Australia: The Land Where Time Began
- 5.3-1.6 Ma
After the sudden cooling phase at the close of the Miocene, The Miocene Terminal Event, there was a warming phase between 5 of 3 Ma. It is believed that part of the West Antarctic ice sheet may have melted during this warm phase. The cooling trend then resumed, the Arctic ice cap starting to form about 3 Ma. This was the first time the Arctic Ocean froze.
The fluctuations of wet-dry, cold-warm phases as the ice caps expanded and contracted began to come in rapid pulses. The largest extent of the Arctic ice cap at this time was only half of that it achieved during the Pleistocene.
In the Pliocene the Panama Isthmus rose, the blockage of the flow of the equatorial currents in the sea that resulted contributed to the increased severity of the cooling phase. The regions to the north of Australia were being deformed as Australia moved north, more land being elevated above sea level and making migration from Asia easier, especially during glacial phases when the continental shelves were exposed. About 5 Ma rodents arrived in Australia. Prior to their arrival the only placental mammals were bats, which the Riversleigh deposits show arrived in the Miocene or Late Oligocene.
The drying trend that began during the Middle Miocene became more rapid after the warm phase at the start of the Pliocene, at which time very different subregions developed and the central deserts spread. When the northern ice cap became formed about 3 Ma ice regimes became firmly established.
Anticyclonic high pressure zones moved north bringing winter rainfall to the southern and southwestern margins of Australia. The establishment of a Mediterranean type of climate led to the evolution of a highly individual flora, with 80 % of species being endemic, in southern Western Australia. A similar climate developed in the southwestern Cape of South Africa developed at the same time, with parallel development of the flora, both evolving from a similar Gondwanan stock, resulting in a unique flora on both continents.
Both the regions, in Australia and South Africa, lie on the same southwest positions on the respective continents, both floras being unique. Having evolved in isolation the genera of the 2 floras are distinct, but the structure and content at the family level, having evolved from of the vegetation of the same origin on the 2 continents, and with the same environmental pressures, the 2 floras demonstrate a high degree of similarity.
At this time plate movements brought the northern part of the continent into the hot, humid climatic zone, allowing the northern areas to avoid the aridification trend continuing over much of the continent. Refuge areas were provided by the high country along the east coast and by the elevation of the New Guinea mountains. rainfall and conditions remaining suitable for the survival of closed forest and rainforest species, as well as marsupials and birds and other forest animals.
As tectonic processes go, the Kosciusko Uplift that raised the eastern highlands was a gentle process. the rejuvenated coast-ward flowing river systems cut the deep river valleys that later became the "drowned valley" systems when the sea level again rose, characterising the modern coastline.
Throughout the Tertiary the nature of forests had been changing, but it was in the Pliocene that open forests developed on a large scale with a groundcover of grasses and Asteraceae (Daisy family). Already arid areas had been taken over by the Pliocene. The kangaroos, the Australian equivalent of the African antelopes, evolved to inhabit the open plains. In the Pliocene Myrtaceae dominated the forests. The modern style Eucalyptus have been shown to be less dominant at this time, the pollen record indicating Baeckea, Backhousia and Tristania were more common. It is believed the Eucalyptus-style pollen were probably composed of Angiophora, Syncarpia and Metrosideros, and Eucalyptus formed a less dominant element of the vegetation. The dry country was being colonised by Acacia trees and shrubs as they adapted to the dry conditions, and the Casuarina (Desert Oaks) were adapted to arid areas.
The aridity of Australia exceeded that of the present, but it was about 200,000 years ago that actual desert conditions were reached. It is believed there were probably virtual deserts in pockets throughout the Neogene, that resulted in a highly specialised desert flora and fauna. When the desert areas expanded these desert-adapted forms were ready to colonise the desert areas as they expanded.
Among the megafauna of this time were Diprotodon, Giant Kangaroos and Wombats, large Running Birds, and many other animals that included the trunked Palorchestids, and many "normal" appearing Marsupials, Birds and Mammals.
Mary E. White, The Nature of Hidden Worlds, Reed, 1993
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