Meet the ‘echidnapus’, a bizarre blend of the world’s strangest creatures

From the electrosensitive bill and venomous spur of a platypus to the echidna’s thorny armour and four-pronged penis, the zoological quirks of modern-day monotremes have long attracted international awe and scientific wonder.

Now, relying on evidence that has emerged from the opal fields of Lightning Ridge in outback NSW, Australian scientists including Tim Flannery have claimed a previously unknown array of these extraordinary egg-laying mammals once dominated the prehistoric Australian landscape before this continent became the land of marsupials.

Six monotreme species have emerged from the opalised fossils of the 100 million-year-old site, and three are completely new to science. The newly described species represent at least five different taxonomic families and include the enigmatic “echidnapus”, a creature that had features of the platypus and echidna.

Six ancient species of monotreme have emerged from Lightning Ridge opalised fossils, including the “echidnapus” (bottom left).Credit: Peter Shouten

“What it shows us is that 100 million years ago, we had six kinds of monotreme that were living at the same time in the same place. And we have no other mammals from this site,” chief scientist of the Australian Museum Professor Kris Helgen said.

“So this gives us this amazing first-time window into a time in Australia’s past where you have, essentially, a world of monotremes.”

Marsupials arrived 54 million years ago from South America, said Professor Flannery, lead author of the research and an honorary associate of the museum.

“I don’t think we’ll ever make a discovery like this again, because the broad overview of mammal evolution on the various continents is pretty well understood now,” Flannery said. “So to find a whole new era when a group of mammals dominates that was previously unexpected. This is a bit of a landmark discovery in that way.”

The tiny opalised jaw and teeth fossils contain rich information about ancient species’ lives and diets.Credit: James Alcock/The Australian Museum

The echidnapus, Opalios splendens, was slightly bigger than a modern platypus and was semiaquatic, but it had a slender snout somewhere between a platypus and an echidna. It’s too old to be the genuine ancestor of both creatures, Flannery said, but it provides an idea of what the common ancestor of surviving monotremes might have looked like.

Alive at the same time as the dinosaurs of the Cretaceous period, the echidnapus would have roamed cold, wet, mossy forests on a large floodplain that drained into a vast inland sea.

Lightning Ridge would have been further south during the echidnapus’s reign, close to the Antarctic circle. Early monotremes may have adapted the ability to detect the electrical impulses of their prey – as a platypus still does – to find insects during the dark winter months when daylight was scarce.

Another newly described monotreme, Parvopalus clytiei, was a tiny, 100 gram tree-dwelling creature that possibly resembled a modern phascogale or antechinus. The third new species is more like a proto-platypus. Dharragarra aurora had lower molars that have remained basically unchanged through time. Juvenile platypus still have the same teeth, but now they fall out as the animals mature.

The fossil record shows that platypus retained their teeth through 95 million years of history and evolved to lose them a few million years ago. Why is that?

The answer may be the water rat, the researchers propose in their new paper, which is published in palaeontology journal Alcheringa.

The native water rat, or rakali, arrived in Australia about 2 million years ago as a freshwater mammal of a similar size to the platypus, which inhabited the same ecological niche.

As the rakali specialised in things that crunch, the platypus may have evolved to eat soft and slippery food and lost its teeth.Credit: Getty/Museums Victoria

Perhaps as rakali snapped up hard-bodied prey such as mussels and crayfish, platypus evolved to hunt softer animals including worms, larvae and shrimp, and no longer needed sharp teeth.

“Those leathery pads in a platypus jaw are much better at holding soft, slippery food than a tooth is. So maybe platypus lost their teeth at the last moment in evolution, really,” Flannery said.

“We put that hypothesis out there to get some young palaeontologists off their backsides and looking for more fossils. They can have the pleasure of disproving us if they find the right ones.”

Lightning Ridge fossil hunter Dr Elizabeth Smith and her daughter, Clytie, found the tiny jaw bone fossils the new research is based on. Opalised fossils are useful because the translucent opal can offer insights into the inner anatomy of a preserved bone.

The opalised fossil jawbone of Steropodon galmani, another monotreme discovered at Lightning Ridge that was the first Mesozoic mammal discovered from Australia.Credit: Australian Museum

But fossils can be easily lost or ground up in the opal mining process. The tiny echidnapus jaw bone was pieced together from several fragments.

“Opal fossils are rare, but opalised monotreme fossils are infinitely more rare, as there’s one monotreme fragment to a million other pieces,” Elizabeth Smith said.

“We hope our discoveries may help the scientists solve the many unanswered questions of the evolution of our monotremes, the only egg-laying mammals on earth.”

Smith is a co-author of the new paper, published on Monday along with Flannery, Helgen, and researchers from Monash University and Museums Victoria.

The Examine newsletter explains and analyses science with a rigorous focus on the evidence. Sign up to get it each week.