More than 40,000 years ago, Australia’s tropical northeast was home to species of giant birds, reptiles and marsupials, including the world’s largest kangaroo, standing 2.5 metres tall, a seven-metre-long freshwater crocodile and giant lizards.
Discovery of their remains, fossilised during the Pleistocene, is reported in the journal Nature Communications, along with unique insights that help unravel some of the long-standing mystery of what drove megafauna extinction in Sahul (Australia and New Guinea).
“It is the first reliable glimpse into Australia’s tropical lost giants when people first arrived and spread across the continent,” says Scott Hocknull from the Queensland Museum, who led the 12 years of research in collaboration with several Australian universities.
It wasn’t the people who killed them off, according to meticulous analyses, but severe climatic conditions and environmental corrosion including loss of water flow, increased drying, fires and reduced grasslands.
“We cannot place humans at this 40,000-year-old crime scene,” Hocknull says. “Therefore, we find no role for humans in the extinction of these species of megafauna.”
The fossils were discovered in 2008 by the indigenous Barada Barna people during a cultural heritage clearance at South Walker Creek near Mackay, which reveals a diverse and rich ecosystem inhabited by at least 16 species of megafauna – 13 of them now extinct.
They were uniquely preserved for a tropical site, which normally erodes fossils, thanks to swift extermination of animals by crocodiles and a rapid flood in the most well-conserved and youngest site.
The researchers painstakingly excavated the remains, piecing them together and aligning them with the age and environment at the time they lived.
They included leaves, seeds, pollen, insects and molluscs, providing evidence of a forest that housed megaherbivores, carnivorous reptiles and predators such as giant wombats (Diprotodon), a six-metre long monitor lizard (Megalania) and a marsupial “lion” (Thylacoleo).
Hocknull is confident the colossal 274-kilogram kangaroo (from the genus Macropus sp.), the largest yet found that would make modern-day kangas look like toddlers, is a new species, but diagnostic pieces of skeleton such as teeth are missing to confirm.
Despite its size and evidence of surviving a long-standing bone infection, the kangaroo was no match for one of the colossal crocodiles, bearing two puncture marks on its tibia that bear witness to its demise.
As far as megafauna extinction, the timeframe coincided with persistent regional hydroclimatic and environmental deterioration. “Such a combination of factors proved fatal to the giant land and aquatic species,” says Hocknull.
The age and location of the fossils contradict theories that people hunted them to extinction across the continent, as these events did not correspond with human migration.
While the fossils that survived past pandemics and turbulent climates tell us a bit more about the “cataclysmic” past of one of Earth’s oldest continents, what relevance do they have now?
“To many, a bunch of old bones would seem like poorly chosen prophets of the future,” Hocknull writes.
But they do tell a portentous tale, particularly relevant now with the unprecedented planetary and climatic changes wrought by humans.
“Not since the time of the dinosaurs has Australia been home to such magnificent giants, and yet within a geological instant they were gone forever,” says Hocknull.
“Our study shows that the giants succumbed to major environmental change and this is a lesson that we can learn from.”
It’s also simply fascinating.
“The megafauna in Australia were the largest animals to dominate the environment after the extinction of the dinosaurs 65 million years ago,” says Hocknull, “and to think that we had a range of mega-reptiles and mega-mammals both predators and prey attaining gigantic size and living with the first people to call Australia home beggars belief.”
For those whose imaginations are captured by the weird and wonderful critters that roamed amongst modern humans, the bones have been scanned and transformed into models that people can interact with in 3D online.
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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