Around 50,000 years ago, a giant flightless bird roamed the open woodlands and lake edges of southern Australia.
This mihirung paringmal (giant bird), sometimes called a “thunder bird” (Genyoris newtoni), weighed five to six times that of an emu, was 2 metres tall, and had a huge beak.
But what caused the demise of the species isn’t entirely settled.
As with much of Australia’s extinct megafauna, the sticking point centres on the relative influence of people versus climate change.
Now new evidence has found the giant birds had another struggle to contend with.
Analysis of fossils found stuck in the salt-lake beds of the Lake Callabonna fossil reserve in South Australia shows some of the animals were suffering from a painful bone infection when they died.
Lead author of the research published today in Papers in Palaeontology, PhD candidate Phoebe McInerney from Flinders University, said the infection would likely have caused pain and weakness in the afflicted animals before their deaths.
“We had some large cavities throughout the bones and we also had some distorted bones,” Ms McInerney said.
“The surface of the bones is really frothy and doesn’t have the smooth look of healthy bones.”
The abnormal growths and cavities, which were found on the chest, leg and foot bones of four individuals out of a total of 34 animals the researchers looked at, point to osteomyelitis — a severe bone infection that can spread via the bloodstream or tissue.
It’s found in birds today including domestic chickens and turkeys, as well as people, and often arises from an injury to the bone or an infection elsewhere in the body.
But it’s rare in wild birds to find multiple individuals of a population suffering osteomyelitis, and points to external pressures causing the disease cluster, Ms McInerney said.
“Having a single infection type in 11 per cent [of the population] is highly unusual,” she said.
“We were able to use dating of the fossil site [to see that it coincided] with really severe drought.”
The disease is thought to become more prevalent when an external stress, such as drought, compromises the immune system.
Blitzkrieg, climate change, or both?
The mihirung that the well-preserved fossils once belonged to are thought to have become stuck in mud on the edge of a lake.
It’s possible that the weakness and pain caused by the osteomyelitis made it more difficult for those individuals to get out of the mud, and may also go some way toward explaining the cluster of fossils found with signs of the infection.
Dated to between 54,200 and 50,400 years ago, the animals died during a period when severe droughts were becoming more frequent, and swathes of the continent were being reshaped; inland lakes and forests were disappearing, and grasslands and desert were becoming increasingly dominant.
Other species of fossilised megafauna have also been found at the site, Ms McInerney said, and point to a very different landscape than the one there today.
“We also have evidence of diprotodon getting stuck, and giant tree kangaroos as well.
“Now there is just nothing. You stand there and look over this flat plain and you have a few salt bushes — it’s pretty much just desert.”
When it comes to the extinction of Australia’s megafauna, there are a few competing hypotheses.
At one extreme, the blitzkrieg hypothesis argues that the extinction of megafauna all around the world likely happened as humans spread out, encountering native and easily-hunted prey as they went.
At the other end of the scale, it’s argued that the extinction of megafauna was caused by climate change.
Research published last year in Nature Communications looked at megafauna extinctions in northern and eastern Australia, including at South Walker Creek near Mackay in Queensland.
Despite being over 1,000 kilometres from Lake Callabonna, the two sites are connected, according to the lead author of that paper, palaeontologist Scott Hocknull from the Queensland Museum.
“Even though they’re thousands of kilometres apart, they both share the same catchment,” Dr Hocknull said.
“The lakes at Lake Callabonna all drain from Queensland — it’s all part of the Lake Eyre basin.”
But the megafauna extinctions at the South Walker Creek site are thought to have occurred thousands of years after those at Lake Callabonna.
This is consistent with a trend of aridification that is thought to have started in Australia’s interior, spreading outwards toward the coast, Dr Hocknull said.
But it’s not consistent with the pattern of dispersal of people across Australia.
The evidence of environmental stress in the Lake Callabonna population of Genyornis newtoni, prior to the extinction of the species Australia wide, suggests climate factors were heavily influential at that location.
So does that shift the debate one way or the other?
“A few years ago there was definitely two camps: either climate change or humans did it,” Dr Hocknull said.
“Most of us working in this area have always considered that neither one, nor the other, did it.
“The research suggests that each catchment in Australia needs to be treated individually.”