Five huge fossils of extinct megapodes – a group of large birds that includes turkeys – have been discovered in Australia. Among them are the remains of a turkey the size of a kangaroo, which was capable of flight.

The ancient bush turkey Progura gallinacean, which weighed 8 kilograms, roamed the Australian landscape 2 million years ago, in the company of marsupial lions and diprotodons.

The giant prehistoric bird is thought to have buried its eggs in warm sand or soil, like Indonesian megapodes do today. While their nests were in the ground, when not caring for their eggs, the turkeys would have roosted in trees, the researchers say.

The bones were discovered in recent excavations by researchers at Flinders University. Two of the new species were found in the Thylacoleo Caves by the Nullarbor Plain in southern Australia.

The discovery clears up questions about the origin of the modern Malleefowl, another type of megapode that is much smaller. Some scientists have proposed that the larger turkeys, first discovered in the 1880s with more found in the 1970s, were in fact the same species as today's Malleefowl. They proposed that the birds became smaller and smaller over evolutionary time – a process called dwarfing – until they reached the size of the modern turkeys.

But this picture has now been overturned. The ancient birds were a unique, extinct species.

"We compared the fossils described in the 1880s and the 1970s with specimens discovered more recently, and with the benefit of new fossils, differences between species became really clear," said Elen Shute, one of the researchers who discovered the fossils.

"What's more, we have found bones of Malleefowl in fossil deposits up to a million years old, alongside bones of three extinct species of various sizes, so there's really no evidence that dwarfing took place."

The discovery hints at a much more diverse history for Australia's large birds than was previously imagined.

"These discoveries are quite remarkable because they tell us that more than half of Australia's megapodes went extinct during the Pleistocene," said Shute.

Shute's colleague Trevor Worthy added: "Our research shows how little we know of Australia's immediate pre-human birds. Probably many smaller extinct species also await discovery by palaeontologists."