Ancient bones found in opal fields have given scientists a first-ever insight into a "civilisation" of echidna and platypus-like creatures that roamed Australia 100 million years ago.
The opalised jaws, unearthed in the NSW outback town of Lightning Ridge, date back to when monotremes inhabited the continent, research published on Monday suggests.
"In one snapshot we see six different egg-laying mammals living together in Lightning Ridge over 100 million years ago," Tim Flannery, a mammologist at the Australian Museum said.
"All of them are holding potential evolutionary destinies that can go off in different directions and all of them are deep distant ancestors and relatives of the current living monotremes."
The platypus and the echidna are the only surviving monotremes - egg-laying mammals unique to Australia and New Guinea.
While Australia was known as a land of marsupials, the discovery of the fossils was the first indication that the continent was also previously home to a range of monotremes, Prof Flannery said.
"It's like discovering a whole new civilisation," he added.
Evidence of the so-called "age of monotremes" was discovered by a team of scientists from the Australian Museum in Sydney, Museums Victoria and the Australian Opal Centre.
Elizabeth Smith, from the Lighting Ridge-based opal hub, made the landmark discovery of the rare "opal fossils" with her daughter Clytie.
The fossils, one of which scientists noted looked like a platypus-echidna mix or an "echidnapus", showed Australia was once a land of "furry egg-layers".
"It seems that 100 million years ago, there were more monotremes at Lightning Ridge than anywhere else on earth, past or present," Ms Smith said.
