An ancient monotreme dubbed the “echidnapus”, which shares platypus- and echidna-like characteristics, is one of three extinct species of egg-laying mammal unveiled by palaeontologists today.

Another of the creatures shares close similarities with modern platypus, and could represent the oldest-known monotreme to have a platypus-like body.

The trio of fossils are between 100 and 96 million years old and were found in the opal fields of Lightning Ridge, New South Wales.

This discovery brings the tally of monotreme species found at Lightning Ridge to six, making the area the most diverse monotreme site on Earth.

Researchers behind the fossil finds, which were published in Alcheringa, speculate the diversity of species at Lightning Ridge could represent a long theorised “monotreme radiation”, an evolutionary phenomenon where a multitude of new species split off from a single ancestor, heralding an “age of monotremes” in Australia.

But not all palaeontologists are convinced there is enough evidence yet to make the “age of monotremes” claim definitively.

The story a jaw bone can tell

Two years ago, Australian Museum mammalogist Tim Flannery, lead author of the new study, was looking through the institution’s collection when he came across pieces of five jaw bones in a drawer.

“Which I recognised pretty well straightaway as being from monotremes,” he said.

“That was astonishing, because over the last, I think, 40 years, we’ve only found four fossilised monotreme jaw bones from Lightning Ridge. So this was like a bonanza.”

Different angles of the jaw bone pieces of the newly described species Opalios splendens.(Supplied: Australian Museum/Alcheringa)

The jaw bones had been collected more than 20 years ago by Lightning Ridge miners, given to study co-author and Australian Opal Centre palaeontologist Elizabeth Smith and her family, and then passed onto the museum.

Two jaw bones belonged to the already-described Steropodon galmani, but new species were identified from the other three:

  • Parvopalus clytiei, which may have been a land-based animal similar to a brushtail phascogale
  • Dharragarra aurora, which shares similarities with modern platypus
  • Opalios splendens, otherwise dubbed the “echidnapus”.

Australian Museum chief scientist Kris Helgen, also a study author, said O. spendens had narrow facial features like an echidna but other aspects of the jaw bone were similar to platypus.

DNA studies estimate platypus and echidna diverged from other monotremes between 18 million and 55 million years ago.

But Professor Helgen said whether echidnapus, which has been given its own family Opalionidae, was the exact ancestor of modern monotremes was hard to pin down.

Teinolophos trusleri is the earliest monotreme known from Australia’s fossil record.(Supplied: Australia Museum/Peter Schouten)

Queensland University of Technology evolutionary biologist Matt Phillips, who was not involved in the study, said based on the limited evidence available, the dentistry of O. spendens tied it closer to the platypus family, Ornithorhynchids, than the oldest-known monotreme Teinolophos trusleri.

“But it would be great to find dental remains for some corroboration,” Professor Phillips said.

“The paper is significant for substantially expanding knowledge of Australia’s Cretaceous mammal fauna, including hinting at the dominance of monotremes among the larger-sized members of that mammal fauna and showing that monotremes were once far more diverse.”

When monotremes ruled?

Only five species of monotreme remain today: platypus, short-beaked echidnas and three long-beaked echidnas.

According to a genetic study, the egg-laying monotreme subclass of mammals might have diverged from mammals that give birth to live young, called therians, about 187 million years ago, when Australia was located near the south pole as a part of the supercontinent Gondwana.

But when it comes to understanding how modern species evolved, how many kinds of monotremes there were over history, and why most died out, the fossil record is sparse.

So far, 16 extinct monotremes have been found from dig sites in Australia, and two from Argentina.

Now that six species have been found in the same place from the same time period in Lightning Ridge, Professor Flannery and his colleagues believe there is credence in the monotreme radiation theory.

“The discovery really has revealed a very fundamental fact about Australia, which was that during the late Cretaceous, Australia was a land of monotremes with a very diverse group of monotremes existing there,” he said.

Professor Flannery said the new species were just the beginning of the radiation story, and each new specimen found would help fill the picture of what Australia was like during what he called an “age of monotremes”.

But Flinders University palaeontologist Rod Wells, who was not part of the study, does not think there is enough evidence yet to declare an age of monotremes in Australia.

These findings are quite interesting and suggest the possibility of a monotreme radiation in Gondwana,” he said.

“It may have been at least as diverse as the later Australian marsupial fauna … but I would need more evidence.

“Although that is going to be hard to come by in Australia. Perhaps sub-Antarctic Islands or South America will yield the older fossil-bearing sediments.”

Murrayglossus was active in the Pleistocene which lasted from 2.58 million to 11,700 years ago.(Supplied: Australia Museum/Peter Schouten)

Professor Wells said he hoped the Lightning Ridge discoveries would spur more exploration of less traditional environments for fossils to piece together the monotreme story.

One of the most glaring unanswered questions is what happened to monotremes between 60 and 26 million years ago, a period from which no monotreme fossils have yet been found.

Whether large monotremes survived the last mass extinction 66 million years ago, and how species diversity might have changed with the rise of the marsupials, is still a mystery.

But with Lightning Ridge confirmed as a monotreme hub, Australian Opal Centre palaeontologist Elizabeth Smith hopes to see more targeted digs in the region.

“There’s certainly fossil ground that’s very prospective and needs to be looked at,” Dr Smith said.

“It’s something we’ve been considering for a very long time, but it’s a matter of funding.”

Get all the latest science stories from across the ABC.

Posted , updated