It is hard to stand in this dry, spinifex country and believe it was once dripping rainforest, but the fossils insist. Twenty-five million years ago, as the fragments of the ancient supercontinent Gondwana drifted apart and Australia crept north, this was a lush, wet forest teeming with life. The fossiliferous limestone laid down here, near the Gregory River, recorded that world and every world that followed as the climate dried and the rainforest gave way to grassland. More than 250 species have been described from the roughly 100-square-kilometre area, many of them known from nowhere else on the planet. Scientists, mostly from the University of New South Wales, have pulled fossils from more than 200 separate sites, and they are still digging.

The cast that emerges from the Riversleigh stone is the stuff of nightmares and wonder. The marsupial lion stalked here, a pouched predator armed with gigantic blade-like premolars, the most specialised meat-cutting teeth any mammal has ever evolved. Giant pythons coiled through the forest, alongside crocodiles and lizards. And there were the great flightless birds palaeontologists nicknamed the "demon ducks of doom," hulking relatives of waterfowl that towered over a person. These were not gentle ancestors of today's gum-tree fauna. They were the apex hunters and giants of a continent evolving in isolation, and Riversleigh is the keyhole through which we glimpse them.

Some Riversleigh fossils rewrite what we thought we knew. The site yielded the 15-million-year-old skull of Obdurodon dicksoni, an ancient platypus that, unlike its toothless modern descendant, still carried functional molar teeth in its bill, a window straight into the evolution of one of the strangest animals alive. In 1993, researchers cracking open a previously unknown cave found the skulls of Nimbadon, a sheep-sized, tree-climbing relative of the wombat that thrived around 15 million years ago and died out, perhaps twelve million years back, as the climate turned against it. The cave deposits proved especially rich in bats; ancestors of the recently extinct thylacine surfaced here too, including a well-preserved skull of Nimbacinus dicksoni that let scientists reconstruct how the animal hunted. Even the koala's long retreat from rainforest into dry eucalypt scrub is written in these bones. The fossils are sorted into four faunal zones spanning the Late Oligocene to the Late Miocene, a stacked calendar of deep time running from roughly 28 million years ago to about 5 million.

Riversleigh's importance is recognised at the highest level. In 1994 it was inscribed on the World Heritage List as part of the Australian Fossil Mammal Sites, sharing the honour with the Naracoorte Caves in South Australia, two places that together chart the evolution of Australia's unique animals across tens of millions of years. The fossil field lies within Boodjamulla National Park, on the country of the Waanyi people, whose connection to this land long predates any scientific expedition. At the public D Site, a walking track threads past illustrated signs and weathered limestone, letting visitors stand where the marsupial lions hunted and feel, in the silence and the heat, the unimaginable depth of time beneath their feet.