Creatures great and small: Origins of monotremes revealed

Monotremes are iconic Australasian species – as the only egg-laying mammals alive today, the platypus and echidna continue to fascinate us. But the origins of these species have continually raised questions for scientists – why aren’t they more present in the fossil record and why can we only find them in Australia and New Guinea?

A team of international scientists led by the Australian Museum’s Chief Scientist and Director of AMRI, Professor Kristofer Helgen, and Honorary Associate Professor Tim Flannery have answered these long-standing questions in a recent publication. AM scientists with Museums Victoria, Monash University, Swinburne University and the Smithsonian Institution in Washington, D.C. examined every known significant monotreme fossil to chart their history and evolution. This new research shows that monotremes are the last survivors of a diverse set of fossil species that once roamed the southern continents. In particular, the team investigated the oldest and smallest known monotreme Teinolophos trusleri – classifying it in a new mammalian family– as well as the largest egg laying mammal that ever lived: a gigantic extinct echidna from Western Australia for which a new genus, Murrayglossus, was named.

We sat down with Professor Tim Flannery to learn more about this ground-breaking research:

Congratulations on such a wonderful and impactful publication! This study asks some significant and long-standing scientific questions – could you please tell us more about how this started?

The project really came about due to lockdown, but so many of these questions actually came to me in a dream! I had a moment to think about my basic science, what I was doing for years at the Museum, and there were a few questions that really fascinated me with the monotremes. Where did they come from? Why are they restricted to Australia when they evolved at a time when the continents were joined – shouldn’t they be everywhere? Where are the echidnas in the fossil record? The oldest echidna fossils in the fossil record are less than two million years old. And having the opportunity to work with, and discuss such big questions, with talented curators, researchers and collection managers has been fantastic.

One of the big discoveries that came out of this research was classifying the oldest, and smallest, known monotreme – why don’t we see this species all over the world?

The very earliest monotremes (about 130 million years ago), date from a time when south-eastern Australia was within the Antarctic circle. We know from studies of the Early Cretaceous environment that there was seasonal darkness (three months at least), and polar forests. So, if you’re a tiny animal trying to find food in total darkness in a perpetually wet forest, it makes sense you would develop an electro-sensitive ‘bill’ or ‘beak’ (which is what characterises the monotremes). We found evidence that this ‘mini monotreme’, that was around the size of a shrew, had developed this incredible survival adaptation to search for insects in moss and snow in total darkness, in a high-latitude area. But think how restrictive this environment would have been – this helps explain why this animal didn’t migrate to other southern continents. That was the first thing we solved: why these monotremes were restricted to Australia and where they came from.

So, a huge and tiny discovery at the same time! Another big discovery was delving into the origin story of the echidna, could you please tell us more?

For the echidna question, it was really Professor Kris Helgen who hit the nail on the head. Kris and I went back and forth, asking where are echidnas in the fossil record? Were they in some unknown habitat, were they just really rare or is there preservation bias? We ruled out every possibility, and finally Kris said – New Guinea!

Millions of years ago existed proto-New Guinea, a series of continental fragments, before it developed into a single giant island (the Bird’s Head’s Islands are the best documented). During this period, this area had extremely shallow seas and it seems feasible that a land mass was joined to Australia. About 50 million years ago, these land masses separated, and a sub sample of Australian fauna became isolated on an island in what would become New Guinea – imagine a platypus-type creature on a tropical island without many rivers but ever-wet forest. This creature may leave the river systems and forage on the land, which is what the echidna did.

We know from echidna embryonic development that the echidna evolved from an animal that had a bill like a platypus, and we think they evolved on that island. We have evidence form around two million years ago that there was a big faunal interchange between that island in proto-New Guinea and Australia – so at that time, we think that Australian species went to that island and the echidna came back into Australia.

And the results were pretty spectacular!

Yes! In the process of doing this work, we named a new family of monotremes (those tiny creatures from the polar environments) and we discovered the largest monotreme that ever lived, a gigantic echidna (about 30kg) from south-west WA. People have known about it for a long time, but no one had described or named the genus. We named the genus Murrayglossus after Dr Peter Murray, a palaeontologist based in Tasmania who has researched fossil echidnas for decades. His fundamental work helped us work out what this giant echidna was – and he was very happy with the news, it’s his favourite animal in the whole world!

Many of our scientists have identified that a huge challenge in taxonomy is that we don’t know what we don’t know – we need to put time, expertise and funding into describing our biodiversity. You mentioned people knew of this species but hadn’t described it, why do you think this is?

To understand the origins of the monotremes, you have to do this at the right time. For us, we were in lockdown and some important geological papers had been published where new taxa had been described. As we had months of serious thinking and analysis, we could put the story together – this was lucky for us and having that time was a luxury, that only came about to lockdown.

It was great getting into the science again, such a blessing! This paper describing the fundamental of monotremes has also opened up other avenues of research, which we are working on.

Be sure to stay tuned for more!

Professor Tim Flannery, Honorary Associate, Australian Museum.

Meagan Warwick, AMRI Project and Communications Officer, Australian Museum.