Extract from ABC News
A kangaroo bounding across the landscape is a picture of efficiency; a locomotive wonder honed over millions of years of evolution.
They're exquisitely tuned to hop quickly across long distances. With super-stretchy tendons and powerful hind legs, and balanced by a muscular tail, they deftly weave through scrub and over fences.
"The humble kangaroo is one of nature's marvels," University of the Sunshine Coast comparative biomechanist Christofer Clemente says.
"Nowhere else in the world do we see a large animal that hops like a kangaroo."
Just a few hopping kangaroo species can be found in Australia today, along with a handful of wallaroos and wallabies and a couple of tree-dwellers.
But for millions of years, a whole bunch of other roo types roamed the continent and neighbouring lands before dying out.
They included giants, such as Procoptodon goliah, which disappeared around 40,000 years ago.
This huge stubby-faced species tipped the scales at 240 kilograms — that's two-and-a-half times the weight of the heftiest contemporary red kangaroo (Macropus rufus) — and stood 2 metres tall.
But were those burly beasts nimble hoppers, like their modern counterparts?
Thanks to ancient fossils and modern technology, scientists are piecing together a picture of how the now-extinct marsupials moved and lived.
And, perhaps, why they died.
Virtual hoppers
To understand how long-dead kangaroos got around, it's worth examining why today's ground-dwelling roos, such as eastern greys (Macropus giganteus), are such remarkable jumpers.
Crucial to their bounciness is their Achilles tendon, a thick, tough cord of connective tissue that connects lower leg muscles to the heel bone.
When a hopping kangaroo's foot strikes the ground, their Achilles tendon lengthens and stores energy, like a stretched elastic band.
Then as they push their foot off the ground, the Achilles recoils and twangs back into place, helping propel their leg up and forwards into the next hop.
By storing and releasing energy in their Achilles with each bound, kangaroos can bolt long distances with relatively little effort.
But it also means a kangaroo's Achilles, which is almost as thick as a human finger, must deal with huge forces passing through it.
And the heftier the roo, the more force that tendon takes.
So is there a maximum weight that a kangaroo Achilles can take before it fails?
To find out, Dr Clemente, University of Queensland biomechanics researcher Taylor Dick, and PhD student Lauren Thornton removed an Achilles tendon from a young male eastern grey that had been hit by a car.
Using equipment that measures the strength of materials like metal and concrete, they stretched the tendon to see how much force it could take before it tore.
They then fed this information into a computer model to simulate how heftier kangaroos might move.
A quarter-tonne kangaroo, according to simulations, could hop — but only just, and at 9.4 kilometres per hour. No faster, no slower.
In contrast, eastern greys can motor along comfortably at around 20 kph for kilometres on end, and accelerate to 65 kph for short distances.
But while hopping at just one speed isn't exactly practical, when it comes to how the most massive kangaroos travelled, "looking at the kinematics, we can't rule out hopping", Ms Thornton says.
Skeletal secrets
Could more clues about how ancient giant roos moved lie in their remains?
Unluckily for some individuals, they fell into a cave or got stuck in thick mud and died.
But luckily for comparative anatomists like Murdoch University's Natalie Warburton, their teeth and bones were preserved and discovered, and can be examined today.
At first glance, an extinct kangaroo's skeleton looks fairly similar to eastern grey bones, "but it's very, very chunky", Dr Warburton says.
And telltale clues within the remains of a Procoptodon and a 50,000-year-old short-faced kangaroo — a close relative of Procoptodon — strongly suggest giant kangaroos did not hop.
For instance, the pelvis and knee joints were much broader in the extinct species than the eastern grey.
Big joint surfaces are typically found in animals that shift their weight from one leg to the other as they walk.
And extinct kangaroos' hip bones and vertebrae suggest it had a more upright posture and shorter tail than an eastern grey — perhaps better suited to walking.
Tracks through time
According to Museums Victoria vertebrate palaeontology collection manager Tim Ziegler, "independent lines of evidence all seem to roughly be pointing in the same direction, that there were these giant striders in the Australian landscape".
Now, evidence of a giant strider in the Australian landscape may have been uncovered in the form of footprints.
What appears to be a trackway of extinct kangaroo prints was found on Pirlatapa and Adnyamathanha country at Lake Callabonna, around 600km north of Adelaide.
Today, it's a dry, windswept, salty pan that fills with water every now and again.
More than 50,000 years ago, the region looked completely different, and was covered with mostly permanent freshwater lakes, Flinders University palaeontologist Aaron Camens says.
That amount of fresh water meant plenty of vegetation grew, such as eucalypts and she-oaks. This, in turn, attracted animals — including giant kangaroos.
But the climate changed and lakes started disappearing. Animals, perhaps venturing into the thick, sticky mud to drink from dwindling water supplies, got stuck, died and fossilised.
In some instances, animal footprints were also preserved in the sludge. As a foot or claw or paw squished into the mud, crystals of the mineral gypsum grew in the compressed sediment.
Over time, these gypsum-crystallised footprints were covered by silt — and then exposed, thousands of years later, by water and wind eroding the surface, for palaeontologists such as Dr Camens to find.
"They look a bit like blobs, essentially, and the only way that you can tell necessarily that you're looking at a trackway is by the repetition of the pattern," he says.
In 2023, he spotted a series of around 17 elongated blobs emerging from the sand.
There were two potential explanations for the trackway, Dr Camens says.
"You've either got an animal with four feet, and the way their [smaller] hind- and forefeet land forms one elongated print, or you have something like a kangaroo with a long footprint bipedally striding.
"In this particular case, the prints were well enough preserved that we could tell it wasn't two oval prints overlapping. It was definitely one long print."
If the prints were made by an extinct short-faced kangaroo, could it have been swimming? Modern hopping kangaroos move their legs independently while swimming, kind of like doing doggy paddle.
Was the Lake Callabonna track-maker simply going for a dip, and tapping the ground with its feet as it went?
Well, no, according to Dr Camens.
"Characteristics … such as foot shape demonstrate that the tracks were not made underwater, so we can definitely rule out swimming."
He and his colleagues are yet to analyse the tracks to find out more about the animal that made them, such as its size and speed as it wandered across the sticky mud.
But, Dr Camens adds, "one of the great things about trace fossils [such as footprints] is they're an actual record of behaviour.
"And so if an animal is moving in a specific way in a trackway, it's not a theory anymore.
"We have actual evidence for how that animal moved."
While we still don't know exactly why short-faced giant kangaroos died out, being a strider may have contributed to their demise.
They likely couldn't move as far or as fast as hopping kangaroos, so may have been more vulnerable to local pressures such as drought and disease.
The arrival of human hunters, too, could have accelerated their downfall.
And today the continent is left with mobs of nimble hoppers bouncing across the landscape.
Not a single giant loper to be seen.
The two-part Catalyst program Megafauna: What Killed Australia's Giants? begins Tuesday, June 25 at 8:00pm on ABC TV. Catch both episodes on ABC iview.
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