Scientists measure the sands of time to age Shirley the diprotodon fossil

Palaeontologist Adam Yates has been studying the fossil of a wombat-like marsupial known as the diprotodon. (ABC News: Xavier Martin)

More than a decade ago, an outback cattle station worker who was out searching for crocodiles stumbled across a different type of prehistoric creature.

“They thought they’d found a dinosaur, just because there were these big bones that were petrified in this riverbank,” Darwin historian Jared Archibald said.

Half buried in the red soil of a large riverbank at Auvergne Station, 300 kilometres south-west of Katherine, was the rare fossil of a giant wombat-like marsupial that roamed Australia thousands of years ago.

The now-extinct diprotodon is believed to be the largest marsupial that ever lived, and this fossil is the only one to have ever been found in the Northern Territory.

Teams were called in to dig the heavy bones out of the ground, carry them up a steep five-metre embankment, and transport them hundreds of kilometres to the museum in Central Australia, where they’ve been sitting, encased in protective plaster.

Until now.

She’s known fondly as Shirley, and scientists have finally discovered her age.

“As well as being the most northerly diprotodon found in Australia, it is one of the oldest ones,” Mr Archibald said.

The massive marsupial dates back to between 112,000 and 123,000 years old.

Shirley’s age determined by OLS dating

Lee Arnold is an associate professor in earth sciences at the University of Adelaide and a specialist in geochronology, tasked with finding out Shirley’s age.

He said Shirley was so old, scientists weren’t able to use the typical method of radiocarbon dating.

Using a technique called optically stimulated luminescence (OSL) dating, Dr Arnold measured the age of individual sand grains from around the fossil.

“Using OSL dating, we were able to say when that sediment was last exposed to daylight, and therefore when the fossil that was contained within those sediments was also deposited in that environment,” he said.

“So it gives us a kind of indirect way of determining the age that the fossil was formed at that site.”

It took the team more than a year to get an accurate date, as thousands of individual grains of sand had to be measured.

But Mr Arnold said the “painstaking time” was worth it, as it could lead scientists to new discoveries about Australia’s history and why diprotodons went extinct.

Adam Yates, a palaeontologist from the Museum and Art Gallery of the NT, was quick to point out that although the diprotodon was technically not a wombat, wombats were their closest living relatives.

Dr Yates said the diprotodon was the “largest marsupial that’s ever existed”.

“It was about the size of a white rhinoceros, and probably would have looked a bit like a rhinoceros, but without the horns,” he said.

Dr Yates said Shirley the diprotodon was “large, four-footed, slow-moving and a plant eater”.

Although scientists have found evidence of diprotodons in sites across Australia, Dr Yates said little was known about how these animals existed in the north.

He said the scientific world was very interested in finding out the age of Shirley the diprotodon because of where she was discovered, and the fact she was the most northerly diprotodon ever found.

“We wanted to know if the ones up here are of similar age to diprotodons found elsewhere … it’s older than most,” Dr Yates said.

Rather than just being a fossilised set of bones, knowledge of her age means scientists and historians can begin studying the specimen further.

Dr Yates is now embarking on the long and arduous process of preparing Shirley’s skeleton for display at the megafauna museum in Alice Springs.

That will involve carefully opening the protective plaster jackets that were constructed around the skeleton in the field during the excavation, and using handheld tools to carefully clean off all the adherent clay and rock.

Although most of Shirley’s remains were found, some key parts are missing.

“We don’t have a skull and we don’t have its limbs, but we have its pelvis, one of its limb bones and a rib cage and spinal column,” Mr Archibald said.

That Shirley was discovered at all, he said, was a major win for science.

“It would not be there now, it would have eroded away and just been lost and gone, and no one would have known it was there,” he said.

“So it’s actually very lucky happenstance that everything’s come together and allowed scientists to start working out what was happening at that time period here in the Northern Territory.”