Dirt story: the past in their hands

Were it not for satellite technology, Dr Michael Archer doubts he would be on an isolated outcrop in north-west Queensland, ankle deep in red dirt.

Were it not for satellite technology, Dr Michael Archer doubts he would be on an isolated outcrop in north-west Queensland, ankle deep in red dirt.

Covered in sweat bees - the only creatures besides us foolish enough to endure the midday heat - the palaeontologist holds up a baseball-sized rock. "This is a time capsule," he says.

To the untrained eye, it is just congealed dirt. But to the group of scientists who gather around, the specimen holds clues to a crucial, yet virtually unknown, time in our continent's history. On the rock's surface, flecks of white bone are barely visible. The experts quickly identify them as the limbs and teeth of several different groups of primitive mammals, which first appear in the fossil record more than 20 million years ago.

For the past week, Archer, a professor at the University of NSW, and a team of bone collectors have extracted piles of silt-covered rocks from a small pit, the first yield of a new fossil deposit uncovered last year beside the Riversleigh World Heritage fossil site 200 kilometres north of Mount Isa.

The vast deposit, known as New Riversleigh, is so remote the team fly in each day by helicopter. "No European would've set foot on this hill before we got here," says Archer. Were it not for one of Archer's former masters students, Ned Stephenson, who was using remote sensing data from satellites to study the region, it would likely have remained that way.

So far, the team have recovered dozens of specimens, many yet to be identified, including species of bat never seen before, a couple of large wombat-like diprotodontids, small, primitive kangaroo, and an ancient ring-tailed possum. "This place is bone city," says Archer.

The treasures from this first site, named Wholly Dooley because of the reaction it prompted in the diggers, add to the impressive collection of fossils recovered from the Riversleigh area since its first major excavation in the late 1970s.

Since then, Archer, Associate Professor Sue Hand, retired researcher Henk Godthelp and a dedicated group of scientists and volunteers have unearthed the remains of countless ancient birds, reptiles and the ancestors of almost every modern group of Australian mammal. "Nowhere else in Australia, and very few places in the world, do you get a continuous record of animal communities spanning such a long period, from now until 25 million years ago," says Hand.

Between then and now, northern Australia experienced a dramatic climate change that transformed it from a landscape of lush rainforests, like the Amazon today, to one that is predominantly arid. This shift can provide insights into how today's mammals may cope as the globe warms, says Hand.

While Riversleigh provides a spectacular and detailed record of how our environment evolved, and how species such as the kangaroo, koala, wombat and possums adapted (or ended), over this period, there remains a significant gap in understanding.

Between roughly 15 million to 5 million years ago, a period know as the late miocene, researchers believe the continent reached a tipping point - the tropical rainforests opened into woodlands and the country began to parch. Until now, few fossil specimens appeared from this time. "This new site seems to represent a relatively young deposit, with rocks from this critical age," he says.

Rick Arena, a seasoned fossil hunter, has spent the past week leading a team of explorers through new Riversleigh. With its vast stretches of spinifex, punctured by the odd anorexic shrub, it's a hard slog. While satellite data led the team to the deposit, technology cannot replace the traditional method of locating fossils - a sharp eye and lots of walking. So far this trip, the exploring team has located eight new sites with fossiliferous rocks.

Ancient snail shells offer the first clue that bones may be nearby. "That tells us we're looking at rocks of the right age," says masters student Bok Khoo. Flow stones, rocks that have been shaped by water, are the next telltale sign. Almost all of Riversleigh's bones have survived through time because their owners were petrified in water containing the mineral calcium carbonate.

"It's dissolvable, but readily re-precipitates into solid limestone rock," says Arena. "That's why everything here has been preserved." Even the bones of cattle that die in the nearby Gregory River quickly become fossilised, he says.

The trigger for this unique set of conditions began 500 million years ago, when Australia was part of the giant supercontinent Gondwana, and covered by an ancient ocean. Over many millions of years, the calcium carbonate shells of dead marine animals formed thick deposits of limestone on the sea floor, which remained buried long after the ocean drained away.

"But 26 million years ago, huge forces were at work," says Arena. Now separated from Antarctica, the Australian continental plate slammed into a chunk of Pacific Ocean crust, in the region where New Guinea sits today. The collision forced parts of northern Australia to warp. Known as the Pine Creek Upwarp, this exposed these long-buried layers, providing a source of calcium carbonate. When animals then died in the region's mineral-rich lakes, their remains were preserved when the limestone re-solidified. As time passed, acid rain slowly whittled away parts of the soluble rock, forming extensive networks of caves and crevices just below the Earth's surface.

"Stuff falls into these spaces and re-solidifies into solid rock," says Arena.

Which brings us to Wholly Dooley pit. "We think this is part of a cave deposit," says Archer, drawing his finger across two large boulders.

A well-executed blast by explosives expert Lizard Cannell uncovers more fossils, which Archer's inspects and declares them further support to his hypothesis that Wholly Dooley may fill some of the gaps in the continent's history book.

Archer's interpretation is based mainly on the teeth the team have recovered, all of which appear to be worn down. "That probably means there was more dust and grit in their diet. And that means the environment was drier."

The work of trying to understand what happened at this site has only just begun. The 1.8 tonne of rocks the team recovered are on their way to the university's labs, where fossil preparators will spend months dunking them in weak acid to dissolve the limestone and reveal the specimens. As Lizard Cannell says: "Understanding this place is like trying to put together a puzzle without the picture on the box," he says.

Tony Wright's column returns next week.

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