As the bones encased in rock rotted, hydrous silica seeped into the crevices, solidifying into opal and preserving precious details for 100 million years. The resulting fossils now provide evidence that there may indeed have been an age of monotremes before other mammals came to dominate.
“It’s like discovering a whole new civilization,” says Australian Museum palaeontologist Tim Flannery.
“Today, Australia is known as the land of marsupials, but the discovery of these new fossils is the first indication that Australia was once home to a diversity of monotremes.”
Only five of these rare mammals still cling to existence: one platypus and four species of echidna shared by Australia and Papua New Guinea. But because of their reptile-like egg-laying characteristics, these animals have long been thought to have evolved before placental mammals like us and marsupials.
“It can be estimated that wherever they originated, monotremes predated marsupials in Australia, and that the old Australian monotreme fauna may have been at least as diverse as the later marsupial fauna…” speculated naturalist Philip Jackson Darlington 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 time period to six, proving that there really was some diversity among these furry eggs. The new finds range from the size of a small opossum to the size of a cat, Flannery told James Woodford The new scientist.
Australia’s Lightning Ridge in northeastern New South Wales, where the fossils were found, now has the most diverse range of monotreme fossils on record, all from the Cenomanian period, roughly 100 to 95 million years ago.
“Four species are known from a single specimen, suggesting that the diversity remains underrepresented. This discovery adds more than 20 percent to the previously known diversity of monotremes,” Australian Museum paleontologist Matthew McCurry says.
Among the new fossils is the toothy “echidnapus” (Opalis splendens), which, as its nickname suggests, shares the characteristics of both echidnas and platypuses. It has a long, narrow snout like an echidna, but also some platypus traits, such as complex electroreception.
“The story of how our egg-laying mammals evolved is ‘toothy to toothless,'” says Australian Museum mammalogist Kris Helgen.
“The oldest monotreme, Teinolophos trusleri, which came from Victoria 130 million years ago, had five molars in each jaw. At Lightning Ridge, we see that 100 million years ago, some monotremes still had five molars, but some of them have up to three.”
Modern echidnas have no teeth, and platypuses lose theirs before they are fully grown.
Flannery and colleagues also describe the smallest known monotreme, Parvopalus clytieiand dharragarra aurora, which has a similar jaw to the modern platypus, in their new paper. These join 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 indeed have been alone among the dinosaurs of the time. However, a larger fossil assemblage that includes additional sites will be needed to confirm this theory.
This research was published in Alcheringa: Australasian Journal of Paleontology.