Joshua Sokol

Wombats and koalas stand out as bizarre animals even in a continent famed for bizarre animals. They are also each other’s closest relatives.

Koalas munch on eucalyptus, resemble living teddy bears and, like Australia’s other imperiled native fauna, they need occasional rescuing. Wombats poop in cubes — yes, cubes — that they leave out and even stack to mark their territory. As for the animal itself, picture chonk incarnate, a burrowing ball of fuzz and fat powered by muscular little stub-legs.

Now multiply that five times. That’s the size of a new long-lost member of the same animal group, Mukupirna nambensis, a mega-wombat that tipped the scales at well over 300 pounds. Scientists believe it scrounged around in the rainforest soil of Australia some 25 million years ago.

“I would compare it to a black bear,” said Robin Beck, a paleontologist at the University of Salford in England, who described fossils of the wow-inducing wombat on Thursday in the journal Scientific Reports.

The hefty species is the newest member of a supersized menagerie. For millions of years up to the present day, big, unique marsupials flourished on Australia and New Guinea, isolated from the rest of the world.

Koalas and wombats are the only surviving remnants of an otherwise extinct group called the vombatiforms, “wombat-like” animals that were more diverse than any other type of marsupials.

Beyond the new Mukupirna, other extinct vombatiforms got even bigger, like Diprotodon, a herbivore that weighed almost as much as a rhinoceros. Or a mysterious horse-sized animal that had a trunk. Or the “marsupial lion,” a carnivore comparable to African lions today.

“You would think they could do anything, they could take over the world,” said Vera Weisbecker, an evolutionary biologist at Australia’s Flinders University who was not part of the research team. “They didn’t, so that’s frustrating.”

Credit…Julien Louys/Griffith University and Robin Beck/University of Salford

Because very few high-quality fossil sites have been discovered on the continent, scientists have long struggled to learn about these lost animals.

Mukupirna, which means “big bones” in the Diyari language spoken near where it was found, languished for decades before being studied. It was dug up in July 1973 by paleontologists working on the dry bed of Lake Pinpa in South Australia.

“All of us were desperately anxious to find any ancient fossils of Australian animals, because they are so rare,” said Michael Archer, then a doctoral student on the expedition who now works at the University of New South Wales in Australia.

The original team held a thin metal rod to drive down into the clay. Sometimes it passed through cleanly. Other times they would hear a clang from buried bone. Nearer to the surface, they uncovered and later published an alien ecosystem: teeth from lungfish, possums and freshwater dolphins; bones from flamingos, kangaroos that galloped instead of hopping and a supersized koala, among others.

Big fossils like Mukupirna that clanged against the pole were wrapped in plaster and shipped to the American Museum of Natural History in New York. There it sat for over four decades, until Dr. Beck and his colleagues obtained high-resolution pictures and looped in Dr. Archer.

From its skull, the team found teeth that resembled those of a baby wombat, allowing them to include Mukupirna in a family tree of other large wombat relatives. They also found the animal’s arms would have made it an efficient digger, allowing it to scratch around for roots and tubers, although it probably would not be able to burrow like modern wombats.

“That would be one hell of a burrow,” Dr. Archer said.

Other teams are now excavating the same Lake Pinpa site, so more fossils may still be uncovered. But as for whether Mukupirna shared the cubic poop of modern wombats, science hasn’t yet offered a definitive answer.

“Sadly, neither the poo nor the intestines have been preserved,” Dr. Beck said. “I couldn’t rule this out.”

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