Australia: The Land Where Time Began
The Rhacopteris Flora of Primitive Seed-ferns - Middle to Late Carboniferous and Early Permian - about 325-285 Ma
By the Middle Carboniferous, the combination of the onset of a glacial phase and movement of Australia from the tropics towards the South Pole, was strongly affecting the climate, and hence the vegetation of the continent. The Giant Clubmoss Flora was gone and during the Late Carboniferous the low diversity Rhacopteris Flora of primitive seed-ferns, had evolved that could survive the rigorous conditions that existed as an ice age took hold globally, and even in Australia a continental ice sheet covered about half of the continent by the glacial maximum at the end of the Carboniferous.
After the close of the Early Carboniferous Australia's break from Pangaea picked up speed as it headed for the South Pole. By the Late Carboniferous it was situated between 60o and 75o S and was rotated 90o. It remained in the polar regions for about 50 million years until the end of the Permian. As Australia was in the polar regions at the time of the glaciation, it also had glaciers that covered the southern half of the continent. The adjacent parts of Gondwana were also covered by glaciers.
It appears the glaciers began on the volcanic highlands along the eastern coast, and probably on the central highlands. By the end of the Carboniferous a continuous ice sheet is believed to have existed the spread from the South Pole north to cover the southern half of Australia. The ice sheets reached their maximum extent at the boundary between the Carboniferous and the Permian. Not all accept that there were continuous ice sheets. See Australian Glaciation. The marine transgressions that occurred in the Early Permian were the result of the melting of the ice sheet at the end of the glacial period and the climate began to warm. It would have taken some time after the disappearance of the glaciers for the crust to rise by isostatic rebound after the weight of the glaciers was removed.
By 285 Ma the continent had reached it present size, the eastern part of the continental crust having been added to the older core blocks by crustal movement and volcanic activity. Marine incursions occurred in the Late Carboniferous and Early Permian when much of Victoria and South Australia were covered by shallow seas. In Western Australia the Canning and Carnarvon Basins were also inundated at this time.
The flora of Australia was affected by the increasing cold as the ice age tightened its grip. At this time the Giant Clubmosses that had managed to survive from the Early Carboniferous finally died out, never having achieved the luxuriance of the Northern Hemisphere forms. After the disappearance of the Giant Clubmoss Flora there is a hiatus in the fossil record, not because there were no vegetation, but because of widespread volcanic activity in the south of the Sydney Basin which disturbed the deposition of sediments. Some petrified wood has been found, such as Pitus, wood from large gymnosperms. It is believed that early gymnosperms probably grew during these times of strong seasonality and alpine glaciation, as their relatives still do at present in sub-Arctic regions.
The best developed Rhacopteris Flora that is known comes from New South Wales. As a result of the extreme climatic conditions in Australia at the time, the Rhacopteris Flora is impoverished when compared to the flora of the Early Carboniferous of the Northern Hemisphere. The plants of this flora had, isolated from their Northern Hemisphere relatives, adapted well to the cold conditions.
In Australia, the Rhacopteris Flora is characteristic of the Late Carboniferous. In the Northern Hemisphere, similar plants are characteristic of the Early Carboniferous. As with much of the biota, the Australian plants are closest to those of South America. Again, it is climate that determines the relationship.
The Rhacopterids were Seed-ferns that had a primitive leaf structure and venation, Rhacopteris occurring throughout the Late Carboniferous. 2 other forms of Seed-ferns appeared by the middle of the Late Carboniferous, representing opposite extremes of the form range. There was a series of intermediate forms between a type with large variously lobed pinnules and a type in which the pinnules were deeply dissected.
Those with foliose pinnules are assigned to the form-genus Botrychiops and those with very finely dissected pinnules to Dactylophyllum. The intermediate forms are very difficult to assign to form-genera. Some believe the intermediate forms are simply a hybrid swarm produced by the 2 distinct genera interbreeding. A characteristic of these plants was the large aphlebiae, spathe or bract-like leaves that are thought to have possibly been protective structures for the developing fronds of pinnate leaves.
Aphlebiae come in a variety of forms, they can be large, undivided leaves, or forms which have a lamina that is partly pinnate, similar to the normal foliage of the plant. The leaf-like aphlebiae are of similar size and shape to leaves of the Gangamopteris from the earliest Permian that first appear in the fossil record at the times of the interglacials. The aphlebiae have a simple venation, no cross connections between the veins. It is believed possible that the plants with aphlebiae that lost the pinnate stage of their foliage under the extremely cold conditions at the glacial maximum gave rise to the Glossopterid leaf-type of the following Permian. The only requirement for this to occur is a change from the primitive type of venation to the netted form of more advanced plants. Evolution of new characters is known to have occurred in a series of jumps, not in a smoothly morphing fashion.
The Rhacopteris Flora included some Lycopods that derived from the Giant Clubmoss flora that had adapted to the cold climate, some showing the marked banding that results from periods of growth and dormancy that occurs when a plant grows in the wormer part of the year but ceases growth in the colder times of the year.
Botrychiopsis is the only member of the Rhacopteris Flora that is known to have survived to the Permian, being found occasionally associated with Glossopteris.
Maey E. White, The Greening of Gondwana, the 400 Million Year story of Australian Plants, Reed, 1994
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