Australia: The Land Where Time Began
- 23.7-5.3 Ma
Globally the water temperature of the oceans were warmer than in the Oligocene in the first half of the Miocene. This was the only length of time during the Cainozoic that the cooling trend was reversed. In the Tasman Sea the temperatures of the surface waters were a few degrees above the present level. On the southern margin of the continent, the shallow shelf waters were very warm, between 15-20oC, at similar levels to water in the subtropical and tropical areas at the present. It is believed the currents in the oceans were probably sluggish, as they usually are when temperature gradients between the Equator and the Poles were small. Sea levels were rising during the Miocene. It is believed this was mainly because of melting ice, but may also have resulted from the continental collisions in the western Pacific and Asia.
Extensive limestones were formed in the Eucla Basin, Murray Basin, Otway Basin and Gippsland Basin from carbonate oozes that had formed beneath shallow marine transgressions. Throughout the Early Miocene the southern part of the continent had a wet climate. Sealevels began dropping as the ice caps increased. In the Middle Miocene, Bass Strait opened, only to close and open many times as the sealevel fluctuated leading up to and during the latest ice age of the Pleistocene.
When the permanent ice cap formed on Antarctica about 15 Ma temperatures dropped and an increasingly dry anticyclonic circulation began the process of aridification of northern and central Australia. In the arid parts of Western Australia the river systems have been reduced to a series of salt lakes, having been inactive since the Middle Miocene.
At the close of the Miocene the Terminal Miocene Event occurred in which the temperatures plunged and the area covered by the ice cap expanded by a n enormous amount.
There were closed forest communities in the southern parts of the continent for much of the Miocene, similar to those in the Early Tertiary. Nothofagus brassii, Laurels, Conifers and Casuarina were common along rivers and around lakes. There were Myrtaceae, that included some pollen that were eucalyptus-like, and Proteaceae and many other families. This sort of forest extended as far north as Alice Springs in central Australia. N. brassii requires regular moderate rainfall with no dry seasons. After the formation of the Antarctic ice cap, and as the continent moved into drier latitudes, climates changed, rainfall becoming seasonal and N. brassii declined sharply in the Late Miocene.
In the north the vegetation changed, Myrtaceae was then dominant and grasslands appeared in some areas. Saltbush (Chenopodaceae) and Acacia, both of which are arid adapted, appeared in the fossil record.
There is a good fossil vertebrate record form the Miocene in Australia. Footprints of the giant running birds, Dromornithids or Mihirungs, have been found in Tasmanian deposits dated to Early Miocene. Some species of Genyornis were common in the Miocene and Pliocene, and are synonymous with the "mihirung paringmal" or "giant Emus" of the Aboriginal Dreaming. They were still living when the Aborigines arrived in Australia about 60,000 years ago. It is believed they may have survived until about 6,000 years ago. Bones of these birds have been found as large as those of elephant birds of Madagascar, and the broken shells of enormous eggs have been found. They were not ancestral to the Emus, the ancestors of which were living during the Miocene and were much smaller than the present-day Emus.
There were flamingos in central Australia. In the diatomaceous-earth deposits at Chalk Mountain at Bugaldie in New South Wales, an Owlet-nightjar has been found that is related to Frogmouths, the Potoos of South America, and the Nightjars. In the same deposits were Eucalypt leaves and gumnuts that are believed to possibly be Tristania, and flowers of Ceratopetalum, the New South Wales Christmas Bush, as well as many others.
The Diprotodontid Wynyardia was the first land mammal described from Australia, known only from a single skeleton found at Wynyard in Tasmania. Sites in central Australia and the Riversleigh region of northwest Queensland have revealed many marsupial faunas as well as many other animals.
Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
|Author: M. H. Monroe Email: email@example.com Sources & Further reading|