Fossil fragments rearrange marsupial evolution

Two fossil fragments found in Queensland look likely to overturn the conventional theory about the evolution of marsupials, which holds that there was a single migration from the part of the Gondwana ‘supercontinent’ that became South America to the part that became Australia.

One of the fossils, found at the Tingamarra site in south-eastern Queensland, is a 55 million-year-old ankle bone from a mouse-sized marsupial previously known only from South America.

The second is a tooth, which derives from a formerly unknown species that shows similarities to fossils found in South America and, surprisingly, North Africa.

“The origins of Australian marsupials suddenly got a lot more complicated,” said palaeontologist Dr Robin Beck from the University of NSW.

“All the species of modern-day marsupials here are quite closely related. The species represented by the ankle bone belongs to an entirely different group - a group that we know lived in South America but, up until now, we thought never made it to Australia. The tooth is more of a mystery: are its origins in South America, Africa or somewhere else?

“It is impossible to explain the presence of these new fossils in Australia using the single dispersal model. Instead, there may have been multiple movements of marsupials between South America and Australia.”

The similarities between the fossil tooth and those of an extinct species recorded in Tunisia remain the tantalising subject of further research.

For the moment, though, Dr Beck thinks the tooth and the ankle bone - which he described in the German journal Naturwissenschaften - provide fascinating new information about the history of marsupials in Australia.

“They mean that extinction has played a much bigger role in the story than we previously thought,” he said. “Some species became extinct in Australia but survived in South America. And perhaps the opposite is also the case - will there be discoveries in South America of typically ‘Australian’ fossils?”

Dr Beck said climate change might have played a role in the disappearance of the ‘ameridelphian’ marsupials. The Tingamarra fossil deposits date from 55 million years ago, when the climate was very warm.

“It would have been lush rainforest,” he said. “Since then, the climate in Australia has been gradually cooling.”

The work has been published in the journal Acta Palaeontologica Polonica.

Dr Beck is one of 12 early-career scientists unveiling their research to the public for the first in Fresh Science, a national program sponsored by the Australian Government through the Inspiring Australia initiative.