The Stone Menagerie: The Limestone Beds at Riversleigh, in Australia's Northeast, Contain One of the Most Spectacular Assemblages of Vertebrate Fossils Found Anywhere on Earth. Spanning a 25-Million-Year Period, These Ancient Bones Paint a Uniquely Detailed Picture of Everyday Life in Prehistoric Australia

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Beneath the baking hot plains of northeastern Australia there is a natural museum housing such a wealth of fossils that it almost defies imagination. So abundant are the preserved remains in the ancient rocks at Riversleigh in northwestern Queensland that in one instance the jaws and skulls of more than 60 distinct types of mammal were recovered from a single cubic metre of ancient pond floor.

Before palaeontologists started to delve into this remarkable treasure trove, digs across the continent had provided a mere 40 fossil mammal assemblages, comprising roughly 70 species. Together, they painted a pretty incoherent picture of prehistoric Australia, with large numbers of animals left with no known ancestors at all. In fact, the only other continent that had a worse fossil record was Antarctica.

Riversleigh changed all of that. Its rocks have supplied the remains of thousands of animals representing hundreds of different species. Around half of the fossil mammals known from Australia have come from a single Riversleigh site and half of these were unearthed in a single hour.

Fossils were first discovered at Riversleigh in 1901, but for a variety of Reasons--including the fact that it was so difficult to extract them from the limestone--it was another 60 years before anyone returned to the area to look for more. The breakthrough came in 1983, when a particularly rich site known as Gag was discovered. Palaeontologists have since identified 50 new mammal species from this site alone and identified more than 250 individual fossil-rich sites over an area of about 50 square kilometres.

Because of its unrivalled richness, the expanse of time covered by its record and the quality of its fossils, Riversleigh was incorporated into Lawn Hill National Park in 1992 and declared a World Heritage site in 1994.

Today, the driving force behind the preservation and exploration of the site is the current director of the Australian Museum, Professor Mike Archer. According to Archer, the discoveries enable us to see "the other side of the mirror". "What is so important about Riversleigh is that it documents what happened on an isolated continent that was the second natural laboratory in the world, the rest of the planet being the first," he says.

"During periods of low sea level, the northern continents--including Africa and South America--periodically linked up. As animals moved back and forth, you got a kind of homogenisation. This mingling of animals in the other continents has made them, in effect, one natural experiment. This is in contrast to Australia, which for the past 52 million years appears to have been isolated and has never been invaded, except by one or two minor groups," says Archer. "Documenting the history of Australia means you get a second look at what evolution does when it is confronted by a world to be filled by beasts."

But what makes Riversleigh so special? Why are its rocks so full of fossils? The main clue is that the most common rocks in the area are Cambrian limestone--about 530 million years old. The fact that little other rock has formed above this ancient bedrock suggests that during periods when the oceans invaded the Australian interior this area was left high and dry--a refuge that would have become packed with a menagerie fleeing the floods. As these creatures fought, hunted and died in close proximity to one another, their bones would have settled into pools and lakes that had formed in the original limestone. When the seas receded, these water bodies themselves eventually turned into limestone, the bones encased within. Animals falling into the caves and sinkholes that inevitably form in limestone areas would also have helped to concentrate the bones.

The end result is a snapshot of the ancient fauna from hundreds or thousands of kilometres around.

Riversleigh's fossil sites cover three periods: the Oligo-Miocene (25-10 million years ago), Pliocene (approximately five million years ago) and Pleistocene (around 40,000 years ago). …