The first known complete skeleton of Thylacoleo carnifex, the 'marsupial lion', one of hundreds of fossils discovered in caves beneath the sun-scorched Nullarbor Plain in southern Australia.

Credit: Clay Bryce/Western Australian Museum

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ADELAIDE: Australia's gigantic prehistoric beasts were likely wiped out by Aborigine-started wildfires, rather than by a changing ancient climate, according to a new study.

Research published today in the British journal Nature suggests that the prehistoric beasts, or 'megafauna' - which included marsupial lions and giant wombats and kangaroos - that inhabited the Nullarbor Plain in south-central Australia tens of thousands of years ago were well adapted to the new, more arid conditions long before their extinction.

"The Nullarbor discoveries … show convincingly that the Australian megafauna could take all that nature could throw at them for half a million years, without succumbing," said co-author Richard Roberts of the University of Wollongong, near Sydney, Australia.

The Australian megafauna became extinct around 40,000 to 50,000 years ago, and it had been thought that the mass extinction was due to the Australian climate becoming more arid. However, Aborigines also arrived in Australia around this time, causing some scientists, such as Tim Flannery (See "Australia should lead energy revolution", Cosmos Online) in his book The Future Eaters, to posit that the megafauna were wiped out by man.

While the team stresses that more research is needed, the find may lend support to the idea that the megafauna became adapted to an arid climate hundreds of thousands of years before man arrived, and that their extinction was indeed the result of fires lit by Aborigines in order to control vegetation, as has been shown in other parts of Australia.

"The megafauna were astonishingly resilient to the vagaries of the Ice Age climates, and it was only when people arrived that they vanished from the landscape," said Roberts.

Skeletons unearthed by the researchers from several caves beneath the treeless plain (Null-arbor literally means 'no trees') included the remains of nearly 70 species of mammals, birds and reptiles. Among the finds were eight new species of kangaroo and the first complete skeleton of Thylacoleo carnifex: the 'marsupial lion' (See COSMOS magazine, Issue 8, p58).

The hapless animals fell down shafts into the caves, became trapped and died. Over time, the shafts sealed over, preserving a record of the region's fauna between 400,000 and 800,000 years ago - long before the arrival of man, said Roberts. "This created a priceless time capsule giving us an incredibly clear window back to when Australia was locked into a long-term trend of increasing aridity in one of the driest regions of the continent."

To get an idea of the climate in which these beasts lived, the researchers analysed the ratios of isotopes of oxygen and carbon in the teeth of the kangaroo and wombat skeletons. The ratios reflect the isotope content of the vegetation and water they consumed, and the team was able to compare the results with modern animals.

They also looked at the type of animals that lived on the plain - lizards common in open woodland, parrots that would have required trees to nest in and tree kangaroos that today are only found in rainforest.

"Surprisingly, the climate 500,000 years ago was very similar to that of today, although the region must have had substantially more tree cover to support such a high diversity of herbivores," said lead author Gavin Prideaux of the Western Australian Museum, in Perth. The researchers concluded that rainfall would have been similar to current conditions: around 230-260 mm a year.

"Some time during the last 400,000 years, the Nullarbor vegetation changed from the fire­sensitive woodland to the shrub­grass mosaic we see today," said Prideaux. "We think that an increase in wildfires best explains the shift, given that climate change was not a significant factor."