What is believed to be a bat skull fossil. Credit:Tony Walters But to the group of scientists who quickly gather around, the specimen holds clues to a crucial, yet virtually unknown, time in our continent's history. On the rock's surface small flecks of white bone are barely visible. The expert team quickly identifies them as the limb bones 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 from a small pit piles of silt-covered rocks, the first yield of an expansive 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 is flown in by helicopter each day.

''No European would've set foot on this hill before we got here,'' Archer says. 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 has recovered dozens of specimens, many yet to be identified, including a never-before-seen species of bat, a couple of large wombat-like diprotodontids and a ring-tailed possum. ''This place is bone city,'' says Archer, who after decades of digging for fossils, admits he's become a discovery junkie. The treasures from this first site, named Wholly Dooley for the reaction it prompted in the diggers, add to the impressive collection of fossils recovered from the Riversleigh area since its first large 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 and 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 of time, from now until 25 million years ago,'' Hand says. Between then and now, northern Australia experienced a dramatic climate change and transformed from a landscape of lush rainforests, like Borneo and the Amazon today, into a continent that is predominantly arid. This shift is key to our understanding of modern environments, and can provide invaluable insights into how today's mammals may fare as the globe warms, Hand says. 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 disappeared) over this period, there remains a significant gap in scientists' understanding. Between 15 million to 5 million years ago, a period known as the late Miocene, researchers believe the continent reached a tipping point; the tropical rainforests opened into woodlands and the country began to parch.

''We need to unravel what happened in this transitional period. What happened to the animals? How did the environment respond?'' Archer said. Until now, few fossil specimens appear from this time. ''This new site seems to represent a relatively young deposit, with rocks from this critical age,'' he says. A short time later, a petite bearded man appears at the edge of Wholly Dooley pit, his hand filled with explosives. Rick Arena, a seasoned fossil hunter, has spent the past week leading a team of explorers through new Riversleigh. It's a hard slog. This kind of country, with its vast stretches of spinifex, punctured by the odd anorexic shrub, is why Australia is known as the harsh, brown land.

While satellite data might have led the team to the new deposit, the technology cannot replace the traditional method of locating fossils: a sharp eye and lots of walking. So far this trip the exploring team have located eight new sites with fossiliferous rocks. ''It's a lot of detective work,'' Bok Khoo, a masters student at the university, says. Ancient snail shells offer the first tantalising clue bones may be nearby. ''That tells us we're looking at rocks of the right age,'' Khoo says. Flow stones, rocks that have been shaped by water, are the next tell-tale sign. Almost all of Riversleigh's bones have survived through time because upon death their owners were petrified in water containing the mineral calcium carbonate.

''It's dissolvable, but readily re-precipitates into solid limestone rock,'' Arena says. ''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 about 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,'' Arena says.

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 the beginnings of mountain ranges in New Guinea and other places, as well as forcing parts of northern Australia to warp. This dramatic event, known as the Pine Creek Upwarp, exposed these long buried layers of limestone, providing a source of calcium carbonate. When animals then died in the region's mineral-rich lakes their remains were preserved when the limestone resolidified. 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 resolidifies into solid rock,'' Arena says.

Which brings us to Wholly Dooley pit. ''We think this is part of a cave deposit,'' Archer says. He uses his finger to draw a line across two large boulders where he suspects fossils are trapped. Limestone may dissolve in water, but as a solid rock it quickly destroys geological picks. The bearded explosives expert, Lizard Cannell, drills five holes into the boulders and fills them with thin, red tubes of explosives, the kind miners use to connect high-grade explosives.

''We're just going to do a little pop,'' Cannell says. After everyone retreats to a safe distance, licensed shot firer Chris Larkin detonates the charge. When the dust clears, Cannell inspects his work. The rocks have broken in exactly the right places. ''It's like slicing a fruit cake,'' he says. The well-executed blast uncovers even more fossils, which Archer inspects and declares them further support to his hypothesis that Wholly Dooley may fill some of the gaps in the continent's history book. His interpretation is based mainly on the many teeth the team have recovered from the site, all of which appear to be worn-down. ''That probably means there was more dust and grit in their diet,'' he says. ''And that means the environment was drier.'' The discovery of koala bones also suggests gum trees were becoming common in the landscapes, he says.

But 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 back to the university's labs where fossil preparators Dr Anna Gillespie and Dr Troy Myers will spend months dunking rocks into vats of weak acid to dissolve the limestone and reveal the hidden specimens. Loading The task Archer and his team face in studying and interpreting these fossils is massive, a process that will take months, likely years, and made possible with the ongoing support of the local Riversleigh community. As Cannell says: ''Understanding this place is like trying to put together a puzzle without the picture on the box.''