Gigantopterid

Gigantopterids (Gigantopteridales) is the name given to fossils of a group of plants existing in the Late Permian, some 260 to 250 million years ago. Gigantopterids were among the most advanced land plants of the Paleozoic and disappeared soon after the massive Permian–Triassic extinction event 251.4 million years ago. Though some lineages of these plants managed to persist initially, they either disappeared entirely or adapted radically, evolving into undetermined descendants, as surviving life prospered again in much-altered ecosystems. One hypothesis^[1] proposes that at least some "gigantopterids" became the ancestors of angiosperms and/or Bennettitales and/or Caytoniales.^[2]

Gigantopterid fossils were documented as early as 1883, but only investigated more thoroughly in the early 20th century. Some of their most significant evidence has been initially found in Texas, but they might have been present worldwide: another key region for gigantopterid fossils is in China, and the consolidation of all major continents into Pangea would have allowed for easy global dispersal. They were among the most striking and important plants of the Cathaysian flora of Sino-Malaya, also called Gigantopteris flora to reflect this.^[3]

Description

They bore many of the traits of flowering plants, but are not known to have flowered themselves. Gigantopterid plants had simple, bilaterally symmetrical leaf structures, woody stems and spines. They grew new parts by producing shoots, like flowering plants. Judging from the fossil remains, they were probably robust plants with fronds that resembled fern fronds when young. When mature they were more like flowering plant leaves with reticulate venation arranged in a frond. Gigantopteris nicotianaefolia for example is named thus because each of its leaflets resembles a tobacco leaf in shape.

They grew at least over 20 cm (around 10 in), probably over 50 cm (20 in) tall, depending on whether it grew as a scrambling vine (the initial assumption) or erect (nowadays considered more likely). Some apparently preferred wetlands, while others throve in rather arid habitat.^[4]

The vascular tissue contains vessel elements and in at least some taxa resembles that of the Gnetophyta. They had a cuticula similar to that of other seed plants. Some male sporangia (Gigantotheca) and seeds (Gigantonomia, Cornucarpus) have been tentatively referred to this group.^[5] While the sporangia are certainly not flowers, they differ from other sporangia of Permian plants.^[3]

Chemical analysis of fossil remains indicates that gigantopterids produced oleananes, chemical compounds otherwise known to be used by flowering plants, for which they function as a suppressant of insect pests. They are apparently also found in certain modern ferns however.^[2]

Systematics

It is probable that the gigantopterids are a non-monophyletic form taxon. Plant life from their era is very difficult to document and organize. They have been variously allied with the "seed ferns" (another paraphyletic group of early seed plants) Peltaspermales and Callistophytales, Gnetum, and the Magnoliophyta. All that can be said at present is that they were spermatophytes. It is also highly unlikely that they were close relatives of gymnosperms, cycads or ginkgos, because these lineages were already established and distinct in the Late Permian.^[4]

Vegetative leaves of Emplectopteris were at one time included in this group. However, they had ovules attached to the underside of the fronds and are now placed in their own family (Emplectopteridaceae) within the Callistophytales.

Some prefer to refer to the presumed "core" taxa of this group as Gigantonomiales.^[6] However, many experts remain skeptical of the new name, because they consider such a move premature until more is known about the interrelationships of the genera of "gigantopterids" and the relationships of these to other plants.^[3]

Several genera have been named. Those then known were assigned to a family Gigantopteridaceae and numerous subfamilies by Gen-ichi Koidzumi.^[7] It appears as if at least some of his divisions ought to prove useful when a more definite phylogenetic, systematic and taxonomic arrangement for this group is eventually found. On the other hand, the Gigantopteridaceae would with near certainty have to be more narrowly circumscribed even if the Gigantopteridales are by and large verified as a clade; they might even become limited to the two or so genera now placed in the Gigantopteridieae.^[8]

Proposed subfamilies and genera

Cathaysiopteridieae - may be synonym of Palaeogoniopteridieae

• Cathaysiopteris
• Cathaysiopteridium

Gigantopteridieae (including Cardioglossieae)

• Gigantonoclea - includes Cardioglossum and apparently Progigantopteris^[8]
• Gigantopteris

Gothanopteridieae - may be synonym of Cathaysiopteridieae/Palaeogoniopteridieae

• Gothanopteris

Palaeogoniopteridieae

• Gigantopteridium
• Palaeogoniopteris
• Zeilleropteris

Unresolved

• Delnortea
• Emplectopteridium
• Emplectopteris
• Neogigantopteridium - probably Cathaysiopteridieae/Palaeogoniopteridieae
• Vasovinea

See also

Other Permian-Triassic seed plants of unclear relationships are:

• Glossopteridales
• Lyginopteridopsida
• Pentoxyales

Footnotes

References

• Asama, K. (1982): Evolution and phylogeny of vascular plants based on the principles of growth retardation: Part 5. Origin of angiosperms inferred from the evolution of leaf forms. Bulletin of the National Science Museum, Tokyo, Series C (Geology) 8: 43-58.
• Glasspool, Ian; Hilton, Jason; Collinson, Margaret E. & Wang Shi-Jun (2004): Defining the gigantopterid concept: a reinvestigation of Gigantopteris (Megalopteris) nicotianaefolia Schenck and its taxonomic implications. Palaeontology 47(6): 1339-1361. doi:10.1111/j.0031-0239.2004.00425.x (HTML abstract)
• Koidzumi, Gen-ichi (1936): On the Gigantopteris flora. Acta Phytotaxonomica et Geobotanica 5: 130–144.
• Li Xingxue & Yao Zhao-Qi (1983): Fructifications of gigantopterids from South China. Palaeontographica Abteilung B 185: 11-26.
• Meyen, S.V. (1987): Fundamentals of palaeobotany. Chapman and Hall, London and New York.
• Miller, John M. (2007): Paleobotany of Angiosperm Origins. Version of 2007-DEC-10. Retrieved 2007-DEC-12.
• Wang Zi-Qiang (1999): Gigantonoclea: an enigmatic Permian plant from North China. Palaeontology 42(2): 329–373. doi:10.1111/1475-4983.00076 (HTML abstract)

External links

• Image from Li & Yao (1983), showing fossils and life reconstructions of Gigantonomia (denoted F) and Gigantotheca (denoted V). Retrieved 2007-DEC-12.
• Photo of Delnortea abbottiae leaflet, specimen USNM 387473. Retrieved 2007-DEC-12.
• Reconstruction of Gigantopteris leaves. Retrieved 2007-DEC-12.
• Wikiadugi: Version of this article in Cherokee language. Retrieved 2007-DEC-12.