Paleobotany in the Post-Genomics Era
Taylor, Edith L. , Taylor, Thomas N. .
Mosaic evolution: a view from the fossil record of Paleozoic and Mesozoic seed plants.
IN spite of recent advances in our understanding of many gymnosperm groups, especially the availability of molecular data, we can never achieve full understanding of seed plant phylogeny without the paleobotanical record. Because most seed plant diversity is extinct, characters and morphologies that could link modern clades are only found in fossils. We focus on anatomically preserved pteridosperms from Antarctica, including several clades that have been suggested as possible sister groups to the angiosperms. Preserved as permineralizations, these fossils provide data for accurate three-dimensional reconstructions of morphologies that are difficult to interpret in compressed fossils. While some fossil seed plants have characters almost identical to related modern forms, the pteridosperms exhibit mosaics of characters not present in any single extant group. Fossils also provide information on the timing of the widespread appearance of phylogenetically important characters, such as ovule enclosure. Glossopteridales (Permian) exhibit a number of phylogenetically interesting characters, including leaves with reticulate venation, laminar sporophylls partially enrolled to cover the ovules, embryo with suspensor, and unlike other Paleozoic pteridosperms, pycnoxylic wood. They also produced large, multiflagellate sperm, a primitive character among extant seed plants. Petriellales (Triassic) bore ovules enclosed by a laminar sporophyll. If found compressed, Petriellaea would have been described as a fossil angiosperm; permineralization proves that the sporophyll enrolled from tip to base, not conduplicately. Corystospermales (Triassic) presents an even more disparate mosaic of characters. Each ovule is enclosed by a laminar cupule, open only at the micropylar end of the seed. Microsporangiate structures are primitive and similar to Paleozoic seed ferns. Vegetative characters include seasonally deciduous, fern-like fronds, pycnoxylic wood, and short shoots that bear only ovulate organs. These apparent disparities exist throughout the fossil record of gymnosperms, but are especially prevalent in the Mesozoic, a crucial time in the evolution of modern clades.
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1 - University of Kansas, Department of Ecology & Evolutionary Biology, Natural History Museum and Biodiversity Research Center, 1200 Sunnyside Avenue, Lawrence, Kansas, 66045-7534, USA
seed plant phylogeny
Presentation Type: Symposium or Colloquium Presentation
Date: Wednesday, August 2nd, 2006
Time: 11:15 AM