The tale of two of the planet’s oddest animals just took an even stranger turn, thanks to clues from a single fossil that scientists believe belonged to a long-extinct ancestor. This remarkable discovery could reshape our understanding of the evolution of today’s most primitive mammals, shedding new light on their ancient origins and surprising connections.
Native to Australia and New Guinea, the platypus and echidna—known as monotremes—stand out as the only egg-laying mammals.
Monotremes: Nature’s Strangest Mammals and a Glimpse into the Past
The amphibious platypus has a bill and webbed feet, like a duck, and a beaver-esque tail. The small creature spends much of its time hunting for food in the water. The echidna — fittingly known as the spiny anteater — lives entirely on land, is covered in pointy quills and has rear feet that face backward, kicking up dirt as the animal burrows into the ground. Neither animal has teeth, and though they both produce milk, they secrete it through their skin for babies (often called puggles) to lap at, because they lack nipples.
“There’s plenty of weirdness to go around on these little things,” said Dr. Guillermo W. Rougier, a professor in the department of anatomical sciences and neurobiology at Kentucky’s University of Louisville who studies early mammalian evolution.
“They are one of the defining groups of mammals,” Rougier said. “The typical mammal from the time of dinosaurs probably shared a lot more biology with a monotreme than with a horse, a dog, a cat or ourselves.” Therefore, he said, monotremes provide a window into the origins of mammals on Earth.
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Fossil Discovery Sheds New Light on Monotreme Evolution
A new study published Monday in the journal Proceedings of the National Academy of Sciences opens that window a little further. Research led by paleontologist Suzanne Hand, a professor emeritus at the University of New South Wales’ School of Biological, Earth and Environmental Sciences in Australia, reveals the internal structure of the only known fossil specimen of the monotreme ancestor Kryoryctes cadburyi, which lived more than 100 million years ago.
The fossil, a humerus, or upper arm bone, was discovered in 1993 at Dinosaur Cove in southeastern Australia. From the outside, the specimen looked more like a bone from a land-dwelling echidna than a water-loving platypus. But when the researchers peered inside, they saw something different.
“By using advanced 3D imaging approaches, we have been able to illuminate previously unseen features of this ancient bone, and those have revealed a quite unexpected story,” said study coauthor Dr. Laura Wilson, an associate professor at Australian National University.
Ancient Bone Reveals Aquatic Origins of Modern Monotremes
The team found that internally, the fossil had characteristics of the semiaquatic platypus: a thicker bone wall and smaller central cavity. Together, these traits make bones heavier, which is useful in aquatic animals because they reduce buoyancy, so it’s easier for the creatures to dive underwater to forage for food. By contrast, echidnas, which live solely on land, have much thinner, lighter bones.
The finding supports the popular, but unproven, hypothesis that Kryoryctes is a common ancestor of both the platypus and echidna, and that at the time of the dinosaurs, it may have lived at least partially in the water.
“Our study indicates that the amphibious lifestyle of the modern platypus had its origins at least 100 million years ago,” Hand said, “and that echidnas made a much later reversion to a fully terrestrial lifestyle.”
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