Australia's burrowing echidna evolved from a water-dwelling ancestor in an "extremely rare" biological event, scientists said Tuesday in a new study of the peculiar egg-laying mammals.
With powerful digging claws, protective spikes and highly sensitive beaks, echidnas are well suited to a life shuffling through the forest undergrowth, AFP said.
But a team of Australian and international scientists believe many of the echidna's unusual traits were first developed millions of years ago when its ancestors splashed through the water.
"We're talking about a semiaquatic mammal that gave up the water for a terrestrial existence," said paleontologist Suzanne Hand, from the University of New South Wales.
"While that would be an extremely rare event, we think that's what happened with echidnas."
Echidnas and another Australian oddity, the semi-aquatic platypus, are believed to have evolved from a common ancestor called Kryoryctes cadburyi that lived in Australia more than 100 million years ago.
Researchers studied the single known bone fragment left by this ancestor, which was discovered among a trove of fossils at Dinosaur Cove in southern Australia some 30 years ago.
Platypus bones were similar to this ancient ancestor, Hand said, with a thick and heavy structure that provided ballast for diving.
Echidnas, in comparison, had very thin bone walls that made it easier to walk on land, Hand said.
This indicated echidnas were descended from a water-dwelling ancestor but had evolved to live on land, the research found.
It was far more common for prehistoric mammals to go from land to water, Hand said, pointing towards seals, whales, dolphins and dugongs.
The researchers said these findings appeared to be supported by other echidna traits.
Echidnas have backward-facing hind feet that help them shift mounds of soil when burrowing.
These feet may have first developed as rudders helping the echidna's ancestor navigate fast-moving waterways, Hand said.
Echidnas also have a "diving reflex" when submerged in water, which tells their body to conserve oxygen helping them hold their breath for longer.
Echidnas and platypus are monotremes, a rare group of mammals that lay eggs instead of live young.
"We're hoping we'll discover other ancestral monotremes that will help unravel the early history of this most fascinating group of mammals," said study co-author Michael Archer.
The research was published in peer-reviewed journal PNAS.
