Up until now, the accepted understanding about these egg-laying was that they were both descended from a land-bound ancestor. And while the platypus ancestors became semiaquatic, the echidnas stayed on the land, or so the story went. But an analysis of a single humerus bone from Kryoryctes cadburyi — a prehistoric monotreme mammal that lived in what is now Victoria, Australia during the Early Cretaceous epoch — suggests that echidnas evolved from semiaquatic ancestors and that the amphibious lifestyle of the modern platypus had its origins at least 100 million years ago.

“There are about 30 instances where mammals evolved from land-dwelling to live wholly or partly in water, for example, whales, dolphins, dugongs, seals, walruses, otters and beavers,” said Professor Suzanne Hand, a paleontologist at the University of New South Wales.

“But it’s virtually unheard of to see mammals evolve in the opposite direction.”

“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.”

In their new study, Professor Hand and her colleagues examined a single humerus bone of the ancient monotreme mammal Kryoryctes cadburyi.

Using CT and other scanning techniques, they studied the internal microstructure of the specimen.

“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,” Professor Hand said.

“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.”

When Kryoryctes cadburyi lived in southern Victoria around 108 million years ago, monotremes and their relatives dominated Australia’s mammal faunas.

“Australian Mesozoic mammals are rare and are known mostly only from their teeth and jaws,” said Professor Michael Archer, also from the University of New South Wales.

“Kryoryctes cadburyi is so far the only one known from a limb bone.”

“This humerus has provided an exceptional opportunity to gain insights into how early Australian mammals lived, and it tells quite a story–perhaps not one we expected to discover.”

When the researchers looked at the internal structure of the ancient humerus bone, they were surprised to discover it didn’t match the light bones of echidnas.

“The internal structure revealed platypuses have very thick bone walls and a very reduced cavity within the bone for the bone marrow, while echidnas have very thin bone walls,” Professor Hand said.

“The microstructure of the fossil Kryoryctes cadburyi 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.”

There are other oddities about modern echidnas that could be further clues that speak to their semi-aquatic, ancient past.

“The bill of the platypus is well known to have lots of highly sensitive receptors that detect tiny electrical currents generated by prey,” Professor Hand said.

“And while the beak of the echidnas has fewer receptors, people have suggested that these receptors are a leftover of their platypus-heritage, as are remnants of the platypus bill that can be found in the beak of echidna embryos.”

“Another feature that could’ve been inherited from an aquatic ancestor is that the echidnas’ hind feet are turned backwards, much like a platypus’s hind feet which it uses as rudders when swimming.”

“But in echidnas, this feature is used when burrowing, something not seen in other mammals except platypuses.”

“Echidnas have also been found to have a diving reflex triggered when immersed in water that helps conserve oxygen and prolong breath-holding, and a study of a respiratory protein called myoglobin in mammals also suggests a semi-aquatic ancestry for echidnas.”

“A positive charge on the surface of myoglobin is associated with increased capacity of the muscles in the body to store oxygen and increase the length of time a mammal can spend diving.”

“This is high in the platypus, but is also higher than expected in echidnas, even as burrowers.”

The findings were published this week in the Proceedings of the National Academy of Sciences.

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Suzanne J. Hand et al. 2025. Bone microstructure supports a Mesozoic origin for a semiaquatic burrowing lifestyle in monotremes (Mammalia). PNAS 122 (19): e2413569122; doi: 10.1073/pnas.2413569122