"Educated guesswork has long been the basis of most predictions for body mass in fossil mammals," said Wroe. Judging body mass from skeletal material remains can be problematic, he added. Experts estimates can differ by as much as a factor of ten. Wroe noted that many mounted skeletons don't take into account the additional 15 percent that discs and fluid found between spinal vertebrae add to the head and body length in living animals.
To improve upon the usual guesswork, Wroe and colleagues in the University of Sydney's biology and archaeology departments used new predictive mathematical methods. These methods allowed researchers to compare 17 specimens of Diprotodon with 50 different marsupial and mammal species. They compared the relationship between bone size and body mass in four-legged modern marsupials, mice, elephants, and other animals.
Their calculations suggested that an animal with a skeleton of Diprotodon's size would have weighed substantially more than the figure of 2,590 pounds (1,175 kilograms) previous researchers arrived at.
Some scientists have predicted that low levels of soil nutrients in Australia limited the amount of food available to mammals and therefore restricted them to a small size.
The relationship between body size and the environment may not be so clear-cut, however. Wroe noted that the largest living land animals today are African elephants that scrape a living in the continent's harsh Namibian desert. The elephants' large gut allows them to digest plant matter of very low nutritional value that smaller mammals are unable to process.
Mike Archer, vertebrate paleontologist and director of the Australian Museum in Sydney, said: "To have [Diprotodon] one of the largest mammals in the world produced and abundant in Australia, makes it clear that there is nothing intrinsically limiting about nutrient deficiency when it comes to potential size in mammals."
"Wroe has certainly demonstrated that at least one type of marsupial was a lot bigger than had been suspected, throwing some of these ecological hypotheses into doubt," agreed Christine Janis, mammal paleontologist at Brown University in Providence, Rhode Island.
Janis noted, however, that most marsupials found today remain relatively small in size, with no living representatives reaching the weight of a medium-sized antelope or deer. And while the majority of non-marsupial mammals are very small (rodents and bats for example), many weigh well over 220 pounds (100 kilograms), a fact in contrast to marsupials, she said.
Wroe added: "Very large [introduced] feral animals such as camel and buffalo are doing all together too well in modern Australia".
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