Palaeontology
Trove of Australian fossils offers a rare glimpse of the ancient relatives of platypuses and echidnas that lived alongside the dinosaurs 100 million years ago, finds James Woodford
FROM inside an old safe in the Australian Museum in Sydney, palaeontologist Tim Flannery carefully removes a shoe-boxsized container and places it on a table. Inside are more boxes and containers, like a Russian doll.
Once the contents are revealed and spread out, there are nine tiny fragments of fossilised jawbones. If Flannery and his colleagues are right, before us is a glimpse of 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 the island of New Guinea.
The fossils I am looking at were 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 classified and studied. According to Flannery and his colleagues, all nine of these fossils are from monotremes, including three new species. They all lived alongside the dinosaurs 100 million years ago.
The work doubles the number of known monotreme species identified at Lightning Ridge from three to six. One fossil 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 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. The opals of Lightning Ridge have yielded dinosaur, reptile and fish fossils, but no mammals other than monotremes have been found. One of the big mysteries of monotreme evolution is when platypuses and echidnas diverged from a common ancestor. Genetic evidence suggests this happened some 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.
