Ancient echidna relatives may have lived in water, study suggests
This water-dwelling monotreme may have been an ancestor or relative of modern platypuses and echidnas. (Supplied: Peter Schouten)
In short:
An analysis of a 100-million-year-old fossil has found that the creature it belonged to, similar to a monotreme like the echidna and platypus, was water-dwelling.
The study supports the theory there was a period where echidna ancestors were semiaquatic, before they became land-dwellers again.
What's next?
Researchers say learning more about monotreme evolution will help with the conservation of modern echidnas and platypus.
Egg-laying echidnas and platypuses are among the world's most bizarre mammals.
With a spotty fossil record, it's difficult to tell how these monotremes evolved.
But evidence is mounting that the ancestor of the ground-dwelling echidna may have looked more like a platypus — complete with a penchant for swimming.
Palaeontologists have today published an analysis of a fossilised bone in the Proceedings of the National Academy of Sciences that further adds to this evidence.
The bone belongs to a 100-million-year-old "stem-monotreme", which may have been an ancestor or relative of modern platypuses and echidnas.
According to the analysis, the ancient animal appears to have spent a large amount of its life in the water.
The bone was found 30 years ago at Dinosaur Cove in Victoria. (Supplied: Peter Menzel)
Lead author Suzanne Hand, a palaeontologist at the University of New South Wales, said the study could help to inform conservation efforts for modern echidnas and platypuses.
"The platypus looks as though it's been incredibly resilient, having basically the same lifestyle probably for 100 million years," Professor Hand said.
"And then there's the echidna, also capable of adapting to such fundamental change — going from water to land.
"If we can understand how they've evolved and how they adapted their lifestyles like that, that could be important in terms of all the threats they're facing today."
An ancient monotreme bone
The study focuses on a single fossilised bone, found at the Dinosaur Cove fossil site in Victoria in the early 1990s.
The 100-million-year-old fossil was thought to belong to a monotreme species and outwardly resembled the upper arm bone of an echidna more than a platypus's.
The extinct species was named Kryoryctes cadburyi in 2005. To date, no other fossils from the species have been found.
Researchers used a variety of scans, including CT scanning, to study the bone's internal structure.
They compared this to scans of other animal bones, to see how the fossil compared to living species.
They found the bone had thick walls and reduced amounts of marrow, making it most similar to modern semiaquatic burrowing mammals.
"That includes things like the platypus, but also things like Eurasian otters, muskrats and other relatively small mammals," Professor Hand said.
Midsections of Kryoryctes, platypus, and echidna bones, from left to right. (Supplied: Hand et al.)
Thicker bone walls allow the bones to act as ballast, which helps semiaquatic mammals to dive underwater. In contrast, the bones of modern echidnas have finer bone walls, while fully aquatic mammals like dolphins have different structures again.
Mammals first emerged on land, and there are several cases of them evolving to become aquatic or semiaquatic, such as dolphins, seals, and beavers.
But Professor Hand said it was very unusual for a mammal to become semiaquatic, and then return to the land, as the echidna appeared to have done.
"We don't know when echidnas may have returned to the land."
Building the semiaquatic echidna theory
Palaeontologists have theorised that echidnas had semiaquatic ancestors before returning to the land, but Professor Hand said the idea had been "controversial".
"There hasn't really been any fossil evidence to support it or refute it,"she said.
Matthew Phillips, an evolutionary biologist at Queensland University of Technology who wasn't involved in the research, said non-fossil evidence had been building behind the semiaquatic theory.
This evidence includes studies of platypus and echidna genomes, their anatomy, and their proteins.
"It's coalescing around an idea that if you looked at what the most recent common ancestor of platypus and an echidna was, that it was somewhat more platypus-like," Professor Phillips said.
The fossil study, according to Professor Phillips, adds to these "multiple lines of evidence" — although it wasn't absolute proof.
"You rarely get absolute proof of anything in in science. There are other potential explanations, but there's enough evidence coming down with that same conclusion that it's getting fairly difficult to deny."
Tim Flannery, a mammalogist at the Australian Museum, who also wasn't involved in the research, agreed the study made sense.
"It makes a convincing case that it's a semiaquatic, maybe burrowing, organism."
But there were still many gaps in the fossil evidence of monotreme evolution, Professor Flannery said.
As a result, it's hard to tell when echidnas and platypus species finally diverged.
"That's still as mysterious as ever, sadly."