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Lepidodendron sp. (bark, top of photo; root structure (bottom of photo)
During the early Carboniferous Period the Laurasian continents straddled the tropical climate zone and a distinctive Giant Clubmoss Flora flourished within Laurasia’s extensive swamps. The vegetation in these swamps later became the vast coal deposits that characterise the Carboniferous of Europe and North America. In contrast to Laurasia, the Gondwanan continents were much cooler and the Giant Club Moss Flora was not as abundant or as tall as that in Laurasia and there was little accumulation of peat to later form coal. The Giant Clubmoss Flora is characterised by the clubmoss Lepidodendron, horsetails such as Calamites, seed ferns and true ferns. Many species of this flora existed as tree-like forms with some attaining heights in excess of 45m (White, 1986). As Pangea broke up the Gondwanan continents continued to move further south with their climate becoming increasingly drier and cooler. The Gondwana Giant Clubmoss Flora became less abundant in these conditions and it eventually became extinct as cooling progressed with the eventual onset of the mid-Carboniferous glaciation. The bark of Lepidodendron shows a distinctive rhombic shaped pattern that is visible in the image. The regularly spaced circular impressions also visible in the image are indicative of a section of the root buttress of the Lepidodendron tree. -0.1
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-485 Annularia stellate
Annularia is a form genus that refers to the leaves of the horsetail genus Calamites that lived during the Carboniferous and Permian Periods. Horsetails from the Triassic Period are referred to as Neocalamites. Calamites had jointed stems with well-defined nodes and inter-node areas. Leaves (Annularia) and branches originated at the nodes and formed needle - like whorls that are seen as the flattened circular structures in the image. The horsetails formed part of the Giant Clubmoss Flora and in Laurasia reached heights of up to 20 metres. Like other members of the Giant Clubmoss Flora, Calamites thrived in swamp environments and contributed to the formation of thick peat deposits which later formed the extensive Carboniferous coal deposits of Europe and North America. Even though Calamites was tree-like in height, the woody stem was hollow and allowed sediment to accumulate inside it when the stem had fallen over. The internal casts of stems are therefore common as fossils in sediment deposited in deltaic and fluvio-lacustrine environments. Neocalamites forms a locally abundant component of Tasmania’s Triassic coal measure sequence where it is often found in mudstone sequences immediately below coal seams. -0.1
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Alethopteris sp.
During the mid to late Carboniferous Period the climate began to cool over Gondwana as the continents drifted into the high southern latitudes. Significant glaciation occurred over most of the Gondwanan continents which led to the extinction of the Giant Clubmoss Flora that had dominated the vegetation of the early Carboniferous. A group of plants that increased in abundance at this time were the pteridosperms or seed-ferns and they became a prominent component of the flora of the late Carboniferous through to the end of the Cretaceous. Although superficially they resemble ferns with trunks and fronds, the seed-ferns reproduced by seeds that developed from male and female reproductive structures located on the fronds of the plants. True ferns (order Filicales) reproduce by spores. Some of the seed-fern species reached heights over 20 metres but many were of a size comparable to modern ferns and occupied similar environments. The Alethopteris image is an example of part of the frond of a mid- late Carboniferous seed-fern with a distinctive mid-vein extending to the apex of the pinnule. -0.1
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Glossopteris sp.
Glossopteris usually refers to the leaf of a gymnosperm that formed an important component of the flora during the Permian Period in the southern or Gondwana part of the supercontinent, Pangea. Its occurrence within the Permian coal-bearing sequences of India, Australia, South Africa, Antarctica and South America assisted in the understanding that during the Permian these continents were joined together. As the cold climate of the late Carboniferous – early Permian ice age ameliorated, there was a corresponding increase in abundance and diversity of vegetation types. Mosses, ferns and seed-ferns, together with shrubs and trees of Glossopterids were adapted to the waterlogged conditions of the Permian swamps that would later became the coal deposits of the Gondwanan continents. The tongue-shaped leaves of Glossopteris have a net-like pattern of veins and were attached to trees that attained heights of up to 30 metres. Thick accumulations of Glossopteris leaves suggest Glossopteris was most likely deciduous in order to survive the still cold conditions of the early Permian. The woody tissue of Glossopteris trees is similar to that of the modern gymnosperms such as Araucaria, the Norfolk Pine (White, 1986). -0.1
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Dicroidium odontoperoides
The Permian Glossopterid flora was generally well adapted to cool and moist conditions but became extinct by the start of the Triassic. In contrast, a major climate change at the Permian-Triassic boundary, led to the Triassic Period being characterised by a warm to hot climate that had marked seasonality in rainfall. Plant communities, especially in the Gondwana continents, were characterised by the Dicroidium Flora which was dominated by, Dicroidium a forked-frond seed fern that preferred warm, dry conditions. Fossil leaves later ascribed to Dicroidium were first described by Morris (1845) from specimens collected from Tasmania. Apart from some of the early species, the leaves of Dicroidium only fork once but they do show a diverse range in the form of the pinnules between the many species. Taxonomic classification of Dicroidium is further hindered by intermediate forms which show a gradation between some species. It’s ability to adapt to a warming and drier environment is demonstrated by some species having thickened cuticles and reduced leaf size. In arid environments species have spine-like or needle like leaves (White, 1986). Reproductive structures in Dicroidium are found on specialised fronds with seeds being contained within lobed heads at the end of the frond. Spores developed in structures found on separate fertile fronds. Dicroidium species grew as fern- like forms or as small bushes along with conifers, ginkgos, ferns, horsetails and cycads depending on the moisture/shade requirements of the different species. Although Tasmania was located at around 800 south latitude in the Triassic, peat swamps of limited areal and temporal extent led to the later formation of coal seams. Bacon, 1991 suggests the peat swamps developed in a cold temperate, dry forest moorland environment.Dicroidium sp. form a locally abundant component of Tasmania’s Triassic coal measure sequence where it is often found in mudstone sequences immediately below coal seams. -0.1
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Cladophlebis sp.
