“Is that the bluff that’s on the other side of the dig?” asks driver Dr Sam Arman, an affable and scruffy-bearded palaeontologist with the Museum of Central Australia.
Arman raises his left arm to point through the windscreen to a bump on the horizon, but his hand wavers wildly as our Toyota dual-cab ute hits another upsized corrugation on the bumpy track of loose red sand.
Either side of our vehicle stretches a postcard of undulating spinifex country, some 75 km south of Outback capital Alice Springs.
We are hunting a remnant fossil dig, somewhere to the east of the Ghan railway line: though it seems we’re not entirely sure where that might be.
“I reckon we hang a left,” says Arman, taking charge.
The “dig” is at a place called Pwerte Marnte Marnte, fossil home to Mukupirna, a distant relative of the modern wombat, from which parts of a fossilised skeleton were unearthed in 2020 and a scientific paper published.
The site’s name springs from the homeland of the Southern Arrernte tribal group, an Aboriginal community/family living about 100 km south of Alice Springs, near Rainbow Valley.
The fossil discovery sparked international media attention owing to the site’s age — from the late Oligocene, roughly 25 million years old — as well as the number of species buried there, which until now had been missing from the evolutionary timeline.
Hence me tagging along; fortuitously, the museum scientists needed to collect a cache of fossil-bearing rocks that, for want of room, had to be left behind from a previous trip; “all pretty routine”, they’d said.
None of us had any idea the day held its own surprises in store.
Overnight was chilly ahead of cycling to Megafauna Central, a storefront fossil exhibit in Alice Springs’ main tourist mall as part of the broader Museum of Central Australia campus. I arrive early and cold, only to realise my lunch is back in the fridge at home with my flask of water.
Near the labs at the rear of the museum I join two scientists, and an unexpected second hitchhiker, to pile into a Toyota.
Arman is at the wheel beside senior museum palaeontologist Dr Adam Yates, a co-author of the scientific paper on Mukupirna; in the back with me and our baggage squeezes a bright-eyed work experience student from a local college, Sorell.
The highway section gives Yates a chance to background me on the site, but a turn west off the highway puts our conversation abruptly on pause for a bewildering array of bush tracks that prove a rough ride through foothills in country of remarkable beauty.
To our south lies the free-standing sandstone bluffs of Rainbow Valley, quite the spectacle rising from the plain, all part of the James Range and not all that far from where Scottish explorer John McDouall Stuart first reached Central Australia from the south in 1860.
Rainbow Valley has long enchanted visitors to the Red Centre with its cliffs of coloured rock, striped into bands during wetter climates of the past by the action of water; capping the bluff’s top is a dark, hard iron layer, from which pout layers of leached and weathered white beneath.
We reach the dig, park, and scramble down the rail embankment.
A significant find
As is so often the case in palaeontology, the story of the fossil site Pwerte Marnte Marnte, or PMM, is a long one.
The first fossil bone samples surfaced in 2003, after Charles Darwin University horticulturalists Bart Desart and Arthur Dallenburg chanced upon calcareous material containing fragments of bone while working along the Ghan rail line near Rainbow Valley.
The pair found the material in the rail embankment itself, on a remote section of track along the Kulgera to Alice Springs line where earthworks had begun for the line around 1980.
Looking for rock to provide ballast for the track, rail builders discovered suitable material close to the line and excavated a borrow pit; it is still visible once you know what you’re looking for, but long abandoned and grown over, largely indistinguishable from nearby scrub.
Thinking the material significant nonetheless, Desart and Dallenburg collected hand specimens from the embankment and presented them to the Museum of Central Australia at Alice Springs.
Their find quickly sparked the interest of two museum palaeontologists—Drs Peter Murray and Dirk Megirian—who the same year collected a whopping 250 kg of fossiliferous rocks from the site then set about the much longer task of determining their age and significance.
Their efforts were not in vain.
In 2006, the scientists published their results in an article for Alcheringa: An Australasian Journal of Palaeontology.
