By Tim Johnston Thursday, January 25, 2007 SYDNEY Scientists in Australia say they have found a trove of prehistoric animal bones — including those of a large marsupial lion — that provides strong evidence that it was human beings and not climate change that killed 90 percent of the continent's largest animals about 45,000 years ago. In a paper published Tuesday in the journal Nature, scientists led by Gavin Prideaux of the Museum of Western Australian in Perth say they found not only the first complete skeletons of the largest carnivorous marsupial, Thylacoleo carnifex, but also the remains of eight so-far unknown species of kangaroo in a cave system under the arid wastes of the Nullarbor Plain in South Australia. The remains, from about 70 different species, date from between 800,000 and 400,000 years ago, when a "window" to the caves was open for a 10,000-year period. The scientists think the entrance to the deep, vertical-sided caves opened up for a period when the animals fell in and died before being sealed by a boulder, keeping out both air and moisture, which would have accelerated the decomposition of the bones, and scavengers who might have damaged the remains. A group of cavers discovered the location, which has not been disclosed to the public, in April 2002 while exploring. Prideaux said the preservation of the remains was remarkable. "You really get a sense of the animal just collapsing and taking its last breath," he said. But the real importance is scientific. "This is a paleontological Rosetta Stone for the Pleistocene period," he said. Among the most important conclusions is that the rainfall in the Nullarbor Plain when the animals were alive was not significantly different from the rainfall today, leading to the conclusion that it is unlikely that climate change alone explains the extinction of so many species. Professor Richard Roberts, a geochemist at the University of Wollongong who dated the bones, said that by comparing the dates of the remains with evidence of rainfall at the time, the scientists established that different species had managed to survive relatively major changes in the climate. They argue that if the Nullarbor animals were well adapted to dry conditions for at least 400,000 years before they disappeared, then it is unlikely they succumbed to Ice Age aridity. Their conclusions deal a blow to the longstanding argument that the extinction of Australia's giant native fauna, the so-called megafauna, was caused by natural changes in the environment and not humans. "I think we can forget about that as an explanation, because they had already survived the worst that nature could throw at them," Roberts said. "But what they hadn't counted on is the arrival of a completely new species that would eat them." He added, "The only new element was us: man." The major extinction of Australia's megafauna between 40,000 and 50,000 years ago also coincided with the arrival of humans. Roberts dismissed the argument that the relatively small number of people, armed with only the most basic of weapons, could not have inflicted so much damage to so many species. He said low-level hunting of animals with slow reproductive cycles, combined with the widespread use of fire to clear ground and drive animals into traps, would have been sufficient to push animal populations below a number needed to keep species viable. Among the casualties of the megafauna were 3-meter, or 10-foot, kangaroos, Thylacoleo carnifex, or marsupial lions, the Dromornis stirtoni, the largest bird ever known — it weighed half a ton — and the rhino- sized herbivore Diprotodon optatum. The researchers unearthed a dozen complete skeletons of Thylacoleo, enabling them to reconstruct what has turned out to be a remarkable animal. It developed an outside slashing claw on its "thumb," which allowed it to hold down the prey while it worked on killing with its incisor teeth. But Prideaux said that the research team might have only just scratched the surface of what the caves have to offer. Prideaux said there could still be a fossil record going back millions of years just waiting to be uncovered.