Originally, while all the continents were joined together, there were primitive mammals of all sorts in Australia. However, when Australia broke off from Gondwana and started drifting off to the south, the environment started to get drier and sparser. Monotremes and marsupial mammals were well suited for the new harsher environment than the placental mammals. Then, about 34 million years ago, there was a huge world-wide extinction event, possibly caused by large scale bombardment of the earth by objects from outer space such as meteors. This seems to have affected the placental mammals more than it did other creatures. From then onwards monotremes and marsupial mammals seem to have thrived in Australia and placental mammals slowly became extinct.
It is believed that marsupial animals were better suited for Australia's harsh dry environment because of their lower metabolic rate and less demanding reproductive system. Marsupials and monotremes became the dominant mammals of Australia.
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