Early Monotonicity: A Semi-Quantitative Discovery by Paleontologists

The prevailing belief regarding these egg sales has been that both species descend from land-dwelling ancestors. While the ancestors of the platymonds became semi-aquatic, Echidna remained terrestrial. However, a detailed examination of a humerus bone from Kryoryctes cadburyi, a prehistoric monotreme that lived in Victoria, Australia, during the early Cretaceous period, indicates that Echidna evolved from semi-aquatic ancestors, suggesting that the lifestyle of modern paddy field amphibians originated over 100 million years ago.

“Approximately 30 examples of mammals have evolved to live entirely or partially in water, such as whales, dolphins, dugongs, seals, otters, and beavers,” says Professor Suzanne Hand, a paleontologist from the University of New South Wales.

“However, it is exceedingly rare to observe mammals evolving in the opposite direction.”

“We are discussing semi-aquatic mammals that transitioned to a land-based existence. Such an event would be quite unusual, but I believe that’s what occurred with echidnas.”

In their recent study, Professor Hand and her team analyzed a single humerus bone of an ancient monotreme, Kryoryctes Cadbury.

Using CT scans and other techniques, they investigated the internal microstructure of the specimen.

“The external structure of bones allows for direct comparisons with similar animals to clarify relationships, while the internal structure offers insights into the beast’s lifestyle and ecology,” Professor Hand explained.

“Thus, while the internal structure might not reveal the animal’s identity, it can inform us about its environment and living conditions.”

Living in southern Victoria about 108 million years ago, Kryoryctes cadburyi and its relatives dominated the Australian mammalian fauna.

“Mesozoic mammals from Australia are uncommon and primarily known through their teeth and jaws,” states Professor Michael Archer from the University of New South Wales.

“Currently, Kryoryctes cadburyi is known solely from limb bones.”

“This humerus presents an extraordinary chance to gain insight into the lifestyles of Australian mammals, revealing a considerable story, perhaps one we weren’t initially anticipating.”

Researchers were taken aback when they discovered that the internal structure of the ancient humerus differed from that of modern Echidna bones.

“The internal structure shows that the bone wall of platymonds is quite thick with a narrow bone marrow cavity, while Echidna has a much thinner bone wall,” they noted.

“The fine structure of the Kryoryctes cadburyi humerus resembles that found in lizards, with its dense bone serving as ballast, aiding its ability to dive for food, a trait noticed in other semi-aquatic mammals.”

There are also peculiar traits in modern Echidna that might hint at an ancient semi-aquatic ancestry.

“It’s well established that the platypus bill contains numerous highly sensitive receptors that detect slight currents created by prey,” Professor Hand remarked.

“Although Echidna’s snouts have fewer receptors, it’s thought these might be remnants of their aquatic manipulative capabilities, along with traces of platypus-like features found in the embryos of echidnas.”

“Another characteristic inherited from their aquatic ancestors is the backward orientation of Echidna’s hind legs, similar to the rounded feet of the platypus when swimming.”

“In Echidnas, however, this trait is utilized for digging.”

“Our research has shown that echidnas exhibit diving reflexes when submerged in water, which assist in conserving oxygen and prolonging breath-holding capabilities. Studies on a respiratory protein called mammalian myoglobin further suggest echidna’s semi-aquatic ancestry.”

“The positive charge on myoglobin’s surface correlates with enhanced muscle capacity to retain oxygen, thereby extending the duration mammals can dive.”

“This is more prominent in platypuses, but even among burrowing mammals, echidnas show a higher than expected capacity.”

Survey results were published this week in Proceedings of the National Academy of Sciences.

____

Suzanne J. Hand et al. 2025. Bone microstructure supports the Mesozoic origin of monotremes’ semi-aquatic lifestyle. PNAS 122 (19): E2413569122; doi: 10.1073/pnas.2413569122