Scientists have identified one of Australia’s first long-distance walkers: a 250kg marsupial with “heeled hands” that roamed across the continent’s arid interior 3.5 million years ago.
Using 3D scanning, Flinders University palaeontologists have described a new group of ancient marsupial, calling it Ambulator – meaning walker or wanderer – for its specific leg and feet adaptations that equipped it to efficiently roam long distances.
The categorisation is based on their discovery of a partial skeleton in 2017 at the Australian Wildlife Conservancy’s Kalamurina Sanctuary in north-eastern South Australia.
The specimen, now classified as Ambulator keanei, was rare for the number of bones it contained that originated from an individual animal, said Jacob van Zoelen, the study’s first author and a PhD student at Flinders University.
“What was really cool about this specimen is that the foot was encased in concretion, this rock that formed shortly after death,” he said. CT scanning of the rock revealed soft tissue impressions inside, giving researchers an insight into the structure of the animal’s footpad.
Many large modern herbivores such as elephants and rhinoceroses are digitigrades, meaning they walk on the tips of their toes without their heels touching the ground.
Ambulator, in contrast, was a plantigrade, walking with its heels on the ground to distribute weight, similar to humans. “In their hands, they have modified one of the wrist bones into a secondary heel,” Van Zoelen said. “They have a heeled hand.”
These adaptations came at a period when the Australian climate became drier and there was an increase in grasslands and open habitat. “These animals were needing methods of evolving efficient locomotion,” Van Zoelen said.
Ambulator belongs to the diprotodontids, a family of large marsupial herbivores that played an integral part in Australian ecosystems until the last species went extinct during the late Pleistocene, about 40,000 years ago.
Diprotodontids were an incredibly diverse family of animals, Van Zoelen said. “You’ve got ones that are capable of climbing … you have ones with insanely large cheekbones like Euryzygoma, you’ve got ones with really gnarly teeth.”
“The largest herbivores we have today are kangaroos, but some of these animals grew up to 2.7 tonnes.”
One notable diprotodontid was the rhinoceros-sized Diprotodon, the largest marsupial that ever lived.
Ambulator keanei was previously classified in the genus Zygomaturus, but researchers discovered that “while the teeth are very similar, the skull is much more slender, with much smaller cheek muscles than Zygomaturus,” Van Zoelen said.
The research was published in the journal Royal Society Open Science.