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A small bone discovered three decades ago at Dinosaur Cove in southeastern Australia has the potential to significantly alter our understanding of the evolutionary path of echidnas and platypuses.
Historically, it has been believed that these unique egg-laying mammals descended from a terrestrial ancestor, with platypuses adapting to a semi-aquatic life while echidnas remained land-dwelling. However, an analysis led by UNSW suggests that both echidnas and platypuses might share a common ancestor that thrived in water.
Emeritus Professor Suzanne Hand from UNSW School of Biological, Earth and Environmental Sciences notes that mammals have evolved from land-dwelling to aquatic lifestyles numerous times, citing examples such as whales and seals. Conversely, evolution from an aquatic existence back to land is exceedingly rare.
“What we are seeing here is an unusual case, where a semi-aquatic mammal transitioned back to a terrestrial lifestyle, which adds complexity to our understanding of echidna evolution,” she explains.
The Significance of the Humerus
In a study featured in PNAS, researchers detail how a single humerus bone found in Victoria in the early 1990s challenges the established theory of a terrestrial ancestor. This bone, which belongs to the now-extinct species Kryoryctes cadburyi, serves as the only known limb bone from this group of mammals.
The humerus displayed external characteristics similar to those of echidnas, leading some researchers to propose that it might have been an early ancestor to modern echidnas. However, others argue it represents a common ancestor for both echidnas and platypuses, a lineage known as a stem-monotreme. This has led to ongoing debates about whether Kryoryctes lived solely on land or had amphibious traits like modern platypuses.
To address these uncertainties, Prof. Hand and her colleagues examined the fossil’s internal microstructure using advanced imaging techniques. “While comparing external bone structures helps reveal relationships, internal structures offer insights into the ecological and lifestyle adaptations,” she observes.
During its existence around 108 million years ago, Kryoryctes cadburyi contributed to a diverse array of monotremes that inhabited Australia. “Fossils from this period are scarce, typically seen in the form of teeth or jaws. Kryoryctes is unique because it is the only known specimen to include a limb bone,” adds co-author Professor Michael Archer, also from UNSW School of BEES.
This humerus presents a rare opportunity to uncover details about early Australian mammalian lifestyles, contradicting previous assumptions about their evolutionary history.
Upon examining the humerus’s interior, researchers found that its structure did not match the lightweight bones of living echidnas. “The internal analysis suggested a stronger resemblance to the bones of platypuses, which are built to support diving through denser structure,” says Prof. Hand.
The findings bolster the hypothesis that the early monotremes were semi-aquatic. As echidna ancestors transitioned to land, their bones adapted to become lighter. However, the fossil record remains limited, making it challenging to pinpoint the timing of these changes.
“This situation emphasizes the importance of our ongoing research into Mesozoic opalized fossils located at Lightning Ridge in New South Wales, which could help clarify the lineage of these remarkable mammals,” Prof. Archer notes.
Interestingly, some traits found in modern echidnas could provide clues to their semi-aquatic ancestry. Prof. Hand highlights the highly sensitive receptors in the platypus bill that detect electrical currents from prey. Although fewer in echidnas, similar receptors may hint at shared heritage. Additionally, echidnas possess hind feet turned backward, reminiscent of platypus adaptations for swimming, which also assist in their burrowing behavior.
There is also evidence that echidnas exhibit a diving reflex when submerged, conserving oxygen, similar to aquatic mammals. Moreover, a respiratory protein known as myoglobin, found in echidnas, suggests they may have a greater capability for oxygen storage, another indication of possible aquatic roots.
Looking ahead, researchers plan to delve deeper into the histology of the Kryoryctes humerus for more insights. “Understanding bone histology can reveal much about an animal’s lifestyle, but studying a unique fossil requires non-invasive technologies like synchrotron imaging to avoid damaging the specimen,” concludes Prof. Hand.
The research involved collaboration between UNSW, the Australian National University, and several international institutions, further emphasizing the global interest in unraveling the early history of these unique mammals.
More information: 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.
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phys.org