New fossil evidence suggests ancient echidnas lived in water

A small bone found 30 years ago at Dinosaur Cove in Victoria could change what we know about the evolution of two of Australia’s most unique animals, the echidna and platypus.

Until now, it was believed that both species of egg-laying monotremes descended from a land-dwelling ancestor, and while the platypus eventually became semiaquatic, the echidnas stayed on the land.

But new analysis of the bone, which was discovered by a team from Museums Victoria, supports the idea that both echidnas and platypuses evolved from a water-dwelling ancestor.

Associate Professor Laura Wilson from The Australian National University (ANU) co-led the study, working on the 3D shape analysis of the humerus bone.

“A highlight of our work has been the excitement of using a new set of tools to revisit a fossil unearthed decades ago. By using advanced 3D imaging techniques, we have been able to illuminate previously unseen features of this ancient bone, and those have revealed a quite unexpected story,” Associate Professor Wilson said.

According to Associate Professor Wilson and team, the discovery is particularly exciting because while there are around 30 examples of mammals evolving from living on the land to living in the water, such as dolphins, seals and otters, it’s almost unheard of to see mammals evolve in the opposite direction.

The fossil was found in the early 1990s and is the only known bone belonging to the extinct species Kryoryctes cadburyi, another monotreme mammal.

Kryoryctes cadburyi lived in southern Victoria around 108 million years ago, during the Age of Dinosaurs, or Mesozoic Era, when monotremes and their relatives dominated Australia’s mammal faunas.

Lead author, Emeritus Professor Suzanne Hand from UNSW Sydney, said the team used high resolution CT and other scanning techniques to take a closer look at the fossil, in the hopes of answering the question of whether it was a common ancestor of the platypus and echidna, or more closely linked to just one species.

“The microstructure of the fossil Kryoryctes humerus is more like the internal bone structure seen in platypuses, in which their heavy bones act like ballasts that allow them to easily dive to forage for food. You see this in other semiaquatic mammals,” Emeritus Professor Hand said.

“We know that bone histology can say a lot about how an animal lives. This kind of investigation typically requires destructive sectioning of a bone, which is not possible to do with a unique fossil like Kryoryctes. Instead, we are applying powerful scanning techniques, including non-destructive imaging of the fossil, in order to gather even more information to help unravel this ancient mystery.”

The research was a collaboration between Australian institutions and international organisations from Canada, Colombia, France and the UK.

The full study has been published in Proceedings of the National Academy of Sciences (PNAS).