The surprising evolution of the weirdest animals on Earth, according to a new study

Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news on fascinating discoveries, scientific advancements and more.

The story of two of the strangest animals on the planet just got a little stranger, thanks to clues revealed by a lone fossil specimen that scientists now say represents a long-extinct ancestor. The new research could upend what’s known of the evolution of the most primitive mammals alive today.

Found in Australia and New Guinea, the platypus and echidna are called monotremes, and they are unique for being the only mammals that lay eggs.

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.

Peering inside an ancient fossil

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, a senior lecturer at the university’s School of Biological, Earth and Environmental Sciences.

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.”

Evolution of platypuses and echidnas tell the story of mammals

There are well-known examples of animals evolving from land to water — for example, it is believed that dolphins and whales evolved from land animals and share lineage with hippos. But there are few examples that show evolution from water to land. The transition requires “substantial changes to the musculoskeletal system,” Wilson said, including new positioning of the limbs for life on land and lighter bones to make moving less energy-intensive.

A land-to-water transition could explain the echidna’s bizarre backward feet, which Hand said it may have inherited from a swimming ancestor that used its hind legs as rudders.

“I think that they very elegantly prove the suggestion that these animals were adapted to a semiaquatic life very early on,” said Rougier, who was not involved in the study, though he did have contact with the authors during their research.

The primitive history of these unusual animals, he said, is “truly crucial” to our understanding of how mammals (including humans) came to be.

“Monotremes are these living relics from a very long distant past. You and a platypus probably had the last common ancestor over 180 million years ago,” he said. “There is no way to predict the biology of this last common ancestor without animals like monotremes.”

Amanda Schupak is a science and health journalist in New York City.

For more CNN news and newsletters create an account at CNN.com