Ice Age Marsupial Topped Three Tons, Scientists Say

John Pickrell in England
for National Geographic News
October 17, 2003
A giant, wombat-like marsupial that roamed Ice Age Australia, may have
been much bigger than experts previously believed.

The beast, known as Diprotodon optatum, may have been larger than all but the biggest hippopotamus or rhinoceros, according to the first rigorous experimental estimate of its bulk by scientists in Sydney, Australia.

Experts now believe that Diprotodon weighed in at a whopping 6,142 pounds (2,786 kilograms), or nearly 32 times as heavy as the largest marsupial alive today, the red kangaroo (Macropus rufus). Red kangaroos rarely weigh more than 187 pounds (85 kilograms).

Diprotodon has been known in the fossil record to scientists since the 1830s. Previous estimates suggested Diprotodon was slightly bigger than a large cow.

The animal lived during the Ice Ages of the Pleistocene Period—the era from two million to 10,000 years ago. The beast went extinct sometime between 45,000 to 35,000 years ago, soon after human colonization of Australia.

"Incredible Boredom"

Diprotodon survived by browsing on huge quantities of low-quality foliage, similar to today's koala. While not strictly related, the ancient animal appeared "basically [like] a cross between a wombat and a hippopotamus," said University of Sydney paleontologist Stephen Wroe, lead researcher behind the new study.

Diprotodon led a "life of incredible boredom, punctuated by the odd moment of panic when a whopping great marsupial lion crept up on it," Wroe said.

Scientists have long squabbled as to the reason for the pouched giant's demise. Human hunter-gatherers and their burning practices, along with the diseases and pest species that accompanied them, have been implicated in many large-animal declines. Another possible explanation is the drier climate ushered in at the end of the last Ice Age.

A generation of students has been taught that Australian habitats have been too dry, barren, and nutrient-poor to support the kind of large mammals that punctuate the ancient natural history of other continents. But the new body size estimate for Diprotodon, detailed online last month in the science journal Biology Letters, may challenge that theory.

"The [earlier] smaller body mass estimate for Diprotodon, propped up the notion that Australia was a biologically stunted land, unable to support large mammals," said Wroe. "Our work obviously undermines this interpretation."

Tipping the Scales

"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.

Complex Relationship

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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