Evolutionary Origins of the Strange Platypus and Echidna Found at Dinosaur Cove

Scientists may have gotten the evolutionary origins of some of the strangest animals on the planet backward. They’ve long thought that monotremes — egg-laying mammals that include the platypus and hedgehog-like creatures known as echidnas — originated on land. The platypus took the occasional aquatic foray, making it semi-aquatic, while echidnas stayed out of the water, the thinking went.

But new analysis of a bone found 30 years ago may mean that theory requires a revision. A single humerus, originally discovered at Dinosaur Cove in southeastern Australia, indicates that modern monotremes arose from water-dwelling ancestors, according to a report in the journal PNAS.

“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,” Suzanne Hand, a paleontologist with Australia’s University of New South Wales, and an author of the study, said in a press release.

If true, this would provide a rare example of a mammal beginning its evolutionary journey on land, but eventually returning to the water. About 30 mammals — including whales, dolphins, dugongs, seals, walruses, otters, and beavers — started on land, before venturing into water even part time. But a mammalian evolutionary path from water to land is virtually unheard of.

The clue leading to this conclusion comes down to a matter of bone density. Platypus bones are relatively compact, helping the duck-billed, beaver-tailed mammal remain under water with minimal effort. By contrast, echidna bones tend to be lighter and more hollow — consistent with a land-based lifestyle.

The single humerus from the Museums Victoria collection the scientists examined came from an extinct species called Kryoryctes, which lived about 108 million years ago. Ever since that find, scientists have debated whether Kryoryctes was terrestrial or amphibious. Outwardly, the Kryoryctes bone more closely resembled the shoulder of a contemporary echidna. Others thought the species represented a “stem monotreme” — meaning that it was an ancestor to both the platypus and the echidna.

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To help settle the debate, a team of scientists turned to imaging. The pictures have tipped the balance to an aquatic origin.

“The microstructure of the fossil Kryoryctes humerus is more like the internal bone structure seen in platypuses, in which their heavy bones act like ballast allowing them to easily dive to forage for food,” Hand said. “You see this in other semiaquatic mammals.”

If the assessment is correct, stem monotremes originated as semi-aquatic animals, with echidna bodies slowly evolving to favor a land-based existence, while the platypus bones maintained their density, giving those monotremes an aquatic edge.

Precisely when and where this divergence happened remains unclear. There are relatively few fossils of Mesozoic era monotremes. More bones from a number of a variety of those creatures over time could make the answer more definitive.

Read More: Ancient Species Represents Bridge Between Echidna and Platypus

Until more such fossils are found, the group will continue to analyze the single humerus with other methods. They will next focus on the microscopic makeup — or histology — of the bone. Traditionally, this has required taking bone sections from a fossil. That approach is not ideal when there is only one known bone from a single species available.

“Instead, we are applying powerful scanning techniques, including non-destructive synchrotron imaging of the fossil, in order to gather even more information to help unravel this ancient mystery,” said Hand.

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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.