Throughout time Australia has withstood the many climatic and geological changes that have affected it's vegetation and native fauna.
Australia has been isolated from other continents for 45 million years. This isolation resulted in many unique plants and animals evolving. Over this time Australia has changed from a green, forested land to one of the most arid vegetated landmasses on Earth. This change of climate is reflected in the fossil record.
Fossil plants show that at the time Australia separated from Antarctica, much of the land was covered with broad-leaved forests, even Central Australia. Similar forests are found today in wet tropical regions of northeastern Australia and Papua New Guinea.
As Australia moved northwards at 6 to 7 centimetres per year, weather conditions changed and the climate dried out. The centre of Australia became drier and plants living there either became extinct or had to adapt to the drier conditions.
The vegetation in this region changed from forests to woodlands to scrublands and eventually to grasslands.
Eucalypts evolved in Eastern Gondwana at least 60 million years ago, before Australia became isolated. As the climate dried out Eucalypts became the dominant type of tree in Australia.
Mammals have lived in Australia for many millions of years. Australia's most distinctive mammals are marsupials, which carry their young in pouches, and monotremes (the platypus and echidna), which are the only mammals that lay eggs. Placental mammals, for example rats, bats and humans, migrated here much later.
Today marsupials are found mainly in Australia, Papua New Guinea and South America and monotremes only live in Australia and Papua New Guinea.
It is not known where marsupials originally evolved as their fossil record is quite poor. Some scientists think they originated in North America and reached Australia later via South America and Antarctica.
Even before Australia split from Antarctica, its marsupials had begun to evolve into distinctively Australian groups, many of which are now extinct. These include extinct marsupial 'lions' and diprotodons, as well as living possums, koalas, bandicoots, wombats and kangaroos
A group within the marsupialia category is called Vombatiformes, included in this group is the Diprotodon optatum. The Diprotodon was the largest beast, it was an herbivorous wombat-like creature, about the size of a rhinoceros.
There are two conflicting theories surrounding the extinction of the megafauna, such as the Diprotodon. The more accepted theory adopted by scientists is that early human inhabitants hunted them down, the other theory, which is less popular, is the climate change was the cause of extinction.
The first theory, which involved the Australian Aborigines being the cause for the megafauna's extinction is basically based in the timing of the extinction of the megafauna, coinciding the time humans, began to inhabit Australia. These animals were very large and consequently very slow moving, this made them very vulnerable in their environment. However, there is also evidence that may question this theory. It has been found that there was a long overlap between the times that the megafauna existed and humans entered Australia.
The second theory states that extinction simply occurred because of climate change. Australia often experienced many episodes of drought, many animals would have eventually adapted to this, however it is said that maybe there was just one extended drought that just knocked off the megafauna. Dr Judith Field, of the university of Sydney, supports this theory.
For the past 12 or so years Dr Peter Murray and a team of researchers, from the Museum of central Australia, have been excavating fossils from Alcoota, in a remote part of the Northern Territory. Recently they found a large number of 'thunder birds', or Ilbandornis.
These Thunder birds belong to the family Dromornithidae, the largest of which was about three metres tall, weighed about half a tonne, and has been nicknamed the 'giant demon duck of doom'.
Dr Murray stated "Animals tend to gravitate towards the last few remaining water holes during a drought. When you get many animals congregating around one water hole, they eat all the food. Ultimately, if the drought doesn't break, they die of starvation." (Cited at: http://www.abc.net.au/science/features/megafauna/default.htm)