Fossil discovery reinvents story of echidna and platypus evolution

If you’ve always thought echidnas and platypuses were distant cousins who went their separate ways on land and water, think again. A single fossilized arm bone, found in a remote corner of southeastern Australia three decades ago, is rewriting that family history—and possibly changing what scientists thought they knew about how mammals adapt to different environments.

They both belong to the Order Monotremata, a unique order of mammals that give birth to eggs instead of live offspring. The platypus is busy these days racing up and down creeks and rivers, and echidnas burrow around on the ground searching for ants, termites, and worms.

Scientists used to believe a while ago that they descended from an animal which inhabited the land, and platypuses subsequently developed to inhabit water. But new finds indicate otherwise: the origins of echidnas and platypuses may have started in the sea.

The find was directed by UNSW Sydney School of Biological, Earth and Environmental Sciences Emeritus Professor Suzanne Hand and a team of researchers. The fossil represents the incomplete left half of one upper arm bone of a small mammal named Kryoryctes cadburyi. It is estimated to have lived 108 million years ago during the Early Cretaceous era when dinosaurs ruled the earth.

It was found in the early 1990s by Museums Victoria excavators at Dinosaur Cove, a site that was famous for yielding cryptic ancient finds. The bone was at first thought to resemble the bone of an echidna today, and some had believed that it could be an echidna’s ancestor. Others believed that it could be a stem-monotreme, an ancient relative of the platypuses and echidnas.

But it wasn’t until scientists decided to ignore its outside form and look inside using sophisticated scanning techniques that the breakthrough finally came. “Where the outside shape of a bone will enable you to compare it directly to similar animals, its internal structure will tend to reveal something about its lifestyle,” says Hand.

In the humerus, researchers found features like those of no extant echidna. The bone had robust walls with a very small central cavity—features of semiaquatic mammals like the platypus. Dense bones play the ballast function in order to allow animals to dive and stay underwater without breaking through to the surface. Echidnas have lightweight bones more appropriate for digging on land.

The research explains Kryoryctes cadburyi as a semiaquatic digger and perfectly adapted to both life in water and also in soil. That would suggest an amphibious ancient monotreme way of life previously assumed by scientists to be otherwise. If indeed so, echidna and platypus development started in water and echidnas then evolved out of water to a life entirely on land.

“Such a scenario would be a highly unusual phenomenon,” Hand states, adding that while there are some 30 examples of mammals evolving from land to sea—whales, dolphins, seals, and otters, say—there is scarcely a reversion.

The fossil also indicates how little the platypus has changed. UNSW co-author Professor Michael Archer also adds that the line of evolution that has resulted in today’s platypus has exhibited extraordinarily high “niche conservatism” for over 100 million years, i.e., they’ve lived pretty much the same way for a very long period of time. The echidnas are a radical departure from such waterborne origins.

However, the fossil record of monotreme ancestors is limited. Jaw and tooth parts contribute to most Australian Mesozoic mammal fossils. Limb bone Kryoryctes cadburyi is the only one that is from that period, and it gives scientists a unique opportunity to learn about the animals’ habits.

“This humerus has given us an absolute great opportunity to find out about early Australian mammals and how they lived, and it is telling us a story—maybe one that we were not hoping to hear,” he says.

Echidnas are not river-swimming like platypuses but retain the marks of waterborne heritage in their bodies. Firstly, their bills have fewer electroreceptors—sensitive probes that pick up the faint electrical signals from prey—such as in a platypus’s bill. They are perhaps vestigial traces of an archaic foraging system. Even embryonic echidnas retain a weak trace of this heritage: traces of a platypus-like bill in embryonic stages.

Their legs are a hint too. Echidnas, and also platypuses, retain backward-pointing hind legs, though they use them to dig these days and not to swim. Backward-facing foot is not typical for mammals apart from the monotremes. Physiology also digs it out.

Echidnas also have a diving reflex—slowing of heart rate and conserving oxygen during submersion—the same as aquatic mammals. Myoglobin analysis, which measures the amount of oxygen-storing muscle protein, reveals echidnas have more than expected quantities for a burrowing mammal, showing their ancestors had once depended on extensive diving.

Since the Kryoryctes fossil is so unusual, researchers can’t just slice it open to look at its microscopic makeup, or histology. Instead, they’re using high-resolution, non-destructive scanning methods like synchrotron imaging to study the bone at increasingly smaller levels of detail.

The scientists are hopeful that such technologies will reveal more of the patterns of growth, physiology, and lifestyle of the animal without sacrificing the specimen. The research continues, and scientists are also applying their research to other fossil beds like Lightning Ridge in New South Wales. These beds have Mesozoic-age beds which have the potential to yield more monotreme remains, which can reconstruct missing links to this evolutionary past.

The new information rewrote not just our knowledge of the evolutionary history of platypus and echidna, but the general picture of how mammals evolve—or re-evolve—to new continents. If echidnas did inherit a marine ancestor, they belong to an extremely small minority of mammals to do a reverse of the common land-to-sea jump of evolution.

It poses interesting questions about what opportunities and pressures led echidnas onto land. Was it competition for food? Climate change? Or a combination of factors? Without more fossils, the answers remain out of reach. But one thing is clear: the story of these egg-laying mammals is far from finished.

From a single ancient bone, we’re learning that the history of life is rarely a straight line. Evolution can take surprising detours, and sometimes, it even turns around.

Research findings are available online in the journal PNAS.