why marsupials only in australia? - why not in any other continent? (Jun/14/2008 )

I am intrigued as to why Australia ( along with Tasmania, Papua new Guinea) is home to various marsupial mammals and also monotremes like the duck-billed platypus, but these are not found in any other continent. I recently read a book which said that fossils of marsupials have been found in South america. My question is: What conditions in Australia favoured the natural selection of marsupials Vs placental mammals and why were they not selected in the rest of the world( I am assuming marsupials did evolve everywhere else but were outcompeted by placental mammals). So what is or was special about australia?

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It's evolution, the species with (at that time) more efficient biology (not only reproductive system) prevailed. But there are some marsupial species on other continents, but only a few. Opposums of South America are a prominent example for a very successful marsupial species.

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As hobglobin: to expand, the presence and absence of predators play a major role in species survival in a particular geographic location. If you were to bring Kangaroos in Pensyllvania, they would be hunted down in four days . No, seriously, I am talking about natural predators. Also the availablity of a particular food, weather, geography (forests, mountain, hiding place, joggin place) etc aslo play a part. The fact is, evolution works in mysterious ways (not supernatural, though), we are working on it, but with so many parameters to account for and so many local specificiites, evolutionary science would be well, alive and thriving in centuries to come.

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I think opossum of South America is an example for a marsupials outside Australia;

the specialities of climate and geography of Australia should have an inevitable influence on evolutionary development; larger predators were missing; the largest native predator was Thylacinus cynocephalus; the last 150-200 years, dingos contributed to reduce species of non-flying birds and marsupials which shows the influence of missing larger predators

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Tasmania is a part of Australia

Clare (an Australian)

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the oposum also plays "dead", simulating when it is in danger - very well characterized in the movie "over the hedge"

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Thank you , all, for interesting answers! I did not know about the opposum. It makes sense that the absence of effective predators would have made a crucial difference. I also apologise to Clare I had in mind the geographical landmass of australia and not the political entity.

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may be because Australia is isolated from other continents.

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The distance from Tasmania to the rest of Australia is only a matter of a few hundred km (about 350), it's the same continental shelf. [attachment=4829:Aus_shelf.jpg]
Speaking of continental shelves (like the seque?), the other reason why marsupials are restricted to certain continents is they were all part of Gondwana, one of the two supercontinents. Those marsupials that have survived where placental mammals developed did so because of excellent survival traits, like the opossum's fake "death".
As to whether kangaroos would survive outside of Australia, I think that only a top-level predator would be able to take out a full-grown Red or Eastern grey kangaroo. The claws on the hind legs are very large, and the rear legs are very powerful. And the breeding ability of kangaroos is legendary. There are estimated to be more kangaroos now in Australia than there were when the Europeans settled here, mostly because of the increased water supplies we developed for cattle.
Interesting but trivial: the diaphagm of a kangaroo is physically connected to their hind leg muscles, so the faster they jump the harder they breathe. Don't ask me how; if you want to know, look it up.

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human... the top-level predator who like kangaroo meat.

Just kidding...

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