Jan 24, 2007
To animals unfortunate enough to fall in, it was a death trap. To palaeontologists, it was a sensational discovery. Now the first detailed analysis of a spectacular cache of fossilised prehistoric "marsupial lions", giant wombats and kangaroos, owls and parrots discovered in a cave in Australia suggests that humans killed off the continent's megafauna.
The cache, found in the Nullarbor Plain in south-central Australia, contains fossils of 69 species of mammal, bird and reptile, and includes many complete skeletons, including the first of a marsupial lion (see right). There are also eight species of kangaroo that had never been recorded before.
The site was discovered by cavers in 2002, but its size and depth - 20 to 70 metres below the desert - means that it has taken a team led by Gavin Prideaux of the Western Australian Museum in Perth four years to collect and analyse the fossils.
The team found that the animals fell into the caves (pictured below right) between 800,000 and 200,000 years ago. To work out what they had been eating - and hence the kind of climate they experienced - the team analysed the oxygen and carbon content of the tooth enamel from fossils of 13 kangaroo species, including several giants, and one species of giant wombat. The composition was similar to that of the tooth enamel of kangaroos and wombats living today on the parts of the Nullarbor that are extremely dry, indicating that the fossil animals had also lived in an arid climate (Nature, vol 445, p 422).
However, the wide variety of herbivore species represented by the bones - 23 species of kangaroo alone, some of which could climb trees - suggests that the Nullarbor must once have had much more varied vegetation than the few species of shrub it sports today.
"It's astonishing. I never imagined tree kangaroos on the Nullarbor Plain. Australia's arid zone was clearly once capable of supporting a much wider variety of browsing animals," says Tim Flannery of Macquarie University in Sydney. Climate may play a smaller role in determining species diversity in Australia than has been thought, he says.
Australian megafauna died out roughly 40,000 years ago, and Prideaux says the discovery that they survived in an arid environment undermines one of two popular theories for what killed them off - namely, that ice-age aridity was responsible. That leaves the second theory, which suggests that the giant kangaroos and wombats were wiped out by the actions of humans, either through habitat destruction or hunting, says Prideaux.
That conclusion is supported by other discoveries by teams led by Prideaux in the Naracoorte caves in south-eastern Australia. Fossils here showed that many species of megafauna survived the recent ice ages - except the last one, which occurred after humans had arrived (Geology, vol 35, p 33).
"We're never going to find a diprotodon [one of the largest extinct marsupials] with a spear in it, but this is as close as you can get to nailing the argument," says team member John Long of Museum Victoria in Melbourne.
Not everyone is convinced, however. "You can say that a species was arid-adapted 200,000 years ago, but you can't then extrapolate to 40,000 years ago and say 'So humans must have done it,'" says Judith Field of the University of Sydney. "It's far too simplistic."
Field argues that archaeological finds from Cuddie Springs in south-eastern Australia, the only place where human and megafauna remains have been found in the same place, do not show that the animals were hunted.
She prefers the idea that different species were driven to extinction at different times and places. Combinations of events, including the stress of the later, more severe ice ages, could have been responsible, she says.