Why are there so many marsupials in Australia?

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These marsupials live Down Under (the wallaby, tasmanian devil, wombat, kangaroo with its joey, quokka and koala) but marsupials didn't actually begin their evolution there.
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▼ Australia is the kingdom of marsupials, home to furry kangaroos, koalas and wombats. The continent has so many marsupials, it raises the question: Did these pouch-bearing mammals arise Down Under?
The answer is an unqualified (or "un-koalafied") no. Marsupials were around for at least 70 million years before they made it to Australia, according to Robin Beck, a lecturer in biology at the University of Salford in the United Kingdom.undefined
"Marsupials absolutely categorically did not originate in Australia," Beck told Live Science. "They are immigrants."
In comparison to most mammals, marsupials are odd. Unlike placental mammals, such as humans, dogs and whales, marsupials give birth to relatively underdeveloped young that continue to grow a ton in the mother's pouch.
"The young are born alive, but they're very poorly developed," Beck told Live Science. "They basically crawl to their mother's nipple, which is often in a pouch, and they basically clamp on the nipple and stay there, feeding on their mother's milk for long periods of time — usually, several months."
Marsupial homeland
And it turns out, the oldest known marsupials are actually from North America, where they evolved during the Cretaceous period after splitting off from placental mammals at least 125 million years ago, Beck said.
These ancient marsupials appeared to flourish in North America, populating what was then the supercontinent Laurasia with about 15 to 20 different marsupial species, all of which are now extinct, Beck said. It's unclear why these marsupials did well. But for some reason, at about the time that the nonavian dinosaurs went extinct, about 66 million years ago, the marsupials made their way down to South America. At that time, North and South America weren't connected as they are today. But the two continents were very close, and a land bridge or a series of islands may have linked them. This connection allowed all kinds of animals to expand their stomping grounds.
Once in South America, marsupials and their close relatives had a field day, diversifying like crazywithin 2 million to 3 million years after arriving, Beck said. For instance, marsupials and their close relatives evolved into bear- and weasel-size carnivores, and one even evolved saber teeth. Others evolved to eat fruits and seeds.
"What's happening in South America is they're evolving to fill the kinds of niches that in the northern continents certainly were filled by placental mammals," Beck said. (▪ ▪ ▪)

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