Meet the ‘Echidnapus,’ an Extinct Creature That Resembles Both the Echidna and Platypus of Today

The species is among three newly identified monotremes, or egg-laying mammals, discovered from fossils in Australia that are shedding light on the odd animals’ evolution

Illustration with several different types of egg-laying mammals in nature
Newly examined fossils suggest monotremes—egg-laying mammals—were once much more abundant in Australia than they are today. Peter Shouten

Today, egg-laying mammals are rare oddballs, with creatures like the platypus and echidna standing out in the animal kingdom.

But it wasn’t always this way: Australia may have once been dominated by a diverse group of egg-laying mammals, known as monotremes, scientists report in the journal Alcheringa: an Australasian Journal of Palaeontology.

While revisiting old fossils, researchers recently identified three previously unknown extinct species of monotremes—including one, nicknamed the “echidnapus” and officially called Opalios splendens, that showed a mix of characteristics found in modern platypuses and echidnas. This unusual species roamed Australia during the Cretaceous period.

“Long before Australia became the land of pouched mammals, marsupials, this was a land of furry egg-layers—monotremes,” study co-author Elizabeth Smith, a paleontologist with the Australian Opal Center, tells BBC News’ Tiffanie Turnbull. “It seems that 100 million years ago, there were more monotremes at Lightning Ridge than anywhere else on Earth, past or present.”

The fossils were discovered about 25 years ago in the Lightning Ridge opal fields, a region of New South Wales known for producing valuable black opals. At the time, Smith and her daughter were sifting through an opal mine’s discard pile when they stumbled upon fossilized teeth and jawbones.

The pair donated the fossils to the Australian Museum, where someone stashed them in a drawer and forgot about them. Two years ago, researchers re-discovered the fossils and decided to investigate.

They identified six total species: the three new ones, plus three previously identified extinct monotremes, reports Science News’ Carolyn Gramling.

“It’s like discovering a whole new civilization,” says study co-author Tim Flannery, a mammalogist at the Australian Museum, in a statement.

The “echidnapus” likely had a narrow snout similar to an echidna, but its overall anatomy was more akin to a modern platypus, though the “echidnapus” was slightly bigger. The other new species are Parvopalus clytiei, a tiny monotreme about the size of a rat, and Dharragarra aurora, the earliest known platypus species.

four images of teeth and three illustrations; the teeth are long and narrow, some are numbered by the researchers
Images and diagrams of teeth from the "echidnapus," or Opalios splendens. G and H are not to scale. Flannery et al., Alcheringa: an Australasian Journal of Palaeontology, 2024

Rod Wells, a paleontologist at Flinders University in Australia who was not involved in the study, tells the Australian Broadcasting Corporation’s Peter de Kruijff that he “would need more evidence” before getting on board with the idea that the country was once dominated by monotremes.

Still, the fossils may help fill in some of the evolutionary gaps for the unusual mammals—for example, how and why they lost their teeth. While the ancestors of today’s monotremes had teeth, the living species do not and instead grind their food inside their mouths. Scientists suspect the creatures’ diets changed because of the arrival of a new competitor, the Australian water rat, or rakali, reports the Sydney Morning Herald’s Angus Dalton.

Water rats arrived in Australia from New Guinea around two million years ago. They were about the same size as today’s platypuses and likely inhabited the same environments as ancient monotremes.

The water rats may have started feasting on harder animals like crayfish and mussels, which pushed monotremes toward softer foods like shrimp and worms. This, in turn, may have rendered their teeth useless.

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

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