An Australian origin story? Turning mammalian theory on its head

It has long been asserted that ancestors of the placental and marsupial mammals originated in the northern hemisphere – but a new study by Prof Tim Flannery and Prof Kris Helgen at the Australian Museum has rewritten the origin story of modern mammals.

For almost 290 years, it has widely been asserted that Theria (marsupials, placentals and their ancestors), originated and evolved in the northern hemisphere. But a new study by Prof Tim Flannery, Thomas H. Rich, Patricia Vickers-Rich, E. Grace Veatch and Prof Kristofer M. Helgen has turned this theory on its head – and in turn, the team have revealed a profound part of the Australian story.

Theria are a diverse subclass of mammals; many of which today live in the northern hemisphere, where there is also an abundant fossil record. The only living non-therian mammals are the monotremes (platypus and echidna), which are found only in Australasia. The Theria are characterised by their sophisticated molars, known as tribosphenic molars, which are able to crush, puncture and cut through food simultaneously. This adaptation was likely a key component to their evolutionary success.

The study (published in the journal Alcheringa) examined fossilised tribosphenic molars from Madagascar, South America and India, which date to the early and middle Jurassic. The discoveries are particularly significant because these fossils predate the oldest firmly dated tribosphenic molars from the northern hemisphere by 50 million years. The researchers were able to show that, while clearly part of the same lineage, considerable evolutionary change occurred between the Jurassic fossils and related forms from the Early Cretaceous of Australia. The oldest of the Jurassic fossils had structures in their jaws and premolars that resembled those of very primitive mammals from the Triassic which are ancestral to both the Theria and the Monotremata, while the Australian Cretaceous fossils share characteristics with both the Jurassic southern hemisphere fossils and the modern northern hemisphere Theria.

This research provides evidence that Theria may have originated in Gondwana, evolving there for 50 million years before they migrated from Australia to Asia via volcanic island arcs. Within 20 million years of reaching Asia, the Theria had spread to Europe and North America where the first marsupials evolved. Much later, both marsupial and placental mammals would reach Australia.

But why did the early Theria die out in Australia, and when? One possibility is that they went extinct when the marsupials arrived in Australia, via Antarctica, around 55 million years ago. The fossil record is too incomplete to say what happened on the other southern continents, but it is clear that none of the early southern hemisphere therians have survived, with all lineages with living representatives descending from species that migrated from Australia to Asia. ‘It is astonishing to think that a part of the evolution of the Theria occurred in Australia,' says Helgen, noting that the discovery was entirely unexpected.

Professor Tim Flannery, Honorary Associate, Australian Museum.

Professor Kris Helgen, Chief Scientist and Director, Australian Museum Research Institute.

Meagan Warwick, AMRI Project and Communications Officer, Australian Museum.