Echidnas just got weirder. As well as being quill-covered mammals that lay leathery eggs, it now seems that these land-living animals evolved from water-dwelling ancestors.
Until now, it was presumed that echidnas are descended from a land-bound ancestor, but a new study published in PNAS suggests a different story.
Kryorycytes cadburyi was an ancient egg-laying mammal, and ancestor of the echidna, that lived around 108 million years ago in what is now southern Victoria, Australia. It is known only from a single chocolate-coloured upper arm bone (which explains the name cadburyi), dug up at Dinosaur Cove on the south coast of the state in the 1990s.
Researchers from the University of New South Wales used powerful scanning techniques to study the bone’s internal structure, only to find that it was more platypus- than echidna-like.
Echidnas and platypuses, which are the only two living species of egg-laying mammal, are closely related. Echidna bones have thinner walls with bigger bone marrow cavities, whilst platypus bones have thicker walls with smaller bone marrow cavities. The comparatively heavy platypus bones are thought to act like ballast, which helps the animal when it is diving for food.
The results of the new study suggest that the early ancestors of echidnas and platypuses were semi-aquatic. Then, somewhere along the line, these animals moved onto the land, where their bones became lighter as they adapted to a new way of life.
There are around thirty instances of aquatic or semi-aquatic mammals that evolved from land-dwelling ancestors. This includes whales, dolphins, dugongs and seals. But it’s very unusual for mammals to evolve in the opposite direction.
“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,” says palaeontologist Suzanne Hand, who led the study.
Modern echidnas also have other oddities which may pay point to a watery past.
Their hind feet, for example, are turned backwards, much like those of the platypus which are used as rudders when swimming. “But in echidnas, this feature is used when burrowing, something not seen in other mammals except platypuses,” says Hand.
Echidnas also have a diving reflex that is triggered when they are under water, and which helps them to hold their breath and conserve oxygen, and a particular version of a respiratory protein called myoglobin, which has been linked to an aquatic lifestyle.
So just when you thought it couldn’t get any weirder, the echidna doesn’t fail to disappoint.
Main image: the dig at Dinosaur Cove where the Kryroryctes humerus bone was found. Credit: Peter Menzel
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