Scientists Find Evidence of 'Age of Monotremes'

There are exactly five existing egg-laying mammals, or monotremes: the platypus and four echidna species, all native to Australia and New Guinea. According to new research, they're the last survivors of a diverse set of species that roamed the southern continents during the time of the dinosaurs. The Lightning Ridge opal fields of New South Wales, once a wet forest bordering an inland sea, have given up fossils of three distinct monotreme species, doubling the number of monotremes known to exist in this part of the world between 96.6 million and 102 million years ago. "In some other contexts, six species might be quite modest–but these account for a third of the known genera of monotremes in the 130-million-year history of the order," per IFL Science.

This is the first indication of "a great diversity of monotremes" in Australia at a time "when there were no other mammals living on the continent," mammalogist Tim Flannery, an honorary associate of the Australian Museum and author of the new study, tells Australian Geographic. "It's like discovering a whole new civilization," he adds. Indeed, researchers say the fossils point to a previously unknown "Age of Monotremes" during the Cenomanian Age of the Cretaceous Period. "It seems that 100 million years ago, there were more monotremes at Lightning Ridge than anywhere else on Earth, past or present," says paleontologist Elizabeth Smith, who discovered the fossils with her daughter about 25 years ago, per the BBC. The fossils were donated to the Australian Museum but "sat forgotten in a drawer until about two years ago," when Flannery rediscovered them.

The three new species show combinations of features unlike those in other monotremes living or dead. One species, Opalios splendens, "sits on a place in the evolutionary tree prior to the evolution of the common ancestor of the monotremes we have today," says study co-author Kris Helgen. It retains features of the earliest known monotremes but also has characteristics of modern monotremes. "Its overall anatomy is probably quite like the platypus, but with features of the jaw and snout a bit more like an echidna. You might call it an 'echidnapus,'" says Helgen. Another new species, Dharragarra aurora, comes from "the oldest fossil identified as a type of platypus," per IFL Science. Little is known about the third new species, Parvopalus clytiei, identified through the smallest of the fossils found. (More discoveries stories.)