In 2002, a team of cavers exploring the vast, treeless Nullarbor Plain of southern Australia made what some palaeontologists described as the “find of the century” – a dozen skeletons of the extinct marsupial ‘lion’, Thylacoleo, as well as the bones of giant wombats, short-faced kangaroos and thylacines.

Until then, no complete skeleton of Thylacoleo was known to science, so the Western Australian Museum launched “Operation Leo” to recover these fossils, supported by the Rio Tinto WA Future Fund.

Now in early 2007, the first analysis of these subterranean treasure troves -- led by Dr Gavin Prideaux, Rio Tinto Research Fellow at the Western Australian Museum -- has appeared in the latest issue of the prestigious international journal Nature.

Fossils from the caverns, dubbed the Thylacoleo Caves, show that the Nullarbor was once home to at least 69 species of mammals, birds and reptiles, including 23 different types of kangaroo, ranging from pint-sized bettongs to 3-metre tall giants. Eight of the kangaroo species are entirely new to science, and include an unusual wallaby with large ‘brow-ridges’, and, ironically (in the now treeless Nullarbor Plain), two tree-kangaroos – modern relatives of which inhabit rainforests in tropical Australia and New Guinea.

Expedition crews led by Western Australian Museum palaeontologist, Dr John Long (now Head of Science at Museum Victoria in Melbourne), spent a month camped in the middle of the Nullarbor during 2002, 2003 and 2004.

“We collected hundreds of specimens during each expedition and were stunned by the amazing preservation of the fossils. Many of the skeletons are complete. As palaeontologists, we spend most of our time trying to identify and reconstruct extinct animals from fragments. All of a sudden, it was information overload,” said Dr Long.

Fossils were dated using changes in the Earth’s magnetic field, uranium-series dating and optically stimulated luminescence by a team of geochronologists including Professor Richard ‘Bert’ Roberts from the University of Wollongong. The researchers found that animals fell into the caves between about 400,000 and 800,000 years ago, through the same narrow entrances that cavers abseil through today. Since then, the caves have been sealed for much of the time, which accounts for the pristine condition of the bones. “The dates place the animals in a period when Australia was locked into a long-term trend of increasing aridity in one of the driest regions of the continent”, Professor Roberts said.

The researchers reconstructed the ancient Nullarbor environment from oxygen and carbon isotopes contained in the enamel of kangaroo and wombat teeth, and from the modern geographic ranges of species in the fossil fauna that are still living today.

Surprisingly, the climate half-a-million 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.

“Some time during the last 400,000 years, the Nullarbor vegetation changed from fire-sensitive woodland to the shrub/grass mosaic we see today. We think that an increase in wildfires best explains the shift, given that climate change was not a significant factor,” Dr Prideaux said.

The study also has implications for the debate over what finally drove the megafauna to extinction 40,000-50,000 years ago. The authors argue that if the Nullarbor animals were well adapted to dry conditions for at least 400,000 years before they disappeared, then it is unlikely they succumbed to Ice Age aridity.

“Our work removes another pillar of support from the idea that the megafauna were driven to extinction by climate change, especially given that most of the large species were not Nullarbor specialists – they were widespread across much of Australia. But the jury’s still out on whether or not the change in the Nullarbor vegetation is correlated with the arrival of humans, as seems to have been the case in other parts of Australia,” said Dr Prideaux, who earlier this month published a separate study in the journal Geology revealing that megafauna from the Naracoorte Caves in southeastern Australia were resilient to climate change for half-a-million years before humans arrived.

The palaeontologists now have their eye on a deep sediment pile in one of the Thylacoleo Caves, which may preserve a record extending back millions of years. Study of bones from the various layers will help build up a detailed view of the evolution of the Nullarbor animals, and their responses to an increasingly dry climate.

“The Thylacoleo Caves are undoubtedly one of Australia’s most important fossil localities. It’s been a privilege to work on these sites, and we can’t wait to get back out there to start the next phase of the project,” Dr Prideaux said.