A Single Prehistoric Bone Might Rewrite the History of the World’s Strangest Mammals Analysis of the fossil suggests that the only two egg-laying mammals, platypuses and land-based echidnas, both descended from a semi-aquatic creature New research suggests that land-based echidnas descended from semi-aquatic mammals.  Gunjan Pandey CC BY-SA 4.0 via Wikimedia Commons Between backward-facing-feet, nipple-less milk secretion and egg-laying, platypuses and echidnas are some of the strangest mammals on Earth. While the amphibious platypus spends much of its time in water, the echidna—also known as the spiny anteater—makes its home on land. Now, new analysis of a bone found three decades ago in Australia suggests that both animals evolved not from a creature that lived on land, as was previously thought, but from a water-dwelling animal. A new study published Monday in the journal PNAS, upended the widely held theory about monotremes—the most ancient living order of mammals, now down to just platypuses and echidnas. “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,” study co-author Laura Wilson, a researcher at the University of New South Wales’ School of Biological, Earth and Environmental Sciences in Australia, tells CNN's Amanda Schupak. The humerus bone unearthed in the 1990s dates back to between 103 million and 108 million years ago, and is the only fossil example of the extinct monotreme Kryoryctes cadburyi. The creature was an early common ancestor of platypuses and echidnas and the scientists used advanced scanning techniques to take a look at the bone's internal microstructure. “While the external structure of a bone allows you to directly compare it with similar animals to help work out the animal’s relationships, the internal structure tends to reveal clues about its lifestyle and ecology,” Suzanne Hand, a paleontologist at the University of New South Wales and a co-author of the study, says in a university statement. “So the internal structure doesn't necessarily give you information about what that animal actually is, but it can tell you about its environment and how it lived.” While on the outside, the bone appears similar to echidna bones, the internal structure of the bone was more like that of the platypus. The bone had thick walls and a narrower bone marrow cavity. The thicker walls function as ballast that helps semiaquatic animals dive. In contrast, echidna bones are much lighter. This “indicates that the amphibious lifestyle of the modern platypus had its origins at least 100 million years ago,” Hand tells Schupak, “and that echidnas made a much later reversion to a fully terrestrial lifestyle.” While dozens of examples exist of marine and semi-aquatic mammals evolving from land-based mammals—such as whales, dolphins, seals and otters—the vice-versa scenario is much rarer. "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," Hand explains in the statement. Researchers don't have enough fossil evidence to estimate when echidnas made their return to land. Nevertheless, monotremes can help scientists better understand the earliest stages of mammalian evolution, says Guillermo Rougier, a researcher in anatomical sciences and neurobiology at Kentucky’s University of Louisville who also wasn't involved in the study. "The typical mammal from the time of dinosaurs," he says, "probably shared a lot more biology with a monotreme than with a horse, a dog, a cat or ourselves."   Matthew Phillips, an evolutionary biologist at Queensland University of Technology who wasn't involved in the study, tells ABC's Ellen Phiddian that the recent research adds to previous evidence suggesting that both of today's surviving monotremes descended from semi-aquatic mammals. However, it's not proof. "You rarely get absolute proof of anything in science," he points out. "There are other potential explanations, but there's enough evidence coming down with that same conclusion that it's getting fairly difficult to deny." Get the latest stories in your inbox every weekday.