The largest marsupial that ever lived was even bigger than we thought, Australian scientists say.
By Jonathan Amos
BBC News Online science staff
New information on the immense wombat-like Diprotodon optatum indicates it reached more than two and a half tonnes on average - nearly double some previous estimates.
Human hunting does not tell the whole story (Image by BBC Bristol Design)
If that was the case, researchers say, it gives the lie to a popular theory that weak vegetation growing on Australia's poor soils during the last ice age stunted the continent's big beasts.
With its gigantic bulk, D. optatum would also have been a mighty handful to hunt, suggesting that rapid overkill by humans was probably not the main cause behind its extinction more than 30,000 years ago.
Size for size
The new assessment of this pouched behemoth is provided by Dr Stephen Wroe, from the University of Sydney, and colleagues.
They did their calculation by comparing the beast's fossil remains with the known bone dimensions and body masses of several living marsupials and other mammals.
Lived 1.6m to 40,000 years ago
Two metres tall at the shoulder
Grazed on leaves and shrubs
Closest kin: Wombat and koala
"The equation we used to predict the weight included animals of the size of a mouse up to a big bull elephant," Dr Wroe told BBC News Online.
"This is considered to be among the most reliable methods. It's great if you have the whole skeleton - as we have D. optatum. It doesn't work with the many species for which we only have teeth."
The analysis pointed to the average D. optatum tipping the scales at almost 2,800 kilograms.
Have short gestation periods
Newborns are underdeveloped
Body pouch to carry young
260-plus species identified
This would have made the creature heavier than the largest rhinoceros. Only elephants among living terrestrial mammals can claim to be more massive.
Australia had a remarkable and distinctive collection of ice age beasts.
Its megafauna, as researchers like to call them, even included a marsupial lion (Thylacoleo carnifex).
But the nagging suspicion in scientific circles is that the Australian beasts were never quite in the same heavyweight league as the creatures found elsewhere in the world during this period in Earth history.
It has been suggested that poor soils on the continent produced weak vegetation; the herbivores were smaller and so too were the carnivores who preyed on them.
"Our research flies in the face of this argument," Dr Wroe said. "Go to Africa and you will find the biggest elephants are in Nambia, in the desert area, which has extremely low productivity. The smallest elephants are in the forest.
"This idea that if you throw more food into the system the animals will get bigger is over-simplistic in my view."
Whatever the truth about this size argument, what is certain is that right across the globe all of the great ice age animals went extinct very quickly.
By 11,000 years ago, the end of the Pleistocene Epoch, virtually all the beasts had been wiped from the face of the Earth.
We - the human species - are one of the chief culprits for this mass extinction. As our kind spread around the globe, it is thought we systematically hunted out many of the big game.
But Dr Wroe doubts this can fully explain the demise of D. optatum and some of the other Australian megafauna species.
Long gone: Woolly rhino, mammoth and giant elk
He said the weapons technology required to bring down these big animals efficiently was not evident on the continent until 6,000 years ago, long after the beasts had perished.
"I wouldn't say early aboriginals couldn't kill them but they would have been less efficient hunters of this species than say the Clovis Indians, with their stone spear-points, were of mammoths in North America.
"This data doesn't take humans off the hook but it does suggest the reasons behind the extinction of this species are likely to be more complicated than some have said."
Dr Wroe said that major climatic changes taking place in Australia at that time had to be a consideration.
The D. optatum research is published in Biology Letters, a journal of the Royal Society, the UK's academy of science.