As the bones locked in the rocks rotted away, water-based silica seeped into the crevices, solidified into opal and preserved precious details for 100 million years. The resulting fossils now provide evidence that there may really have been an age of monotremes, before other mammals took over.
“It’s like discovering a whole new civilisation,” says Australian Museum paleontologist Tim Flannery.
“Today Australia is known as a land of marsupials, but the discovery of these new fossils is the first indication that Australia was previously home to a diversity of monotremes.”
Only five of these rare mammals still exist: one platypus and four echidna species, distributed between Australia and Papua New Guinea. But because of their reptilian egg-laying ability, these animals have long been thought to have evolved before placental mammals like us and marsupials.
“One might suspect that monotremes, wherever they came from, radiated into Australia before marsupials did, and that the ancient Australian monotreme fauna was at least as diverse as the later marsupial fauna…” naturalist Philip Jackson Darlington speculated in 1957.
Although this theory is widely accepted, fossil evidence has remained scarce.
Now three newly discovered monotremes bring the total number of fossil species known from this one place and period to six, proving that there really was some diversity among these hairy egg layers. The new finds range from the size of a small possum to that of a cat, Flannery told James Woodford at New scientist.
The Australian Lightning Ridge in northeastern New South Wales, where the fossils were found, now has the most diverse monotreme fossils ever recorded, all from the Cenomanian period between about 100 and 95 million years ago.
“Four species are known from a single specimen, indicating that diversity is still underrepresented. This discovery adds more than 20 percent to the previously known diversity of monotremes,” said palaeontologist Matthew McCurry of the Australian Museum.
Among the new fossils is the toothy ‘echidnapus’ (Opalios splendens), which, as its nickname suggests, shares characteristics of both echidnas and platypuses. It has the long, narrow snout of an echidna, but also some features of a platypus, such as complex electroreception.
“The story of how our egg-laying mammals evolved is from ‘toothed to toothless’,” describes mammologist Kris Helgen of the Australian Museum.
“The oldest monotreme, Teinolophos trusleri, which dates back to Victoria 130 million years ago, had five molars in each jaw. What we see at Lightning Ridge is that some of the monotremes 100 million years ago still have five molars, but some of them are down to three.”
Modern echidnas have no teeth, and platypuses lose them before they reach full maturity.
Flannery and colleagues also describe the smallest known monotreme, Parvopalus clytieiAnd Dharragarra aurora, which has a similar jaw to modern platypuses, in their new paper. These join the three previously discovered ancient Lightning Ridge monotremes: Kollikodon ritchiei, Steropodon galmani, And Stirtodon elizabethaethe largest known monotreme.
So far, researchers have found no other signs of mammals from this period, suggesting that monotremes may have been alone among the dinosaurs of that era. However, a more extensive fossil collection that includes other locations will be needed to confirm this theory.
This research was published in Alcheringa: an Australasian journal of paleontology.