8 August 2011

What killed the megafauna?

A giant marsupial skeleton could answer the question that has plagued scientists since Charles Darwin discovered the remains of a giant ground sloth in 1830.
Diprotodon optatum

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.”

PREVIOUSLY, SOME SCIENTISTS had suggested that the decline of the Australian megafauna coincided with the arrival of Aborigines about 60,000 years ago. The theory was that they then hunted the megafauna to extinction over the following 15,000 years and profoundly changed the vegetation through frequent use of fire.

In 2006, Barry Brook and Chris Johnson from Flinders University published a study in Alcheringa: An Australasian Journal of Palaeontology, suggesting that selective hunting of juveniles could have caused the megafauna’s extinction.

By targeting the juveniles, the Aborigines would have killed off many of the next generation, causing extinction within a few thousand years.

Archer, however, argues that the first humans to arrive in Australia didn’t bring spear points, dogs to assist with hunting, or tools that could have cleared or altered the land. There is also no evidence for frequent burning in eastern Australia prior to 6,000 years ago, suggesting that Aboriginals may not have impacted the Australian landscape enough to cause the megafauna extinctions.

To the north of Australia, there is evidence that indigenous hunter-gatherers coexisted with the megafauna. Aborigines are believed to have entered Australia via Indonesia, which had its own megafauna species.

But in spite of human settlement, most of these survived long after the arrival of humans, such as the Komodo Dragon, a three-metre-long lizard which survived volcanic eruptions, climate change and the arrival of at least two species of human, Homo erectus and our species Homo sapiens.

Other megafauna species, like Tapirs, Sun Bears and the Bali Tiger, also survived human incursion. “The megafauna were there while humans went through the archipelago and were there after humans entered Australia,” Archer says. “There is no evidence to indicate that the first Australians did other than live compatibly with the already declining megafauna as they did with the rest of the Australian biota from that point on.”

In 2010, Gavin Prideaux from Flinders University in Adelaide suggested, however, in a study published by the Proceedings of the National Academy of Sciences, that a single-factor explanation for megafauna extinction may be overly simplistic, and that both the climate and the impact of humans played a role in their disappearance.

Tight Cave in southwestern Western Australia contains megafaunal remains dated to 40,000 years ago, with a second cave nearby containing human remains from 49,000 years ago. Prideaux suggests that the extinction of the megafauna would have occurred well before the most extreme climatic phase, 18,000 years ago, and that while climate brought about the decline of the megafauna, the arrival of humans was probably the decisive factor.

The underlying issue in both arguments for the cause of these extinctions was probably the megafauna’s large size. Like today’s large herbivores, megafaunal species would have needed access to plentiful food and water, says Scott Hocknull of the Queensland Museum in Brisbane. Rapidly diminishing vegetation and water sources could not sustain such large animals during the last glacial period, when the ice sheets locked up much of the world’s water and the vegetation decreased as a result.

As the climate became increasingly arid, the vegetation and water sources that the megafauna relied on would have dried up. “It may simply be that size was the key to their downfall,” he says.

Their huge size would have also influenced their reproductive rate, limiting the number of offspring they could produce. Slow growth and breeding rates would have made it difficult to adapt to new environmental conditions.

If the arid climate and their size drove the megafauna to ‘islands’ of isolated vegetation and waterholes, they would have been vulnerable to the arrival of a new predator. Hocknull says, “bring in the human factor and you bring in a whole other layer of complexity.”

Palaeontologist Aaron Camens, from the University of Adelaide, suggests that like many large herbivores, Diprotodon would have had to invest a lot of time and energy in reproduction. “Even kangaroos are usually restricted to one young at a time,” Camens says. “A Diprotodon might only produce half a dozen young in its lifetime.”

Like the dinosaurs, the megafauna left behind a mystery when they died. While their size was probably a contributing factor, there are also strong arguments for either the increasingly arid conditions or the arrival of humans during this period of intense climate change as the primary factor that pushed them over the brink. Depending on its age and what its bones reveal, the Diprotodon skeleton from the Leichhardt River could provide a vital new clue that may help to resolve this prehistoric mystery.

Laura Boness is an intern at Cosmos magazine and has a degree in environmental science.

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