The site had proved a gold mine of vertebrate fossils yielding a wealth of new material across several major lineages of Australian marsupials, and the 25-million-year age estimate made, placing them in an era from which little evidence had so far been unearthed in Australia.
Freed after millennia from their grave of hard limestone, the assemblage of found species was dubbed the Pwerte Marnte Marnte Local Fauna, and included a cousin of the modern wombat, a forerunner of the crocodile, an early koala, and evidence of several emu-sized birds.
Set among the much older Palaeozoic hills of the James Range, the fossilised bones and teeth—albeit now mostly shards—were found in a riverine deposit of sands and gravels dropped by an ancient stream flowing through the range during the Oligocene.
Some remains were more popular than others with fossil fans; Mukupirna fortidentata was identified from its teeth and described as a cross between a wombat and the extinct Thylacoleo, or “marsupial lion”, an attractive mix to be sure.
Thought to share an ancestor with the modern wombat, M. fortidentata is estimated to have weighed about 50 kg; in fact, the palaeontologists uncovered 35 different Mukupirna specimens at the PMM site, from some 2000 hours of painstaking laboratory work.
Tales from a forested riverbank
On the edge of a dug ledge of mixed clays and limestone—surrounds of the 2003 dig and the more recent dig of 2020—Yates sits to conjure the wetter climate when the now dry river channel at his feet had allowed crocodiles to flourish and supported a forest of trees homing a bevy of arboreal creatures.
“The site has produced, believe it or not, quite a broad range of animals” he begins.
“Amongst the commoner creatures here are the large flightless birds, a number of species, but the most common one is about the size of an emu. There was a slightly larger one here at the site, but those bones are quite rare.
“Next most common are the crocodiles, so there’s a species of crocodile in the genus Baru, that’s related to the really big Baru we get from Alcoota. This Baru is older and a bit more slender-jawed … not quite as massive as the Baru that came later. And that’s probably part of the fact that this is a very early species of Baru and it’s only just begun to specialise towards eating big animals. And it probably wasn’t having to deal with animals quite as big as the later Baru had to tackle .”
Yates’ fine words conjure an appealing scene. But the reality of low scrub atop reddened clay and hardened earth of the current site is somewhat drab by comparison, making it hard to imagine the site might have more treasures to divulge.
“The site is really significant because of its age,” he continues. “The animals that are buried here are likely very early representatives of branches of the animal tree that we find in younger sediments, things like the giant flightless ‘dromornithid’ birds, the Baru crocodiles and various other families like the kangaroo family, the diprotodontid family, the koala family.
“But all of these things that are present at this site are species that haven’t been found previously.
“They have less evolutionary novelties added to their anatomy, so … look like they come from very early on in the family tree of these different groups of animals.
“And so that leads us to suspect that this site is particularly old, [and] we know from other sites in other parts of Australia early on in the history [that is] well before 20 million years ago.”
Nearby, using a mattock swung with practiced ease, Arman tries more digging: Is it worth extending the pit? Can there be more ancient secrets buried at Pwerte Marnte Marnte?
An unexpected find
As Arman excavates more red clay and dust there is little reward for his efforts, until just when we have all turned away, he cries out.
I switch my phone to video and run to his side, where he is prising loose a slab of rock about half a metre across and 150 mm thick.
“That’s a good one,” says Arman, eyeing the slab. “How do we get it into the car?”
“All four of us?” I laugh.
Arman turns the slab over, spots something: “Oh, look at that,” he says, pointing to a finger-sized protuberance from its underside.
“Ohhh yess,” says Yates, leaning in, clearly enthused: “that’s a nice trochlea, the knuckle joint at the end of a foot bone of an emu-sized bird.”
And there it was: perhaps PMM had a few secrets left to divulge after all?
Don’t miss PART 2: The long, winding road to our past
Join us for a peek inside the Flinders University laboratory of palaeontologist Arthur Crichton, lead author on the Mukupirna research. It is in this lab, and with almost geological slowness—as tends to be the case for all fossil investigations—that the evidence from Pwerte Marnte Marnte is being analysed for his doctoral thesis. What has he found? When might he get to Arman’s slab, if ever?