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For more than a century, scientists have been unsuccessfully hunting for skull fossils for the thunder bird species Genyornis newtoni. About 50,000 years ago, these titans, also known as mihirungs, from an Aboriginal term for “giant bird,” tramped through the forests and grasslands of Australia on muscular legs. They stood taller than humans and weighed hundreds of kilograms.
The last of the mihirungs went extinct around 45,000 years ago. The only skull, found in 1913, was incomplete and badly damaged, raising questions about the giant bird’s face, habits and ancestry.
Now, the discovery of a complete G. newtoni skull has resolved this longstanding mystery, giving scientists their first face-to-face encounter with the massive mihirung.
And it has the face of a very strange goose.
G. newtoni was about 7 feet (2 meters) tall and weighed up to 529 pounds (240 kilograms). It belonged to the family Dromornithidae, a group of flightless birds known from fossils found in Australia.
Between 2013 and 2019, a team of paleontologists unearthed a G. newtoni fossil jackpot in southern Australia’s Lake Callabonna, discovering multiple skull fragments, a skeleton and an articulated skull providing the first evidence of the bird’s upper bill. This bonanza shed new light not only on G. newtoni, but also on the entire dromornithid group, connecting it to modern waterfowl such as ducks, swans and geese, scientists reported Monday in the journal Historical Biology.
Though scientists have known about Genyornis for well over a century, the new fossils and reconstruction supply critical missing details, said Larry Witmer, a professor of anatomy and paleontology at Ohio University who was not involved in the research.
“The skull is always the prize simply because so much important information is in the head,” Witmer said in an email. “It’s where the brain and sense organs are located, it’s where the feeding apparatus is located, and it’s typically where the display organs (horns, crests, wattles and combs, etc.) are located,” he said. “Plus, skulls tend to be dripping with structural characteristics that give us clues about their genealogy.”
In the new study, “the authors milked these new fossils for all they had,” said Witmer. The researchers not only modeled the bones in the skull; they also analyzed placement of jaw muscles, ligaments, and other soft tissues that hinted at the bird’s biology.
“This latest discovery of new Genyornis skulls has really helped fill in the blanks,” Witmer said.
‘Very goose-like’
The newfound skull takes center stage in a digital reconstruction, supplemented by other skull fossils and data from modern birds, and it offers previously unknown clues about G. newtoni’s appearance, said lead study author Phoebe McInerney, a vertebrate paleontologist and researcher at Flinders University in South Australia.
“It is only now, 128 years after its discovery, that we can say what it actually looked like,” McInerney said in an email. “Genyornis has a very unusual beak which is very goose-like in shape.”
Compared with the skulls of most other birds, G. newtoni’s skull is quite short. But the jaws are massive, supported by powerful muscles.
“They would have had a very wide gape,” McInerney said.
The skull also hinted at G. newtoni’s diet. A flat gripping zone in the beak was suited for ripping up soft fruits and tender shoots and leaves, and a flattened palate on the underside of the upper bill may have been used for crushing fruits to a pulp.
“We knew from other evidence that they likely ate soft food, and the new beak supported that,” McInerney said. “The skull also showed some evidence of adaptations for feeding in water, maybe on freshwater plants.”
This suggestion of underwater feeding is unexpected, given G. newtoni’s massive size, Witmer said.
“Maybe that shouldn’t be too surprising given that dromornithids like Genyornis are related to the group including ducks and geese, but Genyornis was six or seven feet tall and weighed maybe as much as 500 pounds,” Witmer said. Additional fossil discoveries could help resolve whether such adaptations were unused features inherited from aquatic ancestors, “or whether these giant birds were wading into the shallows in search of soft plants and leaves.”
‘A strange amalgamation’
The reconstruction helped scientists resolve the conflicted lineage of dromornithids, placing them within the waterfowl order Anseriformes, the study authors reported. Based on bone structures and associated muscles, dromornithids were likely close relatives to ancestors of modern South American screamers, ducklike birds inhabiting wetlands in southern South America.
While G. newtoni had a gooselike beak, its face wasn’t a perfect match to those of modern geese, said study coauthor and avian paleontologist Jacob Blokland. A researcher with the Flinders Palaeontology Group at Flinders University, Blokland illustrated reconstructions of the skull and of G. newtoni in life.
“It surprised me how superficially goosey it looked, with its large spatulate bill, but definitely unlike any goose we have today,” Blokland said in an email. “It has some aspects reminiscent of parrots, which it is not closely related to, but also landfowl, which are much closer relatives. In some ways it appears like a strange amalgamation of very different looking birds.”
For the new reconstruction, Blokland began with the bony external ear region, “as there were several specimens that preserved this part,” he said. From there, he constructed a scaffold that was consistent across multiple skull fossils. Some areas of the reconstruction were based on skulls belonging to other dromornithids or to modern waterfowl, and anatomical studies of modern birds hinted at how muscles and ligaments might move the bones.
One previously unknown detail was a wide triangular bony shield called a casque on the upper bill, which may have been used for sexual displays, the study authors reported.
Big, flightless emus and cassowaries (which are not close relatives of thunder birds) currently roam Australia, but cast a far smaller shadow than the long-lost mihirungs, which still loom large in the popular imagination, McInerney said. There is much about the anatomy of these extinct giants that is yet to be discovered, she added, such as how inner ear structures associated with head stabilization and locomotion may have been affected by gigantism and flightlessness.
And while the new perspective on G. newtoni is the most accurate to date, additional fossils will more sharply focus the portrait of this unusual gargantuan goose — the last of the mighty thunder birds — and of its vanished habitat, Blockland said.
“Such a giant and unique bird undoubtedly affected the environment and other animals it interacted with — large or small,” he said. “It is only through study that we can build a bigger picture, and discover what we are now missing.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.