I’m one of those tragics who decided when I was five that I was going to be a palaeontologist. But when I went through my undergrad, the university didn’t offer a specific palaeontology degree. I actually majored in zoology and geology, because when you put rocks and animals together, you get palaeontology.
The Australian fossil record is pretty patchy. The bulk of the evidence shows us that there was a big megafaunal cohort around until about 45,000 years ago. But by 40,000 years ago they were just about all gone – the only megafauna left were our kangaroos, emus and crocodiles.
Some megafaunal taxa have only a couple of representations in the fossil record, so we don’t know much at all about when they went extinct.
Whenever we find something new it’s exciting, but the holy grail isn’t so much finding a specific animal, but fossils from a specific time period. At the moment, we’ve got a reasonable record of the modern Australian groups of animals from about 25 million years ago. But the molecular evidence suggests that many of those families originated maybe 35 million years ago. We don’t have any fossils representing what’s going on in that missing time period.
A big part of the challenge is that our Australian fauna have evolved in complete isolation for at least 50 million years. If we found a site from that particular time period, 35 to 40 million years ago, it would be absolutely amazing. It would clarify so much of our understanding of the early evolution of the modern groups of Australian animals. Unfortunately, there are very few sediments of the right age preserved in Australia.
Some of the places we go to are very arid environments. I’ve just been out around Cooper Creek, which is one of two main rivers that flow into the eastern side of Lake Eyre in South Australia. The current drainage pattern is eroding through the sediments from what we call palaeochannels, so we are seeing good potential for fossils to be exposed.
You really do get to love the desert environments. If you’re out around Lake Eyre or Lake Frome, at first it seems like there’s a bit of scrubby saltbush around and not much else. But after spending time there, you get to know the desert flora and the desert landscapes, and you develop a much better appreciation for how those ecosystems function.
It’s kind of our job to try and imagine what these landscapes looked like all those millions of years ago. The evidence certainly shows that a lot of Australia has been a lot more heavily forested than it is today– go back 20-25 million years ago and it was rainforest in parts of Central Australia. Even more recently, when we look at places like Lake Callabonna, northeast of the Flinders Ranges, we’re finding thousands of megafauna animals that got stuck in mud there. The fauna there show us that the vegetation was very different to what it is today, just to be able to support them.
Other people have looked at the lake-level sequences from about 50,000-45,000 years ago and shown that Lake Eyre and Lake Frome went through a catastrophic drying phase. Prior to that they were almost permanently full of fresh water. That would have completely transformed the landscape. You can imagine how much vegetation that would support, and the flow-on effects for the rest of the ecosystem.
But it’s the fossils there that absolutely make it exciting for us. New technologies make it possible to carry out a bunch of new analyses – a whole range of different scanning technologies, ranging from field-based to lab-based. These can be surface morphology, or various different kinds of X rays or CT scans, plus the really high-powered things like synchrotrons and neutron beams, which allow us to look inside of bones, or even look inside the concretions that contain fossils.
We have potential now to image and investigate fossils in a completely non-invasive way. It’s very rare that soft tissue would be preserved, for example, and in the preparation of the bones any existing soft tissue impression could easily be destroyed. But the new imaging technology allows us to have a permanent three-dimensional record of the soft-tissue impression too, as long as there’s some kind of density difference.
I work a lot with fossil footprints. Quite often they’re in remote areas, or not easy to get casting supplies out to. But there’s a technology called photogrammetry, which uses computer software to turn a series of photos into a three-dimensional image. With a camera in the field and a bunch of good photos, we can create an accurate three-dimensional record of those trackways.
We’ve had a whole range of exciting finds. I’m writing up a project at the moment where we actually found some trackways from a bipedally striding kangaroo – this was a kangaroo that was walking rather than hopping.
In figuring out what made the tracks, we go through a whole sequence of forensic investigations. We look at the sediments, their age, the environment, and then we look at the fossil record and what that tells us about the fauna at the time, and what we know about the shapes of the different feet of the animals that were there. Through a process of elimination, we can figure out what the animals were that made the tracks, or what possible species, and go from there.
Basically, we were able to eliminate everything except the kangaroos. There have been a couple of studies in the past, based on observations of skeletal morphology, that suggested that some of these extinct kangaroos were bipedally striding. But this is the first evidence directly proving that some of them moved in that way.
These trackways are about 3.5 million years old, but it’s been proposed that the group that they belong to, the big short-faced sthenurine kangaroos, all walked bipedally until they died out about 40,000 years ago.
There are two competing hypotheses why they disappeared – either driven extinct by climate change, or hunted by the first people to arrive here. While we’ve got evidence at a number of different sites to show that the megafauna were able to withstand some variation in climate, on the flip side we don’t have any direct evidence of humans hunting megafauna in Australia. So there’s a lack of evidence either way. Which means we have plenty of work to do!
As told to Graem Sims for Cosmos Weekly.