Ancient bone discovery suggests aquatic origins for echidnas

Ancient bone discovery suggests aquatic origins for echidnas
by Simon Mansfield
Sydney, Australia (SPX) May 01, 2025
A fossilized bone unearthed three decades ago at Dinosaur Cove in southeastern Australia may rewrite the evolutionary history of monotremes, the group of egg-laying mammals that includes echidnas and platypuses.

Long held theories proposed that monotremes shared a land-dwelling ancestor, with platypuses later adapting to aquatic life while echidnas remained terrestrial. However, new research led by Emeritus Professor Suzanne Hand at the University of New South Wales (UNSW) suggests both species may have descended from a semi-aquatic forebear.

The pivotal finding centers on the analysis of a single humerus bone attributed to the extinct species *Kryoryctes cadburyi*, discovered in the early 1990s. The fossil, long debated by scientists, was previously thought to be more closely related to echidnas due to its external form. Yet, a new internal examination reveals otherwise.

Using CT and other advanced scanning technologies, researchers studied the bone's microstructure and found it bore more similarities to that of platypuses than echidnas. The heavy bone walls and reduced internal marrow cavity suggest aquatic adaptations, consistent with traits seen in modern semi-aquatic mammals such as otters and beavers.

"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," said Prof. Hand.

This contradicts previous assumptions and indicates that the echidna lineage may have undergone a rare evolutionary transition from water to land. Co-author Professor Michael Archer highlighted the significance of the find, noting that mammal fossils from Australia's Mesozoic era are exceptionally rare and usually limited to teeth and jaw fragments. *Kryoryctes cadburyi* is the only known example from a limb bone.

Further support for this aquatic heritage comes from several traits in modern echidnas. These include remnants of platypus-like bills in embryos, backward-facing hind limbs also seen in swimming platypuses, a diving reflex that conserves oxygen, and elevated levels of the oxygen-binding protein myoglobin, which is common in diving animals.

Researchers plan to conduct more detailed, non-invasive studies of the fossil using synchrotron imaging to better understand the microscopic bone structure. These efforts aim to uncover additional insights into the life and environment of early monotremes and resolve ongoing questions about their origins.

Research Report:Bone microstructure supports a Mesozoic origin for a semiaquatic burrowing lifestyle in monotremes (Mammalia)