The massive Diprotodon optatum, from the Pleistocene of Australia, was the largest marsupial known and the last of the extinct, herbivorous diprotodontids.
Credit: Australian Museum
The megafauna were large animals, usually weighing over 44 kilograms, and, in the case of the giant wombat-like Diprotodon optatum, up to three tonnes.
The oldest-known ancestors of this giant appeared in the fossil record 24 million years ago - initially as smaller species - and evolved to immense sizes by the Pleistocene Epoch (some 2.6 million years ago).
Every continent had its own giants, from New Zealand's flightless moa to the woolly mammoths in North America. Australia was home to a plethora of zoological oddities from the marsupial lion to the giant, short-faced kangaroos, giant crocodiles, goannas and the 'thunderbirds', some of which stood over three metres tall.
By 45,000 years ago almost all of these giants had vanished. Some of the smaller species vanished too, but it's the big guys who get most of our attention.
What caused the megafauna to become 'mega' in the first place is a mystery, but why they went extinct extinction is an even greater mystery.
Finding the killer has been a challenge in part because of the limited number of well-preserved fossils. But a nearly complete skeleton of a Diprotodon, recently excavated near Leichhardt River in northern Queensland, may hold a clue to the answer.
Diprotodon was the largest of the Australian megafaunal species, standing nearly two metres tall at the shoulder. In 2010, a team including Scott Hocknull from Brisbane's Queensland Museum, palaeontologist Mike Archer from the University of New South Wales (UNSW) in Sydney and colleagues from Queensland University spotted several bones that suggested they might be part of a complete skeleton hidden in the bank of ancient river sediments on Queenlsand's Floraville Station.
In 2011, they excavated the skeleton and found it to be just as exciting as expected - one of very few articulated and almost complete skeletons of Diprotodon known.
Samples from the sediments containing the skeleton have been submitted for dating, and palaeontologists are hoping that, if dated at less than 60,000 years of age, it may help to unravel why this megafaunal giant species went extinct.
The last ages of the megafauna were ones where the world was in the grip of the coldest periods of the ice ages. Ice core records have shown that in the past 400,000 years, the Earth has gone through four ice ages, also known as glacials, each of which lasted about 90,000 years.
These were interspersed with shorter warmer periods, or interglacials, which lasted about 10,000 years. The term 'glacial maximum' refers to the period when ice sheets in the Northern Hemisphere were at their thickest.
While Australia didn't suffer continental glaciers during the Pleistocene, it did suffer intense dryness as well as cold weather during the glacial maxima. The last glacial maximum lasted from 22,000 to 19,000 years ago. As ice sheets spread, sea levels dropped around the globe, decreasing the available moisture.
In Australia, temperatures dropped by 8°C, rainfall was 50% lower than today's levels and the winds were twice as strong. Ice sheets covered a small percentage of Australia but the impact here was primarily the decrease in rainfall and the dry winds, which caused sand dunes to spread across more than 70% of the continent. While rainfall levels fluctuated throughout the Pleistocene, studies show that a long-term drying trend led to exceptionally arid conditions during the last glacial maximum. Pollen records from Cuddie Springs in New South Wales indicate that grasslands had slowly replaced the woodlands of inland Australia during the Pleistocene.
This would have forced the megafauna into increasingly smaller patches of vegetation in order to survive. "Climate change can have awesome impacts on the biota and it's increasingly clear that the megafauna was in steep decline long before humans arrived," says Mike Archer, a palaeontologist at UNSW.
Scientists arguing against climactic factors as the cause of the decline point out that the megafauna had already survived the arid conditions of several glacial cycles. But Archer says what we didn't realise until recently, was just how bad the aridity was during this final crucial period. The disappearance of the megafauna coincided with what Archer describes as "a ferocious increase" in the drying of the Australian continent 45,000 years ago. "We're convinced now that we probably underestimated the impact of this continent-wide collapse in rainfall."
No spear points? Oh Brother!
Mike Archer, the scientist who suggests that the first homo sapiens to arrive in Australia had no spear points to hunt megafauna seems to be too deeply entwined in his argument to use common sense. In fact, the detractors of the overkill hypothesis generally seem to have an almost "politically correct" aversion to human beings killing anything at all, clinging, perhaps, to the old notion of the noble savage living in perfect harmony with nature!
Whether or not these early adventurers had stone points, any layman using simple intuition could tell you that they were likely very proficient killers, and could bring down big game through cooperative hunting.
Climate change may also have played a role in the demise of the megafauna - but don't be silly or naive about human beings' resourcefulness and love of the hunt.
Agreed... People did it
This extinction and others of a similar nature is covered very well by the book "Guns, Germs and Steel" by Jared Diamond. This book is profound; it'll change the way you think. It won the Pulitzer Prize in 1998 and it's available as an audiobook too.
Diprotodon optatum
The Diprotodon optatom was big.
I know!
Death killed the megafauna!!!!!!!!!!!!!!!!!!!!!!!