Scientists Propose Surprising New Theory On Echidna Origins

A new study of a 100-million-year-old fossil found in an Australian coastal rock suggests that echidnas may have descended from aquatic ancestors.

This is rare—unlike most mammals that moved from land to water, echidnas seem to have gone the other way.

Debate Over the Fossilized Arm Bone: Terrestrial Ancestor or Aquatic Monotreme?

Since unearthing the fossilized arm bone in the early 1990s, paleontologists have actively debated which animal left it behind. They have confirmed it belongs to a monotreme—a group of egg-laying mammals.

Some researchers think the bone came from a land-dwelling echidna ancestor, while others say it belonged to an earlier aquatic monotreme.

New analysis on the tiny humerus bone, which belonged to the prehistoric species Kryoryctes cadburyi, supports an aquatic origin story.

“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,” says paleontologist Suzanne Hand from the University of New South Wales, who led the study.

“The internal structure doesn’t always reveal the exact identity of the animal, but it can offer clues about its habitat and lifestyle.“

Hand and her team used microCT scans to find that the bone has thick walls and a tiny medullary cavity, where blood cells form.

Aquatic Adaptations: Fossil Bone Structure Resembling Platypuses’ Buoyancy-Reducing Traits

Bones with this structure are common in partially or fully aquatic mammals such as sea otters, dugongs, and platypuses. Like a diver’s weight belt, these traits reduce buoyancy, helping the animal stay underwater with little effort.

“The internal bone structure of the fossilized Kryoryctes humerus closely resembles that of platypuses, whose dense bones act as ballast to help them dive and forage underwater,” explains Hand.

On land, heavy bones are a drawback, requiring more energy to move and being more prone to breakage. That’s why echidnas, which are fully terrestrial, have much thinner bone walls.

A Surprising Ancestry: Phylogenetic Clues Point to Aquatic Origins of Echidnas

The researchers ran a phylogenetic analysis to place K. cadburyi in mammal and monotreme evolution. Their findings show it’s a stem monotreme, meaning it shares a common ancestor with both echidnas and platypuses.

This twist in echidna evolution suggests their ancestor was an aquatic, burrowing monotreme that later adapted to life on land. There are some additional hints supporting this idea.

Hand notes that the platypus’s bill contains an abundance of sensitive receptors that detect faint electrical signals from prey.

“Echidnas have fewer of these receptors in their beaks, but it’s been suggested they’re evolutionary remnants from their platypus-like ancestors—just like the traces of a platypus-style bill seen in echidna embryos.”

Echidnas also have hind feet that face backward—a trait that aids in digging, similar to how platypuses use their backward-facing feet as rudders for swimming. This unique foot structure, found in no other mammals, may explain how echidnas travel between islands.

It’s possible that echidnas weren’t simply stranded on land by their aquatic relatives like the platypus—maybe they were the ones who chose to leave the water behind and explore life on land.

“We’re looking at a semiaquatic mammal that transitioned to living entirely on land,” says Hand. “While that kind of shift is incredibly rare, we believe that’s exactly what happened with echidnas.”

Read the original article on: Sciencealert

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