Located 345km south-east of Adelaide, on South Australia’s Limestone Coast, is a World Heritage-listed fossil site that offers a glimpse of how life was long before humans arrived on the continent. Welcome to Naracoorte Caves, which for 500,000 years have acted as natural pitfall traps and predator dens, ensnaring everything from Ice Age megafauna to modern-day species.
Thousands of animals have met their end after tumbling into the caves’ concealed entrances and becoming trapped. Unable to climb out, they perished and their skeletons have mingled with those of cave-dwelling animals dead from natural causes, and bones picked clean by predators and scavengers.
“The caves at Naracoorte have been accumulating animal remains for the best part of the last 500,000 years, more or less continually, so you’ve got a record for vertebrate life within one place that spans the last half-million years,” says palaeontologist Professor Gavin Prideaux from SA’s Flinders University. “A lot of really significant climatic changes occurred over that time: there were the glacial–interglacial cycles, which are the comings and goings of ice ages, so you can see how a really diverse fauna responded to that glacial–interglacial cycling…and what the environment was like at all these successive time periods.”
About 120 animal species are represented in Naracoorte’s fossil and bone deposits, including wallabies, possums, bats, lace monitors, quolls, bettongs, owls, mice and more. “When you look at the Naracoorte fossil assemblages, most of the species that are preserved there are still around now,” Gavin says. “But there’s a whole bunch…[of] larger species – what we’d call megafauna – that aren’t.”
These megafauna fossils are the crown jewels of Naracoorte, and a major drawcard for tourists. They include: Thylacoleo carnifex, the leopard-sized “marsupial lion” that was the largest-known mammalian carnivore to ever prowl Australian ecosystems; Procoptodon goliah, a flat-faced kangaroo that stood 2m tall; modern wombats’ evolutionary relative, Zygomaturus, a hippopotamus-like marsupial herbivore that weighed 500kg; and a 5m-long constrictor snake, Wonambi naracoortensis. Thylacine and Tasmanian devil remains have also been found inside the caves. Collectively, these fossils provide a perspective on the diversity of extinct animal forms known as megafauna.
This ancient and mostly oversized assemblage became extinct 60,000–40,000 years ago. Whatever caused this extinction event remains a hot topic in scientific circles; many researchers are hardline advocates that humans overhunted these animals to extinction, while others blame climate. It was possibly a mix of the two.
“It’s really hard to pull apart what’s likely to be a human-influenced change versus a climatically influenced change, because we know [both] of those factors can drive biotic change,” Gavin explains. “That record prior to 65,000 years ago is super critical. It gives us a baseline that shows how fauna and vegetation respond to climatic change when there are no humans on the scene. Once humans are on the scene and we see evidence of change in the record, we’re better able to decipher what’s likely a human impact versus a climate impact.”
The megafaunal extinction debate looms disproportionately large among scientists; Gavin says it’s “sucking up so much oxygen out of a whole area of research”. Instead, he is more fascinated by the animals themselves. He wants to understand how Australian ecosystems were structured before the arrival of humans 65,000 years ago, and piece together the evolutionary record of these mammals that were biologically distinct from the rest of the world.
“Australia is a unique experiment in mammal evolution because we have all these mammals here that evolved in isolation for 30 million years,” he says. “It’s like this independent experiment; some of them evolved into niches that were somewhat similar to placental mammals elsewhere, whereas others were just doing their own thing.”
There’s still a lot we don’t know about Australia’s megafauna: their diets, population sizes, distribution and ecological roles – alongside the tantalising certainty there are more species out there, waiting to be discovered. “We can get the data [to] work out how big they were, what they ate, how they moved and how far they roamed across the landscape. All that information is attainable – but we don’t have it. So I’ve become sort of more interested in that, rather than the bloody extinction debate,” Gavin says.
“Believe it or not, the current tally is about 73 species of megafauna in the Pleistocene that aren’t any longer here. It’s like going to Africa now and imagining that, suddenly, all the wildebeest, all the zebras, all the elephants and all the giraffes, everything, just disappeared. Their loss from the ecosystem would have a massive environmental impact.”
Modern relevance in a warming world
Naracoorte’s animal fossils and bone deposits allow palaeontologists to peel back layers of time and glimpse into prehistoric environments. Its plant fossils are equally revealing. Scientists can analyse pollen and charcoal deposits stored inside the cave to figure out the types of plants that have grown in the area since the Pleistocene. “Different types of plants have differently shaped pollen, so by studying fossil pollen you can reconstruct the vegetation,” Gavin says. “When you’ve got that, along with animal remains, you get a much better sense of the structure of ecosystems at particular slices of time.”
Scientists can pair this with other geochemical records, such as stalagmites and stalactites, to understand past climate change. Researchers from The University of Melbourne and The University of Adelaide recently examined rainwater stored inside the caves’ 350,000-year-old stalagmites and stalactites, using uranium–lead dating to determine their age. Their findings, published in the journal Nature in February, revealed that Australia’s ice ages received more rainfall than previously thought, suggesting the continent’s glacial periods were more hospitable for plants and animals compared with those in the Northern Hemisphere.
At a time of global warming, these prehistoric climate data have modern relevance. Naracoorte’s ancient fossils and geochemical records provide insights into the ways animals and plants have responded to past climate change – and that might offer clues for the future.
“If we’re trying to make inferences about current global climate change and how that will affect our faunas, we can’t reliably extrapolate [ecological data] from five years or 10 years or 20 years or 50 years ago because it’s not long enough to make a decent inference,” Gavin says. “But that’s where records like those of the Naracoorte caves are really important for understanding what’s likely to happen if the temperature keeps increasing, and what’s going to happen in 100 years, or 500 years, or 1000 years.”
For now, excavations continue inside the caves. Only four of Naracoorte Caves National Park’s 28 known caves are open; the rest are put aside for scientific research. “It’s pretty exciting because we’re discovering new things all the time, whether that’s in the lab or in the museum, or in the field. And it captures people’s attention,” Gavin says. “People are interested in palaentology because it expands their perspective on things and makes them realise that there’s a big picture out there. We certainly get so focused on our own little part of the world, sometimes it’s nice to take a step back and think about the bigger picture.”
