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Study places mass extinction in Australia 46,000 years ago

Study places mass extinction in Australia 46,000 years ago

(June 7, 2001)

A team of Australian scientists had made the best estimate yet for the date of a mass extinction that wiped out most of Australia's large mammals, reptiles and birds including some early kangaroos - data that suggest the disappearance was related to the arrival of humans.

Writing in the June 8 issue of the journal Science, the team - including a University of California, Riverside paleontologist - reports the extinction of many large animals in Australia occurred during an interval of time centered around 46,000 years ago, about 10,000 to 15,000 years after the arrival of humans.

"The reasons behind the extinction of most of Australia's large terrestrial vertebrates during the last Ice Age have been hotly debated for more than a century," said Gavin Prideaux, a postdoctoral researcher in UCR's Department of Earth Sciences. "Up until now, compelling evidence in favor of one possible cause or another proved elusive, because we haven't known precisely when these animals became extinct."

The Australian study dates the mass extinction of "megafauna" - animals weighing more than 100 pounds - to about 46,400 years ago, with a range of uncertainty from about 39,800 to 51,200 years ago. The timing of the extinction rules out an Ice Age drought 19,000 to 23,000 years ago as the cause of the die-off, as some scientists have proposed. "A major hitch with this idea has been finding a way to explain why many animals that were highly adapted to dry conditions would have become extinct during an arid phase," Prideaux said. "Our study shows that all of the largest species had actually disappeared well before this peak in cold, dry conditions."

The researchers were led by Richard G. Roberts of the University of Melbourne. Prideaux, one of two scientists on the team now based at U.S. universities, served as a fossil specialist. Linda Ayliffe, now a postdoctoral researcher at the University of Utah, measured the radioactive decay of uranium-234 into thorium-230 to determine ages for the remains of many of the animals studied.

The mass extinction studied by the team killed all Australian land mammals, reptiles and birds weighing more than 220 pounds and many other animal species weighing more than 100 pounds.

Animals dying off included giant snakes and lizards, large ostrich-like birds and a wide variety of marsupials - animals that complete their development in their mother's pouches. The Australian marsupials that went extinct 46,000 years ago included so-called marsupial lions, as well as range of leaf-eating kangaroos and rhino-sized marsupials.

The team studied the fossils of the animals and the rocks in which they were buried at 27 sites across Australia and one in West Papua, which was connected to Australia when sea levels were low during glacial periods.

Prideaux's work focused on a site containing a rich assemblage of mammals from the latter part of the Pleistocene - an epoch of geologic time between 10,000 and 1.8 million years ago. The site, in the extreme southwest corner of Australia, was a cave into which many animals fell, debilitating themselves upon landing or starving to death with no chance of escape.

Dating of the fossil-rich sediments at that site indicates the last "megafauna" disappeared about 46,000 years ago, corroborating the team's results by inferring that the pattern of extinction in the southwest of Australia was similar to that of the rest of the continent.

Predation and habitat alteration by humans, who arrived in northern Australia 10,000 to 15,000 years preceding the extinctions, were probably decisive in the demise of the Australian "megafauna," according to Prideaux. "As yet, though, we can't say how rapidly the extinctions took place, nor can we rule out climate change in the form of the southward extension of the summer monsoon between 65,000 and 45,000 years ago as a compounding factor," he said.

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