The New Zealand scientist dubbed "Mr Moa" has helped re-write the family tree of Australia's own ancient giant birds - including the horse-sized Dromornis stirtoni that tipped the scales at 650kg.

In a new analysis comparing bird fossils from throughout the world, Associate Professor Trevor Worthy of Adelaide's Flinders University and colleagues have revealed the evolutionary history of the birds - with surprising results.

They have ancient evolutionary roots with the Northern Hemisphere giant Gastornis species, and together form a major lost branch on the evolutionary tree of fowl that chickens and ducks belong to.

This group lived in Australia from 55 million years ago until going extinct about 50,000 years ago.

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When the last one died, a whole order, and some of the most spectacular birds ever to have lived, disappeared.

Worthy, and a team of palaeontologists from Flinders University and Argentina whom he led, have shared their findings in a study just published in the Royal Society Open Science.

How did they end up in different hemispheres?

The scientists suggest small flying birds gave rise to giant flightless fowl twice - once in Australia and again in the Northern Hemisphere.

Worthy said they formed a "neat parallel" to how we now understand flightless ratites, which include ostrich, emu and kiwi.

"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 and evolved 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."

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Despite their great size, the mihirungs and their Northern Hemisphere relatives were gentle giants.

"Mihirungs were herbivores, just like typical ducks and geese," said co-author Professor Mike Lee, Flinders University and South Australian Museum.

"Despite a 500-fold increase in body size, they retained the diets of their much smaller ancestors."

The team found other surprising relationships.

Vegavis, 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 said.

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Moreover, the extinct flamingo-ducks, only newly recognised as having existed in Australia, were shown to also be more primitive than believed till now.

They were shown to be the distant cousins of modern geese and ducks.

Thanks to DNA insights, what we thought we knew about such species and their evolutionary back-story has changed much over recent years.

One landmark study in 2014 revealed how the moa was more closely related to South America's tinamous than its old bushmate, the kiwi, and concluded both moa and kiwi separately evolved to become flightless after their ancestors flew here.

The same year, scientists corrected an uncomfortable theory that the kiwi's ancestor was an Australian immigrant, and set the record straight with findings linking the icon's lineage back to the extinct, 2.3m tall elephant bird, a native of Madagascar.