An Ice Age mega-mystery: the story behind the science

Dr Gilbert Price, School of Earth and Environmental Sciences

My research team and I recently had a new paper published that reconstructs the biology and ecology of the 3,000 kg giant wombat-like Diprotodon of the Pleistocene of Australia. Our study generated a fair bit of media coverage, with reports appearing on the BBC and ABC, and write-ups in both Science and Nature.

In a nutshell, our investigation revealed that Diprotodon was a seasonal migrant, a first for any marsupial, living or extinct. The research has big implications for understanding a variety of aspects surrounding the extinction of the megafauna – the giant land animals that lived alongside Diprotodon including eight-foot tall kangaroos, huge venomous goannas, giant birds, and snakes so big that they that would give the anaconda a run for its money. Most importantly, the new findings tell us more about how Australia’s Ice Age megafaunal ecosystems operated.

This blog post is not so much about the findings and implications of our research, but the story of how the study evolved and why we even did it in the first place.

Scientific beginnings

It all started around 10 or 11 years ago. I was fresh out of my PhD, and moved to The University of Queensland to take up a position working in the Radiogenic Isotope Facility. The lab was then (and still is today) operated by Jian-xin Zhao, a leading geochemist and specialist in uranium-series dating.

I was Jian-xin’s technician, spending most of my time in the lab drilling tiny holes into speleothem (cave ‘decorations’ such as stalactites and stalagmites) for palaeoclimate studies, plus sampling the odd fossil or two for dating.

My PhD training was focused on palaeontology, but here I had an opportunity to develop skills in geochemistry.

Jian-xin encouraged me to explore how I could use the lab to ask the research questions that I was interested in. That is, how did the prehistoric megafaunal ecosystems of Australia work?

One day I came across a paper that examined the potential of seasonal migration in extinct animals such as woolly mammoths. It was an intriguing study that applied a variety of geochemical tools to their fossilised teeth to reconstruct their biology and ecology. The report found that the mammoths under investigation were migratory, moving around their prehistoric ecosystems tracking their preferred food.

I thought to myself that it would be fascinating if a similar study could also be applied to the extinct Australian megafauna.

And here I was in a world-class lab with the tools available to me, plus fossils to test it out.

Putting the plan into action

Fast forward a few years and an Honours student, Kyle Ferguson, joined the project. Kyle’s thesis was designed to ask the question, “Did Diprotodon migrate?”

Kyle spent many hours in the lab drilling powders from a Diprotodon incisor that we borrowed from the Queensland Museum. We initially decided to measure the strontium chemical component of the tooth, with a view that it would allow us to answer basic questions about potential migration.

Kyle passed the samples to our collaborators, Yue-xing Feng and Ai Duc Nguyen, who analysed their chemical composition using various instruments in the lab including our mass spectrometers.

One Friday afternoon, we were presented with the data while we were relaxing at the local watering hole. The results were incredible. Even in their rarest form, it was clear that we had an answer to Kyle’s research question.

Yes, Diprotodon did migrate.

Kyle eventually completed his Honours, but the study was not over. We still needed more data.

Our collaborator Pennilyn Higgins (University of Rochester, USA) supplemented the original dataset with new geochemical information about the animal’s diet and water intake.

Our UQ mentor Gregg Webb encouraged us to explore the distribution of trace elements in the specimen in an effort to check for diagenesis (complications in the tooth that may be caused by fossilisation processes).

We also added new dates that showed that our Diprotodon lived around 300 thousand years ago.

Finally, we had the data we needed. Stats guru Julien Louys (Griffith University) crunched the numbers, and with additional help of geochemist Renaud Joannes-Boyau (Southern Cross University) the story was clear: Diprotodon was a seasonal migrant that tracked its preferred food sources across the seasons.

It’s not the story that we necessarily expected, but it is one that we are hugely excited about. The implications of our finding are massive.

Stories we never scrutinise

You might ask though, why would we even bother to the test the hypothesis about seasonal migration in Diprotodon? After all, no other marsupial has ever been observed or inferred to have undertaken seasonal migration. And that’s despite marsupials, and their broader relatives, the metatherians, having being around for at least 160 million years.

Well, I’m reminded of a quote from the famous palaeontologist, Stephen Jay Gould: “The most erroneous stories are those we think we know best – and therefore never scrutinise or question.”

Here we did question it - using geochemical tools that had never been applied before in this way to extinct Australian animals. And here, we are able to show that the textbooks about marsupial biology and ecology need to be revised in light of this new discovery.

Our study into Diprotodon isn’t over yet. We’re especially interested to find out which other species migrated alongside Diprotodon and whether megafaunal mammal migration was common in other Ice Age ecosystems of Australia.

Back to the lab!

Visit Dr Price’s research blog or keep up with him on Twitter, @TheFatWombat.