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Thursday, November 20, 1997 Published at 21:25 GMT


Who was first: mammal or marsupial?
image: [ Place on Australia's evolutionary tree now in doubt ]
Place on Australia's evolutionary tree now in doubt

Palaeontologists may have to rethink their theories about the evolution of warm-blooded animals, if the findings of a team of Australian scientists are proved correct.

Australia is famous for its bizarre and unique wildlife - in particular the marsupials: animals such as the koala and the kangaroo, whose young are born at a very early stage of development and spend the first part of their life in a pouch.

Stranger still are the monotremes, such as the duck billed platypus and the spiny anteater or echidna - half way between mammals and reptiles, laying eggs but warm blooded and covered in hair.

Until now it has been believed that the monotremes appeared in Australia about 100 million year ago, marsupials appeared about 50 million years ago, and true mammals, whose young are born already well developed, only appeared about 10 million years ago.

But now Australian palaeontologists at the Museum of Victoria in Melbourne have found a fossil jawbone two centimetres long with four tiny teeth, which is undoubtedly at least 115 million years old and which they believe formed part of a placental mammal.

If their analysis, published in the Scientist magazine, is correct, then true mammals lived in Australia long before the arrival of marsupials and around the time of the first monotremes.

If they are right, their find would turn the family tree of warm-blooded animals upside down. But not all scientists will accept such a radical rethink based on a single jawbone.

Some palaeontologists have already joined the argument, arguing that the jawbone and teeth could equally well have belonged to a monotreme, for example a close relative of the egg-laying duck-billed platypus. This creature could have evolved teeth just like mammals because it lived in the same places and ate the same foods. Such parallel evolution is a frequent feature of fossil records.

More finds will be needed before they agree to rewrite the history of mammalian life.

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