Two of the world's lost prehistoric giants, a rhino-sized Australian marsupial and a buffalo-sized South American rodent, were the largest known mammals of their kind and much larger than previously thought, according to two new studies.

The studies, published in this week's issues of the British journal Biology Letters and the U.S. journal Science, re-evaluated the weights of the two extinct creatures, using a new calculation method based on the sizes of their major limb bones relative to those of other animals.

The findings confirm that diprotodontian marsupials - which include kangaroos, wombats and possums and the mouse-sized mountain pygmy possum - have the largest known size range of all the mammals. And that rodents - which include mice rats, squirrels and guinea pigs - now have the second largest size range.

In the first study, a team led by Dr Stephen Wroe, of the University of Sydney, Australia, calculated that the average weight of the massive wombat-like Diprotodon optatum - which only died out about 45,000 years ago - was about 2.5 tonnes, which is more than double the previous estimate.

In the second, a team led by Dr Marcelo R. Sanchez-Villagra, of the University of Tübingen in Germany, found that a semi-aquatic Venezualan Phoberomys pattersoni - jokingly dubbed "guinea-zilla" - which lived about eight million years ago, was by far the largest known rodent and probably weighed about 700 kg, or 15 times more than the largest living species, the South American capybara.

Wroe told ABC Science Online that the new weight estimate for the diprotodon used a method that is widely regarded as among the most reliable because it is the first to rely on an empirical methodology. The method relies on mapping the relationship between bone circumference and body weights from a range of contemporary animals. This is used to develop an equation that can then calculate the weight of prehistoric animals from their fossilised bones.

"Our predictive equation was based on data from 50 species that ranged in size from mice to a five-tonne bull African elephant," Wroe said. "It also included 17 quadrupedal [four-legged] marsupials. In the case of diprotodon we're fortunate in that the animal is known from many complete or near-complete remains.

"The previous figure for body mass of 1,175 kg was an educated guess, no more no less. It was not the product of any empirical methodology. However, in fairness to the authors of this estimate, educated guesswork has long been the basis of most predictions for body mass in fossil mammals. The application of quantitative methodologies is a fairly recent phenomenon."

The new estimate means that the largest diprotodons were roughly equal in size to the largest rhinoceros and that among the living land mammals only elephants are more massive.

A rethink on death of Australian megafauna

Wroe said the finding undermined a long-held theory that Australia was a "wide but biologically stunted land" whose prehistoric megafauna weresmaller than those on other continents because its soils were so unproductive that they could not support very large herbivores or the carnivores that would prey on them.

"Large body size can actually be a response to low productivity," he said. "For example, in modern African savannahs, the highest relative abundance of elephants is actually in areas of very poor soil quality. In fact, the largest living terrestrial animals are the elephants of the Namibian desert, anything but a productive environment."

He also doubts that early humans in Australia - who have been accused of pushing such animals into extinction in an overhunting "blitzkrieg" - would have had technology efficient enough to bring down such large creatures.

In a commentary in Science in relation to the giant rodent discovery, British biologist Dr R. McNeill Alexander, of the University of Leeds, argues that large size can actually be an advantage for animals with a poor quality diet because they can more efficently extract energy from such food.

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