The main reason Australia has such "strange and unusual" animals was its isolation. For millions of years the Australian continent was so far away from any other landmass that there was no possibility for new types of animals to get to it. So the animals that were already on the continent evolved, in isolation, into animals most suitable for the Australian environment.
About 250 millions of years ago the world had just one huge super-continent called Pangaea. Animals and plants intermixed with each other on this huge single landmass.
Around 200 million years ago this super-continent broke up into two continents — Laurasia and Gondwana. When this separation took place monotremes and marsupials were the predominate mammals of the Gondwana region and placental mammals in Laurasia.
Fossil evidence suggests that a vast forest stretched across Gondwana, all the ways from south-eastern Australia, through Antarctica, and into southern South America, and all three types of mammals existed in this forest.
The breakup of Gondwana occurred in stages. About 180 million years ago, in the Jurassic Period, the western half of Gondwana which included Africa and South America separated from the eastern half which included Madagascar, India, Australia, and Antarctica. Then about 140 million years ago Africa separated from South America and the South Atlantic Ocean started forming. At about the same time, India separated from Antarctica and Australia creating the Indian Ocean.
Initially Australia and Antarctica slowly drifted southwards, far away from all the other continents, and became completely isolated from the rest of the world by vast oceans. The other continents however stayed relatively close to each other, and over time collided or joined parts of the old Laurasia landmass. When these landmasses came together monotremes and marsupials were unable to compete with the more advanced placental mammals and became mostly extinct. In Africa and India marsupials became completely extinct. When South America joined North America almost all the marsupials there also vanished and were replaced by placental mammals.
Australia, meanwhile, slowly separated from Antarctica and drifted on its own in a vast ocean – isolated from the rest of the world. The animals and plants which were originally on the Australian landmass no longer had contact with creatures from other parts of the world and continued to evolve independently. The climate also changed, becoming drier and more arid. Monotremes and marsupials, with their less demanding reproductive systems, were more suitable for this new environment and became the dominate animals in Australia. What remaining placental animals in Australia slowly became extinct.
This separate evolution has resulted in some unusual Australian animals. This is why Australian native animals are so different from those found elsewhere in the world.
Australia's long isolation from the rest of the world has allowed its animals to evolve separately from those in other parts of the world, but to fill similar niches in the environment. For example the echidna is an Australian anteater. The Tasmanian tiger (now extinct) was a marsupial wolf. The existence of similar animals in different parts of the world is referred to as "Parallel Evolution".
There are three types of mammals in Australia today. These are monotreme, marsupial and placental mammals.
Australia does not have any native monkeys, apes or hoofed animals such as horses, cattle and deer. This is another result of its isolations from other continents.
Monotremes first appeared between 145–99 million years ago and are the oldest type of Australian mammals. It is not certain if they originated in Australia or elsewhere (a 63 million year old tooth discovered in Argentina belonged to a now extinct platypus).Two out of the five known species of monotremes in the world live in Australia. These are the echidna and platypus.
It was once thought that marsupials originated in Australia. However, recent fossil evidence and genetic research suggests that they may in fact have originated in what is now modern-day China on the ancient landmass known as Pangaea. Marsupials are believed to have arrived in Australia around 50 million years ago via North America, South America and through Antarctica.
Marsupials appeared about 64-65 million years ago and are the second oldest type of mammal found in Australia.
Once Australia separated from the other continents and started drifting away the marsupials stranded on it didn't have any competition from placental mammals. Without competition, the marsupials diverged into over 235 different species found in Australia today. They occupy every niche of the Australian habitat and range from the large red kangaroo to marsupials of the smaller than a mouse.
Placental mammals are relatively recent arrivals to Australia. Bats were the first to arrive getting here about 23 million years ago. Rodents arrived about 5-10 million years ago. These animals reached Australia by flying and crossing the seas that separated Australian from Asia when Australia slowly stated drifting closer to Asia making crossings to the continent possible. These placental mammals make up a very small percentage of the total mammalian population. Humans introduced a number of animals. The dingo was the first of these, arriving in Australia about 5,000 years ago.
Australia has many amphibians and reptiles found nowhere else in the world.
Lizards – There are over 700 species unique to Australia alone.
Snakes – Australia has more venomous snakes than non-venomous ones. The red-bellied black snake is one.
Frogs – Four families of native frogs inhabit the continent.
Crocodiles – Australia has two species of crocodile. The saltwater crocodile is the world's largest and can grow to 1,000 kilos and is known to attack humans. Fresh water crocodiles are much smaller and do not attack humans.
Turtles – There are 35 species of freshwater turtles. Six species of sea turtle also visit the coastlines.
Australia has 800 species of birds, 350 are found only in Australasia.
Megapods – as the Mallee fowl, trace their ancestry as far back as Gondwanan time. These birds are stocky birds which look somewhat like chickens. They have small heads and large feet (that's why the name "megapod" meaning big-feet).
Other birds such as kookaburras are the world's largest kingfishers.