Glossopteris
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| Glossopteris Fossil range: Permian |
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 Fossil of the gymnosperm Glossopteris (number 8, blue-green) found in all of the southern continents show how they were once joined
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Glossopteris (Greek glossa, meaning "tongue", because the leaves were tongue-shaped) is the largest and best-known genus of the extinct order of seed ferns known as Glossopteridales (or in some cases as Arberiales or Dictyopteridiales). Long considered a fern after its discovery in 1824, it was later assigned to the gymnosperms. The genus is placed in the division Pteridospermatophyta. In reality, many of the plant groups included within this division are only distantly related to one another.
Glossopteris should strictly be used to refer to the distinctive spathulate fossil leaves with reticulate venation, however, the term has also been used to refer to the parent plant as a whole. Glossopteris was a woody, seed-bearing shrub or tree, some apparently reaching 30 m tall. They had a softwood interior that resembles conifers of the family Araucariaceae. Seeds and pollen-containing organs were borne in clusters at the tips of slender stalks partially fused (adnate) to the leaves. The homologies of the flattened seed-bearing structures have remained particularly controversial with some arguing that the fertile organs represent megasporophylls (fertile leaves) whereas others have interpreted the structures as flattened, seed-bearing, axillary axes (cladodes).
They are interpreted to have grown in very wet soil conditions, similar to the modern Bald Cypress. The leaves ranged from about 2 cm to over 30 cm in length.
The Glossopteridales arose around the beginning of the Permian on the great southern continent of Gondwana. These plants went on to become the dominant elements of the southern flora through the rest of the Permian but disappeared in almost all places at the end of the Permian. The only convincing Triassic records are very earliest Triassic leaves from Nidpur in India, but even these records are somewhat questionable owing to faulting and complex juxtapositioning of Permian and Triassic strata at Nidpur. Although most modern palaeobotany textbooks cite the continuation of glossopterids into later parts of the Triassic and, in some cases into the Jurassic, these ranges are erroneous and are based on misidentification of morphologically similar leaves such as Gontriglossa, Sagenopteris, or Mexiglossa. Glossopterids were, therefore, one of the major casualties of the Permian-Triassic mass-extinction event.
The profile of glossopterid trees is largely speculative as complete trees have not been preserved. However, based on analogies with modern high-latitude plants Glossopteris trees probably tapered upwards like a Christmas tree. Instead of needles, they had large, broad lance- or tongue-shaped leaves that fell to the ground at the end of summer. The fossilized tree rings in the Glossopteris trees reveal that they grew steadily each summer and abruptly stopped for winter.
More than 70 fossil species of this genus have been recognized in India alone, with additional species from South America, Australia, Africa, and Antarctica. Apart from those in India, only a few fossils from the northern hemisphere have been assigned to this group, but these are not identified with great certainty. For example, specimens assigned to Glossopteris from the far east of Russia in the 1960s are more likely to be misdentifications of other gymnosperms such as Pursongia. Confident assignment of fossil leaves to Glossopteris normally requires their co-preservation with the distinctive segmented roots of this group (called Vertebraria) or with the distinctive fertile organs.
Glossopteris leaves are morphologically simple so there are few characters that can be used to differentiate species. Consequently, many past researchers have considered the Permian Glossopteris flora to be rather homogeneous with the same species distributed throughout the Southern Hemisphere. However, more recent studies of the more morphologically diverse fertile organs has shown that taxa had limited regional distributions and several intra-gondwanan floristic provinces are recognizable. Nevertheless, several species of leaves found in Antarctica are common in the rocks of similar geologic age in India, located north of the equator and half a world away. Seeds, much too large to be windborne, could not have blown across thousands of miles of open sea, nor could they have floated across vast oceans. Observations such as these led the Austrian geologist Eduard Suess to deduce that there had once been a land bridge between these areas. He named this large land mass Gondwanaland (named after the district in India where the plant Glossopteris was found). These same observations would also lend support to Alfred Wegener's Continental drift theory.
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[edit] References
- Brongniart, A. 1828. Prodrome d’une histoire des végétaux fossiles. Paris. 223 pp.
- Brongniart, A. 1832. Histoire des végétaux fossiles ou recherches botaniques et géologiques sur les végétaux renfermés dans les diverses couches du globe. G. Dufour and E. D’Ocagne, Paris 1: 265-288.
- Anderson, H.M. & Anderson, J.M. 1985. The Palaeoflora of Southern Africa: Prodromus of Southern African Megafloras, Devonian to Lower Cretaceous. A.A. Balkema, Rotterdam. 416 pp.
- Chandra, S. & Surange, K.R. 1979. Revision of the Indian species of Glossopteris. Monograph 2. Birbal Sahni Institute of Palaeobotany, Lucknow. 301 pp.
- Davis, Paul and Kenrick, Paul. 2004. Fossil Plants. Smithsonian Books (in association with the Natural History Museum of London), Washington, D.C. ISBN 1-58834-156-9
- Gould, R. E. and Delevoryas, T., 1977. The biology of Glossopteris: evidence from petrified seed-bearing and pollen-bearing organs. Alcheringa, 1: 387-399.
- Pant DD 1977 The plant of Glossopteris. J Indian Bot Soc 56: 1-23.
- Pant, D.D. & Gupta, K.L. 1971. Cuticular structure of some Indian Lower Gondwana species of Glossopteris Brongniart. Part 2. - Palaeontographica, 132B: 130-152.
- Pant, D.D. & Nautiyal, D.D. 1987. Diphyllopteris verticellata Srivastava, the probable seedling of Glossopteris from the Paleozoic of India. - Rev. Palaeobot. Palynol., 51: 31-36.
- Pant, D.D. & Pant, R. 1987. Some Glossopteris leaves from Indian Triassic beds. - Palaeontographica, 205B: 165-178.
- Pant, D.D. & Singh, K.B. 1971. Cuticular structure of some Indian Lower Gondwana species of Glossopteris Brongniart. Part 3. - Palaeontographica, 135B: 1-40.
- Pigg, K. B. 1990. Anatomically preserved Glossopteris foliage from the central Transantarctic Mountains. Rev. Palaeobot. Palynol. 66: 105-127.
- Pigg, K.B. & McLoughlin, S. 1997. Anatomically preserved Glossopteris leaves from the Bowen and Sydney basins, Australia. Review of Palaeobotany and Palynology, 97: 339-359.
- Plumstead, E.P. (1969), Three thousand million years of plant life in Africa. Alex L. du Toit Memorial Lecture no. 11. Trans. Geol. Soc. S. Afr. 72 (annex.): 1-72.
- Taylor, E.L, Taylor, T.N. & Collinson, J.W. 1989. Depositional setting and palaeobotany of Permian and Triassic permineralized peat from the central Transantarctic Mountains, Antarctica. - Internat. J. Coal Geol., 12: 657-679.
