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Australia’s Giant Flightless Fowl’s Far-Flung Family

Australia’s giant mihirungs (Dromornithidae) were flightless fowl that included some of the most massive birds in the world, such as the horse-sized Dromornis stirtoni, which tipped the scales at 650 kg. But what were they related to?

In a new analysis published in Royal Society Open Science, palaeontologists from Flinders University and Argentina have revealed that these birds share their evolutionary roots with the giant Gastornis species in the Northern Hemisphere. Together they form a major lost branch on the evolutionary tree of fowl: chickens and ducks.

This group lived in Australia from 55 million years ago until becoming extinct about 50,000 years ago. When the last one died an entire taxonomic order, and some of the most spectacular birds ever to have lived, disappeared.

How did giant birds disperse across both hemispheres? The scientists suggest that small flying birds gave rise to giant flightless fowl twice – once in Australia and again in the Northern Hemisphere. “They form a neat parallel to how we now understand the ratites (emu, ostrich and kin) evolved,” says study leader A/Prof Trevor Worthy of Flinders University.

“At the base of the family tree of giant flightless ratites on each continent we now know there was a small flying bird like a tinamou. These dispersed across the oceans, settled on a continent, evolving into huge and flightless birds. One became moas in New Zealand, another the ostrich, and yet another the emus and cassowary in Australia. Now we see that the giant fowl share a similar history.”

Despite their great size, the mihirungs and their Northern Hemisphere relatives were gentle giants. “Mihirungs were herbivores, just like typical ducks and geese,” says co-author Prof Mike Lee of Flinders University and the South Australian Museum. “Despite a 500-fold increase in body size, they retained the diets of their much smaller ancestors,” he says.

The team found other surprising relationships. Vegavis, which was previously interpreted as a modern duck from dinosaur-age rocks in Antarctica, was found to be much more primitive, in line with its great age. “This helps bring the fossil history and that inferred from DNA closer together,” Worthy says.

Moreover, the extinct flamingo ducks (Presbyornithidae), whose existence in Australia was only recently recognised, were also found to be more primitive than previously believed, and are shown to be the distant cousins of modern geese and ducks. “This makes sense because they evolved long before typical waterfowl and bear little similarity to them,” says co-author Warren Handley, a PhD student supervised by Worthy.

However, in a surprising twist the team found that the largest flightless bird of South America, Brontornis, is not remotely related to fowl, as some recent studies had advocated. Instead, this 300 kg giant was actually a slower relative of the terror birds (Phorusrhacidae), which replaced dinosaurs as the supreme predators of South America during the Cenozoic era.

“Perhaps the terror birds meant there was no place for sluggish giant fowl in South America. Only the fast-running rheas and kin could live with them,” says co-author Dr Federico Degrange of the Centro de Investigaciones en Ciencias de la Tierra in Argentina.