Fossils of 'big boned' marsupial shed light on wombat evolution

Mukupirna, discovered in the Lake Eyre basin in 1973, was probably five times the size of living wombats

Fossils of a huge, hairy creature with shovel-shaped hands and unusual teeth could hold clues to the evolution of today’s wombats, researchers say.

They say the fossils belong to a new member of a group of marsupials called vombatiforms, and one of the earliest such creatures yet discovered.

“Vombatiforms is this really interesting group; the only living members are the koala and the [three living species of] wombats,” said Dr Robin Beck, the first author of the study, from the University of Salford in the UK.

Beck said extinct members of the group included giant marsupial lions and huge rhino-sized beasts called diprotodons that lived about 2m years ago.

The newly reported animal is believed to be have lived about 26–25m years ago, around the time Australian marsupials first reappear in the fossil record after a dearth following fossils dating back to 55m years ago.

Fossil skull of the Mukupirna.
Fossil skull of the Mukupirna. Photograph: None

The skull and partial skeleton of the creature were unearthed in the Lake Eyre basin of South Australia. “This is a big animal, it is probably around 150kg in weight, so about five times the size of a living wombat,” said Beck.

Its bulk gave the team inspiration for its name: the creature has been called Mukupirna nambensis – meaning “big bones” in the Diyari and Malyangapa Aboriginal languages.

The fossils were discovered in 1973, buried under Lake Pinpa, a salt lake east of the Flinders Ranges. Beck said detailed scrutiny of the fossils had only been completed now.

Palaeontologist Prof Mike Archer, of the University of New South Wales, said the team was lucky to find the extraordinary creature, which was buried under sand and clay. He said a unique combination of weather events revealed the river bed.

“In most years the surface of this dry lake is covered by sands blown or washed in from the surrounding hills,” Archer said. “For the first time that anybody had seen, the sand that covered the clay had blown away. The clay itself, we now understand, is 25m years old. Every now and then, sticking up through the clay would be a larger bone.”

There, along with his colleague Prof Dick Tedford, they found a partial skull and most of the skeleton of Mukupirna.

However, Tedford died before a taxonomy of the creature could be completed, and its remains were kept in the American Museum of Natural History in New York, where Tedford worked.

Archer told Guardian Australia the fossils remained there for decades.

“I didn’t want to feel like a ghoul and go knock on the door and get it out,” he said.

But years later, one of his PhD students, who was “unaware of the history”, asked if he could work on it. Together with other researchers, they completed a classification of the animal.

“It has taken a long time to actually land on the deck now as a described new animal,” Archer said.

Writing in the journal Scientific Reports, Beck and colleagues from the US and Australia report how the analysis allowed them to draw up a new evolutionary family tree of vombatiforms, with Mukupirna representing a whole new family of the animals.

The results reveal that Mukupirna is the closest fossil relative of the wombat family as a whole, suggesting wombats and Mukupirna evolved from a common ancestor.

Mukupirna is not the direct ancestor of wombats, but it does give us an idea of what the ancestor of wombats probably looked like in terms of its teeth and skeleton,” Beck said. One conclusion, he said, was that this common ancestor was probably a digger, since Mukupirna – and wombatsshow adaptations for the task.

“The forelimbs [of Mukupirna] show clear evidence of powerful muscles ,” Beck said, noting that its hands were shovel-shaped, making them effective tools for turning up roots and tubers. He said wombats had even more specialised limbs for digging and were effective burrowers – something Mukupirna is unlikely to have done, not least because of its size.

The discovery of the Mukupirna fossil at Lake Pinpa in South Australia in 1973.
The discovery of the Makupirna fossil at Lake Pinpa in South Australia in 1973. Photograph: None

Beck said the molars of Mukupirna were not capable of growing throughout life as seen in modern wombats. However, its teeth show features that fall between those of wombats and another, previously known, group of early vombatiforms called wynyardiids, shedding further light on the common ancestor of Mukupirna and wombats.

Mukupirna probably evolved from a wynyardiid-like common ancestor that it shares with wombats,” said Beck.

The evolutionary family tree also revealed very large body sizes, more than 100kg, evolved independently at least six times among vombatiforms. However, the common ancestor of all such creatures was probably only the size of today’s koalas. “From this small ancestor we have had multiple origins of large body size,” said Beck.

Archer said: “Of all the animals we are able to compare this with, it is closest to wombats. But it is as different to any wombat as a wombat is to a koala. It really merited its own family established for it.”

Prof Justin Adams, a palaeontologist at Monash University who was not involved in the research, welcomed the study. “It’s not every day that an entire new [family]of extinct mammals is announced,,” he said, adding that the findings on the evolution of large body size among vombatiforms were particularly interesting.

“This will have quite a bit of impact on how we consider the evolution of the group, past ecosystems, and the ecology of these species that led to such significant changes in their body size over time,” he said.

Contributors

Nicola Davis and Naaman Zhou

The GuardianTramp

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