From inside an old safe in the Australian Museum in Sydney, palaeontologist carefully removes a shoe-box-sized container and places it on a table. Inside are more boxes and containers, like a Russian doll.
Once the contents are fully revealed and spread out, there are nine tiny fragments of fossilised jawbones. If Flannery and his colleagues are right, before us is a glimpse into a lost world – a previously unknown chapter of mammal history. Flannery tells me the fossils “represent the first glimmerings we have of the age of monotremes”.
Today, these egg-laying mammals have been almost completely replaced by marsupials and placental mammals. Only two families survive: the platypuses and the echidnas, and they are restricted to Australia and New Guinea.
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The fossils I’m looking at were all collected in the opal-mining town of Lightning Ridge in New South Wales. They have been in the Australian Museum’s collection for decades but have only now been properly classified and studied.
According to Flannery and his colleagues, all nine of these 100-million-year old fossils are monotremes, including three new species and an entirely new family of the egg-laying mammals, which lived alongside the dinosaurs.
The new research doubles the number of known monotreme species identified at Lightning Ridge from three to six. One of the fossils that Flannery is holding, collected by an opal miner in the mid-1980s and named Steropodon, was part of a small tranche regarded as so precious that the Australian Museum purchased it for A$80,000.
The fossils are made of solid opal, formed when silica dissolved in water fills a cavity in rock left by bone after it has rotted away.
When the deposits were laid down in the Cretaceous Period, 100 million years ago, Australia was part of the supercontinent of Gondwana, and Lightning Ridge would have been rainforested and swampy. While the opals of Lightning Ridge have yielded dinosaur, reptile and fish fossils, no mammals other than monotremes have been found there.
One of the big mysteries of monotreme evolution is when exactly platypuses and echidnas diverged from a common ancestor. The genetic evidence suggests that this happened around 50 million years ago.
One of the newly identified species, Opalios splendens, shares traits with both echidnas and platypuses – resulting in the nickname “echidnapus”. It has been placed in a new family named Opalionidae.
Platypuses can detect electric fields with their bills, and this sense helps them to find prey underwater. The echidnapus has canals in the jaw similar to those in platypuses, which carry blood vessels and nerves that enable its electric sense, and the jaw is twisted like that of a platypus. But the beak is narrow and the jaw articulation is long, like that of an echidna.
“We can see it had electroreception,” says Flannery. “But I can’t see how it opened its jaws. Maybe it ate worms that it slurped in. It’s a really weird creature and a bit of an enigma.”
Despite its similarities to both platypuses and echidnas, Flannery says it’s impossible to know whether it was a direct ancestor of the living monotremes, since it lived long before genetic evidence suggests the two groups diverged.
Another one of the fossils, Dharragarra aurora, has a jaw that is very similar to that of a modern platypus. The other specimens are from animals that probably ranged from the size of a small possum to the size of a cat, says Flannery.
The researchers also point out that four of the six Lightning Ridge monotreme species have been identified from single fossil specimens, implying that there is enormous undiscovered diversity.
“If we keep finding more material, then we’ll keep finding more species,” says Flannery. “This will be the last time that anyone comes across an unknown age of mammal evolution.”
, who also worked on the study at the Australian Museum, says it is a big deal to name and describe a new mammal family.
“In Lightning Ridge we see monotremes with teeth [which are absent in modern monotremes] and we have lots of different ways to be a monotreme all living together. It takes 40 million years to birth a mammal family, so these discoveries speak to the fact that there’s already been a long depth of evolution at Lightning Ridge.”
at Flinders University in Adelaide, Australia, says the paper proposes some interesting scenarios, including that Australia’s mammal fauna was dominated by monotremes during the age of dinosaurs before later being usurped by marsupials.
“We need more and better-preserved fossils, and ideally fossils from other sites, to be able to test these fascinating scenarios,” he says.
Alcheringa: An Australasian Journal of Palaeontology
Article amended on 28 May 2024
A statement about the Australian Museum’s purchase of the Steropodon fossil was corrected.
