conifers. Two traits are in the center of debate - the habit of a tree, which requires secondary growth and the existence of a cambium, and heterospory, i.e. the formation of two genders in the gametophyte. Depending, how these traits are interpreted, one arrives at different scenarios. The fact that certain transitions are not represented in the fossile record, might mean that they did not exist, because the respective groups are not directly related. However, it might also simply mean that these missing links have not been found yet. Molecular phylogeny helped a bit, but also could not resolve many points, because the trees depend strongly on the definition of outgroups as illustrated by the Paleoherb - Woody Magnoliid debate. In the meanwhile, the monophyly of seed plants has been questioned. New models propose that the Monocots have derived from heterosporic club mosses (Isoetales) and later gave rise to the Dicots, whereas the gymnosperms have derived from other fern plants. A further blow comes from the discovery that the primordial ferns were already heterosporic, which means that the isosporic condition found in most ferns is not the ancient, but a derived situation. A lot of this controversial debate deals with the timing of specific forms - when did the first seed plants arise? when was the first Angiosperm arise?
Task: You should construct a time line of land plant evolution, where some of the key events or key fossiles are placed in. The result will be a Powerpoint slide with the time line of the last 500 Mya, and the geological stages in the center and at the left the positions of important plant, at the right the positions of important animal forms:
- plants: 1. first mosses, 2. first cormophytes, 3. first seed plants, 4. first Ginkgoales, 5. first conifers, 6. first Angiosperms, 7. first monocots, 8. first dicots
- animals: 1. first vertebrates, 2. first amphibia, 3. first arthropods, 4. first insects, 5. first Coleoptera, 6. first Hymenoptera, 7. first reptiles, 8. first birds, 9. first mammals
- a few introductory slides containing unpublished materials (protected by password) - pdf
- a detailed discussion of the Monocots-first scenario (pages of the Field Museum) - more
- a PNAS article about the sexuality of primordial fern - access through KIT license link