Meet a giant ancestor of wombats

Fossils from a giant marsupial that roamed central Australia during the late Oligocene, 25 million years ago, reveal a new relative of the wombat.

Scientists have named it Mukupirna, meaning “big bones” in the local Dieri and Malyangapa Aboriginal languages, and created a new family for it called the Mukupirnidea, as reported in the journal Scientific Reports.

Mukupirna is a remarkable and surprising addition to the fossil marsupial fauna of Australia; a representative of an entirely new family of wombats that we never knew existed until now,” says lead author Robin Beck from the University of Salford, UK.

The well-preserved skull and partial skeleton were discovered in 1973 along with a rich trove of fossils that a fluke change in local conditions revealed in the floor of Lake Pinpa in northern South Australia.

“It was an extremely serendipitous discovery because in most recent years the surface of this dry lake is covered by sands blown or washed in from the surrounding hills,” says co-author Mike Archer from Australia’s University of NSW, who was on the original expedition led by the late Dick Tedford.

“But because of rare environmental conditions prior to our arrival that year, the fossil-rich clay deposits were fully exposed to view. And this unexpected view was breathtaking.”

The fossils include skeletal remains of many different mammals ranging in size from a carnivorous marsupial the size of a mouse to the giant wombat-like creature, along with teeth and skeletons of extinct fish and bones of different waterbirds including ducks and flamingos.

Mukupirna’s skeleton was initially obscured by clay.

“We found it by probing the dry flat surface of the Lake with a thin metal pole, like acupuncturing the skin of Mother Earth,” says Archer.

Focussing on its fossil bones, Beck and the international team compared them to other living and extinct marsupials, especially those in the vombatiform group that includes koalas, wombats and their extinct relatives.

The 19.7 cm skull of Mukupirna nambensis, seen from below. The front is towards the top of the photograph. Credit: Julien Louys, Griffith University and Robin Beck, University of Salford.

With this information and a powerful supercomputer, they devised a new family tree, “to see where Mukupirna fits in the overall story of vombatiform evolution,” says Beck.

They estimate the creature’s body to have weighed between 143 and 171 kilograms – the size of an American black bear or five times bigger than modern wombats – yet it had the gentle teeth of a herbivore.

Although it most closely resembles wombats, Mukupirna has its own unique features.

The shape of its cheek teeth is midway between those of wombats and those of a fossil group called wynyardiids, indicating wombats likely evolved from these ancestors.

Mukupirna also appears to have been a digger, “with powerfully muscled forelimbs and hands that can work like shovels”, says Beck.

This suggests the lineage leading to modern wombats had already evolved digging adaptions by 25 million years ago. Wombats evolved distinctive features like ever-growing cheek teeth and further digging specialisations much later, which could reflect the evolution of burrowing.

Because of its large size and less specialised digging adaptations, it’s likely that Mukupirna didn’t burrow. “Instead, it may have used its powerful forelimbs to dig up roots and tubers to feed on,” says Beck. “This behaviour is also seen in modern wombats.”

Mapping it onto the vombatiform tree revealed how this family’s body size evolved through time. Adding Mukupirna shows that some had already evolved bodies heavier than 100 kilograms early in their evolutionary history, and that large bodies evolved repeatedly in different lineages.

The largest known member of this extraordinary family of critters was the Dipodotron, which weighed more than 2000 kilograms and lived up to at least 50,000 years ago.

“The living koala and wombats are therefore the last remnants of what was once a much bigger and more diverse group,” says Beck, “and therefore are even more precious and in need of protection.”

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