Scientists Discover That Echidnas Evolved from a Water-Dwelling Ancestor
Echidnas are already one of the strangest animals on the planet: they are covered in sharp spines, they have long sensitive beaks, they lay eggs, and they shuffle through the undergrowth of Australian forests seemingly unbothered by how unusual they are. But a new scientific discovery has made these creatures even more fascinating. According to a study published in the journal PNAS, echidnas most likely descended from an ancestor that lived in the water — and that makes them part of one of the rarest evolutionary stories ever recorded.
To understand why this is such a big deal, it helps to know a little about where echidnas fit in the animal kingdom. They belong to a group called monotremes — the only mammals in the world that lay eggs rather than giving birth to live young. The only other living monotremes are platypuses, semi-aquatic mammals with beaver-like tails, duck-like bills, and webbed feet built for swimming. Scientists had long assumed that both echidnas and platypuses descended from a land-living ancestor, and that only the platypus lineage later adapted to life in the water. The new study suggests the opposite is true: the common ancestor was likely aquatic, and it was echidnas that made the unusual switch to land.
The key evidence came from a single fossilized bone — a humerus, the upper arm bone — belonging to an extinct animal called Kryoryctes cadburyi, which lived approximately 108 million years ago during the Cretaceous period, when dinosaurs were still alive. The fossil was unearthed at a site in Victoria, Australia, called Dinosaur Cove. On the outside, the bone closely resembled those of modern echidnas, which initially led scientists to assume it belonged to a land-dweller. But the research team, led by vertebrate paleontologist Sue Hand from the University of New South Wales, decided to look deeper — literally.
Using micro-CT scanning technology, which works like an incredibly detailed X-ray that reveals a three-dimensional image of what is inside an object, the researchers examined the bone's internal structure. What they found surprised them: the inside of the bone looked far more like a platypus bone than an echidna bone. Modern platypus bones have very thick, dense walls, while echidna bones are comparatively thin and light. Thick, heavy bones act like ballast — similar to the weight belts that scuba divers wear to help them sink rather than float — making it easier for an animal to dive and stay submerged underwater. This strongly suggested that Kryoryctes was a semi-aquatic burrower, comfortable in both water and underground tunnels, and that the ancestors of both platypuses and echidnas shared this aquatic lifestyle.
What happened next, in evolutionary terms, is what makes the echidna story so unusual. Plenty of mammals have transitioned from land to water over millions of years — whales, dolphins, seals, and beavers all have land-dwelling ancestors. But the reverse journey, from water to land, is extraordinarily rare. Sue Hand explained that the more aquatic a mammal becomes, the harder it is to return to land, because the body changes required for life in water — heavy bones, webbed feet, streamlined bodies — work against survival on dry ground. Semi-aquatic burrowers like the platypus, however, are uniquely positioned to tip either way, since they are already adapted to navigate both environments.
Other clues support this theory too. Baby echidnas developing inside their eggs have receptors on their beaks that can detect tiny electrical signals, a feature that is most useful for locating prey underwater. Adult echidnas retain some of these receptors, and platypuses have even more of them. Additionally, echidnas have backward-pointing hind feet — an adaptation used by platypuses as a kind of biological rudder for steering in the water. These features look less like coincidences and more like biological echoes of a watery past. Paleontologist Tim Flannery of the Australian Museum, who was not involved in the research, called the findings another strong piece of evidence in what he described as an increasingly undeniable case for echidnas having platypus-like aquatic ancestors.
Source: Live Science