Thursday, January 7, 1999 Published at 21:11 GMT
Big bird clue to mass extinction
Genyornis newtoni was a ponderous, flightless bird
They have come to the conclusion after studying the fossilised eggshells of an enormous flightless bird called Genyornis newtoni
The theory, put forward in the latest edition of Science magazine, is a controversial one.
For decades, scientists have argued about the likely cause of the mass extinctions that rocked ecosystems around the world during the Quaternary period (1.8 million years ago to the present). These events were mild or absent in some regions of the world, such as Africa and Southeast Asia, moderate in others such as Europe, and extreme in the Americas, Australia, and many oceanic islands.
The Australian mass extinction saw the loss of many large terrestrial vertebrate species - more than 85% of the continent's large animals may have disappeared. They included carnivorous kangaroos and a horned tortoise nearly the size of a small car.
However, the search for an explanation to the mass extinction has always been hindered by the difficulty in nailing down the actual timing of the event, largely because its age pushes the limits of radiocarbon dating.
Gifford Miller of the University of Colorado and his colleagues appear to have jumped this hurdle by using a variety of alternative dating techniques not based on radiocarbon isotopes. They dated the extinction of Genyornis newtoni, a ponderous flightless bird with thick, short legs that weighed around 200 pounds - twice as much as the modern day emu.
They analysed the birds' fossilised eggshells and set the date of Genyornis' sudden disappearance at 50,000 years. This date loosely matches the time of the Aborigines' arrival to the continent as indicated by the most reliable evidence yet available.
This was also a time of moderate climate change, making it unlikely that climate played a role in Genyornis' extinction.
Gifford and his team conducted further tests on the shells to reveal clues about the bird's diet. It appears to have been a relatively picky eater, relying on extensive shrubland for food. Many of the animal species that disappeared around the same time as Genyornis also fed on these shrubs and trees.
The team think that if humans disrupted the natural fire cycle by burning the landscape periodically it could have destroyed the animals' feeding grounds.
"I think we have compelling circumstantial evidence that the Genyornis extinction date is applicable to the vast majority of Australian megafauna (large animals)," said Miller. "There are certainly no secure dates to refute this supposition."
"We suspect the systematic burning by the earliest colonisers - used to secure food, promote new vegetation growth, to signal other groups of people and for other purposes - differed enough from the natural fire cycle that key ecosystems were pushed past a threshold from which they could not recover."