Evrim Yazgin
Cosmos science journalist
A small bone found 30 years ago has been reanalysed revealing perhaps the stem ancestor of platypus and echidnas went through an “extremely rare” evolutionary change.
Platypus and echidnas are the only surviving members of the monotreme family. This includes the only mammals to lay eggs rather than giving birth to live young. For this reason, they are considered relics of early mammal evolution.
Fossils of monotremes have been found in South America and palaeontologists believe they also existed in other southern continents and areas like Antarctica, New Zealand and Africa. Today, monotremes only live in Australia and New Guinea.
Echidnas are burrowing land animals while platypus are semiaquatic. Up until now, it was believed that both animals evolved from a land-based ancestor.
The new study, published in the Proccedings of the National Academy of Sciences journal, upends this belief, suggesting the ancestor of modern echidnas and platypuses was a semiaquatic animal.
“We’re talking about a semiaquatic mammal that gave up the water for a terrestrial existence, and while that would be an extremely rare event, we think that’s what happened with echidnas,” says lead author Suzanne Hand from the University of New South Wales (UNSW) in Australia.
There are dozens of mammals which evolved from terrestrial animals to live wholly or partly in the water. Examples are whales, dolphins, dugongs, seals, otters and beavers.
But mammals going the other way is virtually unheard-of.
The upper arm bone is the only fossil found belonging to the extinct monotreme Kryoryctes cadburyi which lived 106 million years ago during the Cretaceous period – the last in the “Age of Dinosaurs”. It was found in southeastern Australia’s “Dinosaur Cove” in the Australian state of Victoria in the 1990s.
On the surface, the bone resembles echidnas more than platypuses, leading some palaeontologists to suggest it was an ancestor of echidnas. But others thought it might be a “stem monotreme” – an ancestor of both echidnas and platypuses.
Hand and her colleagues used CT and other scanning techniques to find out more about the ancient creature.
“While the external structure of a bone allows you to directly compare it with similar animals to help work out the animal’s relationships, the internal structure tends to reveal clues about its lifestyle and ecology,” Hand says. “So the internal structure doesn’t necessarily give you information about what that animal actually is, but it can tell you about its environment and how it lived.”
The fossil’s internal structure didn’t match the light bones of modern echidnas. It has a dense structure more similar to platypus bones.
“The microstructure of the fossil Kryoryctes humerus is more like the internal bone structure seen in platypuses, in which their heavy bones act like ballast allowing them to easily dive to forage for food. You see this in other semiaquatic mammals,” Hand explains.
This may help explain why echidnas have highly sensitive electrical receptors at the end of their nose – far fewer than the platypus.
Echidnas have backwards-turned feet which it uses for burrowing. Platypuses, too, have backwards feet which it uses as rudders while swimming. Echidnas are also known to have a diving reflex when they are immersed in water.
These traits could be answered by both animals having a semiaquatic common ancestor.