Other components of the Triassic Dicroidium flora were the true ferns from the order Filicales. These ferns include the mid Permian to late Cretaceous genus Cladophlebis. Like modern ferns, Cladophlebis reproduced by spores on the underside of the fronds and would have occupied moist, cool environments similar to those occupied by extant fern species. In the Cladophlebis image, distinctive single forked veins branch off from a central vein along each pinnule. Cladophlebis sp. form a locally abundant component of Tasmania’s Triassic coal measure sequence where it is often found in mudstone sequences immediately below coal seam. -0.1
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Agathis jurassica
The Jurassic was a period in which the Earth’s climate was warm and wet across all latitudes. A relatively uniform vegetation type extended across the world and was dominated by conifers, cycads and ferns. By the mid-Jurassic, faulting and rifting in the Australian region led to the eventual break-up of the Gondwana continents. The formation of rivers and lakes, particularly in developing grabens, was conducive to the growth of extensive forests in these fluvio-lacustrine environments. A mid-Jurassic lake near present day Gulgong, NSW was surrounded by a forest of Kauri Pine (Agathis) with an understorey of other conifers, cycads and ferns. An abundant fish population lived in the lake. Fossils of the fish and the surrounding vegetation are preserved in exceptional detail (Talbragar Fish Beds). Leaves of Agathis jurassica occur in abundance and also demonstrate the close similarity of this species with modern Agathis (Kauri Pine) in the rainforests of Queensland’s Atherton Tableland. -0.1
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Metasequoia occidentalis (top) Grewiopsis alaskana (large leaf at bottom)
During the Cretaceous Period angiosperms or the flowering plants began to evolve amongst the dominant gymnosperm floras. Following the mass extinction event at the end of the Cretaceous angiosperms diversified rapidly and became the dominant flora in many parts of the world. During the Paleocene-Eocene Thermal Maximum global temperatures were as high as 50C above present and sea temperatures, comparable to sub-tropical waters, extended into the high latitudes. A consequence of this was the migration of forests of angiosperms and gymnosperms into polar latitudes. Metasequoia (dawn redwood) was a common conifer within the Paleogene high latitude forests of the Northern Hemisphere. It was deciduous and grew to heights in excess of 40 metres – large petrified trunks and stumps are known from North Dakota in the USA. The image shows the needle-like leaves arranged opposite to each other. The image also shows a typical broadleaf angiosperm leaf, Grewiopsis indicating a mixed floral community of both conifers and angiosperms at the locality where the specimen was found. REFERENCES AND SUGGESTED FURTHER READING
Anderson, J.M. and Anderson, H.M. 1983. Paleoflora of southern Africa. Molteno Formation (Triassic). Vol. 1. Part 1. Introduction. Part2. Dicroidium. Balkema Publ., Rotterdam.
Anderson, H.M; Holmes, W.B. Keith, and Fitness, L.A. 2008. Stems with attached Dicroidium leaves from the Ipswich Coal Measures, Queensland, Australia. Memoirs of the Queensland Museum 52(2), 1-12.
Bacon, C.A. 1991. The Coal Resources of Tasmania. Bulletin of the Geological Survey of Tasmania, 64.
Burrett, C.F. and Martin, E.L.(eds). 1989. Chapter 8 Late Carboniferous-Triassic in Geology and Mineral Resources of Tasmania. Geological Society of Australia Special Publication 15.
Corbett, K.D., Quilty, P.G. and Calver, C.R. (eds). 2014. Chapter 7 The Parmeener Supergroup – Late Carboniferous to Triassic in Geological Evolution of Tasmania. Geological Society of Australia Special Publication 24, Geological Society of Australia (Tasmania Division).
Holmes, W.B.K. 1982. The Middle Triassic flora from Benolong, near Dubbo, central-western New South Wales. Alcheringa 6 (1), 1-12.
Morris, J. 1845 in Strzelecki, P.E. (1845). Physical descriptions of New South Wales and van Diemens Land. Brown, Green and Longmans, London. pp. 422 pp.
Retallack, G. 1977. Reconstructing Triassic vegetation of Eastern Australia: a new approach for the biostratigraphy of Gondwanaland. Alcheringa 1, 247-278.
Tidwell, W.D. 1998. Common Fossil Plants of Western North America (second edition). Smithsonian Institution Press, Washington and London.
White, M.E. 1986. The Greening of Gondwana. Reed Books, Frenchs Forest, NSW WEBSITES THAT MAY BE OF INTEREST
https://uwaterloo.ca/earth-sciences-museum/resources/calamite-fossils
http://www.sciencedirect.com/science/article/pii/S0034666710000400 http://www.academia.edu/1275722/Glossopteris_- _insights_into_the_architecture_and_relationships_of_an_iconic_Permian_Gondwanan_plant
http://austhrutime.com/dicroidium_flora.htm