Some time about 100m years ago in what is now an Australian opal field, a weird, furry, egg-laying, rabbit-sized mammal was gliding through a waterhole across a massive polar floodplain.

This mammal – Opalius splendens but which scientists have thankfully blessed with the nickname “echidnapus” – was among the ancient descendants of one of the planet’s most unique orders of animals, the monotremes.

New scientific research released on Monday showed the echidnapus had characteristics of the last two remaining members of their tribe.

Modern Australia is the stronghold for the only monotreme species – the supremely odd platypus, a nipple-free Aussie mammal with a duck-like bill, and the spiky echidna with its over-stretched snout, which also lives on New Guinea.

But the discovery of echidnapus and two more ancient monotremes in the opal field fossils means at least six monotreme species existed in what is now the far north New South Wales outback.

“It’s like discovering a whole new civilisation,” said Prof Tim Flannery, the lead author of the new research, published in palaeontology journal Alcheringa.

“Today Australia is known as the land of the marsupials, but discovering these new fossils is the first indication that Australia was previously home to diversity of monotremes.

In the region where the fossils were found, “there are no other kinds of mammals. It suggests Australia experienced an age of monotremes when they were the dominant mammal.”