LONDON.- Molecular evidence suggests that wombats have an evolutionary history that stretches back some 40 million years. Despite this, our understanding of them is incredibly poor as their fossil record is limited and patchy.

Dr Pip Brewer has recently described the oldest known wombat fossil as well as a new fossil wombat species in a paper published in Palaeontologia Electronica. She says, 'Studying wombats is a real pain when it comes to their fossil remains.

'This is partly due to Australia itself. It is an old, stable continent where you tend to have net erosion and not much deposition. This means that carcasses are more likely to be eroded away rather than quickly buried and turned into fossils. Wombats specifically are very rare.'

Even as recently as 10 years ago, there was just a single wombat fossil older than 2.6 million years old. This paucity of wombat remains highlights the significance of the site that Pip has been helping to document at Riversleigh, which dates to at least 23 million years ago.

'What is coming out of Riversleigh is telling us pretty much everything we know about early wombat evolution,' says Pip.

Explosive discoveries
The site now is located in the middle of the arid outback in Queensland. But when the wombats were still snuffling around it millions of years ago it was a series of caves in the middle of the rainforest. The hard landscape means that excavators can't exactly use conventional methods to find the fossils.

'They're still digging there,' says Pip, 'using explosives to excavate.

'It's a really rocky, hard environment and you can't hammer it because it won't make much of an impact on the rock. Basically what they do is drill holes into the rock, put detonating cord down them and then detonate it.'

These controlled blasts aim to peel away slabs of rock, which are then airlifted in a net attached to a helicopter to get into the back of a vehicle. The slabs are then taken to a freight train and shipped to Sydney, where they finally go in an acid bath to reveal any fossils trapped within.

The results are revealing a rich Miocene (23-5 million years ago) assemblage of marsupials, providing an astonishingly rare glimpse into the origins of these strange creatures.

Retreat of the rainforest
Modern wombats graze on grass and dig burrows, and are often found in open environments. But the fossils unearthed in Riversleigh seem to paint a different picture of these rotund marsupials.

Back during the Miocene, much of Australia was blanketed in rainforests. This means that many of the continent's most iconic fauna - including kangaroos, koalas and the enigmatic wombats - were evolving in a vastly different environment to where many are found today.

There is even doubt as to whether these early wombats shared some of the basic life histories with their modern counterparts.

Pip asks, 'We know that modern wombats burrow, but did they in the past?

'There has been a lot of debate about whether some of the species were simply too big to burrow, but then we know that giant ground sloths in South America dug tunnels a metre wide.'

With only teeth to refer to the majority of the time, it is almost impossible to answer such questions using the current fossil record. One fossil arm bone from a Pleistocene (2.6 million - 11,700 years ago) wombat with close links to a Miocene form does seem to indicate adaptations to digging. But as Pip notes, 'It could just be an adaptation to rooting around the ground for food. It doesn't necessarily indicate burrowing. Maybe they started off foraging underground before starting to dig burrows.'

While the teeth might not be able to answer the question of burrowing, they can inform Pip about what the animals ate, and therefore what the climate was like at the time.

By studying the wear patterns on the teeth - hampered by the fact that wombat teeth continually grow throughout their lives - the researchers can get a better idea of what the animals were eating and therefore what sort of environment they were living in. This can then be used to track environmental changes over millions of years.

'Wombats are possibly a key to looking at how marsupials responded to this drying out of the environment,' says Pip. 'They're potentially a really interesting indicator.'