Sometimes, a single fragment of bone can shift the foundations of scientific understanding. At Dinosaur Cove in southeastern Australia, almost thirty years ago, a small, seemingly unremarkable fossil humerus was unearthed—a relic from a forgotten era. For decades, it sat quietly in collections, its secrets locked inside, until a recent reexamination led by researchers from the University of New South Wales (UNSW) revealed something astonishing: this little bone could upend everything we thought we knew about the evolutionary journey of some of Earth’s most peculiar mammals—the echidna and the platypus.
Until now, the prevailing view was simple. Echidnas and platypuses, members of the exclusive club known as monotremes, were thought to share a common ancestor that lived solely on land. Over time, according to conventional wisdom, the lineage leading to the platypus adapted to life in water, while the echidna lineage stayed true to its terrestrial roots. But the latest analysis tells a far stranger—and far more fascinating—story.
Meet Kryoryctes cadburyi: A Lone Clue from the Past
The fossil in question belongs to a creature called Kryoryctes cadburyi, named formally in 2005. All we have of this enigmatic species is a single humerus—the upper arm bone connecting the shoulder to the elbow. Early on, scientists noted the external appearance of the bone bore some resemblance to those of modern echidnas, fueling theories that Kryoryctes might be an early terrestrial monotreme, closer to echidnas than platypuses.
Yet some experts had reservations. Perhaps Kryoryctes was not an echidna ancestor at all but a more generalized stem-monotreme—a creature that existed before platypuses and echidnas branched off into their own evolutionary paths. Still, without more fossils to analyze, speculation could only go so far. That’s when Emeritus Professor Suzanne Hand and her team decided to take a deeper look—literally.
Digging Deeper with Modern Technology
Professor Hand and her colleagues employed cutting-edge CT and micro-imaging technologies to peer inside the ancient humerus without damaging it. In the world of paleontology, studying the internal microstructure of bones can yield profound insights about an animal’s ecology, behavior, and environment.
“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,” explains Hand. It was a smart approach: while the fossil’s outward shape hinted at an echidna connection, its inner architecture told a very different story.
What the researchers found was surprising. The internal structure of Kryoryctes‘ humerus resembled that of a platypus far more than an echidna. Platypuses have dense, heavy bones with thick walls and tiny marrow cavities—a feature that helps them stay submerged and maneuver underwater. Echidnas, in contrast, have light, airy bones, better suited for life on land. The Kryoryctes bone, dense and thick, suggested a semiaquatic lifestyle.
This finding flipped the traditional evolutionary narrative on its head: it wasn’t that the platypus lineage alone took to the water while echidnas stayed dry. Instead, both may have descended from semiaquatic ancestors—and it was the echidnas that later abandoned aquatic life for dry land.
The Rarity of Reverse Evolution
Evolutionary shifts from land to water are relatively common in mammalian history. Whales, dolphins, manatees, seals, otters, and beavers all illustrate this transition. Yet the opposite—a return from aquatic to terrestrial living—is exceedingly rare.
“We’re talking about a semiaquatic mammal that gave up the water for a terrestrial existence,” says Professor Hand. “And while that would be an extremely rare event, we think that’s what happened with echidnas.”
The study, published in the prestigious journal Proceedings of the National Academy of Sciences (PNAS), adds to a growing body of evidence suggesting that the evolutionary story of monotremes is far more complicated and fascinating than previously believed.
More Clues Hidden in Modern Echidnas
The revelation about Kryoryctes shines a new light on some of the peculiar traits seen in modern echidnas—traits that may be evolutionary leftovers from a semiaquatic past.
For instance, the beak of an echidna, although quite different from the platypus’s bill, contains electroreceptors. In the platypus, these receptors allow it to detect the faint electric fields generated by the muscle movements of prey underwater. Echidnas have far fewer of these receptors, but their existence suggests a watery heritage.
Moreover, embryonic development provides additional hints. In echidna embryos, remnants of a platypus-like bill are visible before they transform into the more typical echidna beak.
Even the structure of their limbs tells a story. Platypuses use their hind feet, turned backwards, to help steer in water. Echidnas, curiously, also have hind feet rotated backward—not for swimming, but for efficient burrowing. This odd arrangement hints at an evolutionary adaptation repurposed for a very different way of life.
The diving reflex is another relic of aquatic ancestry. When immersed in water, echidnas exhibit a physiological response to conserve oxygen and extend breath-holding—a trait that would be useless for a purely land-based creature but essential for an animal accustomed to foraging underwater.
Finally, the presence of a specialized respiratory protein called myoglobin in echidnas provides a molecular clue. Myoglobin stores oxygen in muscle tissue, enabling extended dives in aquatic mammals. Levels of myoglobin in echidnas are higher than expected for land-dwellers, suggesting they once spent more time submerged than their modern lifestyles would indicate.
The Mystery of Mesozoic Mammals
The time when Kryoryctes cadburyi lived—around 108 million years ago—was a very different world. It was the mid-Cretaceous period, the Age of Dinosaurs, when southern Victoria was a polar environment, shrouded in darkness for weeks at a time during winter. Despite the chill, this region teemed with diverse life, including unique mammals like monotremes.
Australian Mesozoic mammals remain among the rarest finds in the fossil record. Most are known only from fragments of jaws and teeth. The Kryoryctes humerus thus provides an extraordinary window into the lifestyles of these ancient creatures. As Professor Michael Archer, co-author of the study, notes, “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.”
What Comes Next? The Search Continues
Despite these tantalizing findings, the story is far from complete. The fossil record remains painfully sparse, and the exact timeline of echidnas’ transition from water to land remains murky. To fill in the blanks, scientists are intensifying their search, particularly in the opalized fossil fields of Lightning Ridge in New South Wales, where similarly aged deposits might yield more ancestral monotremes.
“We’re hoping we’ll discover other ancestral monotremes that will help unravel the early history of this most fascinating group of mammals,” says Professor Archer.
In addition to fieldwork, researchers plan to delve deeper into the Kryoryctes fossil using non-destructive techniques like synchrotron imaging, which can reveal microscopic details without damaging the precious specimen.
As technology advances, so too does our ability to reconstruct the deep past. Each new discovery peels back another layer of mystery surrounding Earth’s oldest surviving lineage of egg-laying mammals.
An Evolving Picture of Life’s Adaptability
The reexamination of the Kryoryctes cadburyi humerus offers more than a fresh take on monotreme evolution—it challenges our broader assumptions about evolutionary trajectories. It reminds us that evolution is not a simple, linear path but a dynamic, branching, and sometimes reversing journey shaped by countless pressures and possibilities.
Echidnas, once envisioned as stubborn landlubbers who never strayed into water, are now being reimagined as descendants of skilled aquatic foragers who left the streams and rivers behind in favor of dry land. It’s a rare evolutionary gamble that, given the continued survival of echidnas today, clearly paid off.
In the end, the tiny humerus from Dinosaur Cove is more than just a fossil. It is a messenger from a distant past, whispering a story of adaptation, resilience, and the surprising twists and turns of life’s great adventure.
Reference: Hand, Suzanne J. et al, Bone microstructure supports a Mesozoic origin for a semiaquatic burrowing lifestyle in monotremes (Mammalia), Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2413569122. doi.org/10.1073/pnas.2413569122
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