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For more than a century, scientists have been hunting in vain for skull fossils of the thunderbird species Genyornis newtoni. About 50,000 years ago, these titans, also known as mihirungs, from an Aboriginal term for “giant bird,” walked on muscular legs through the forests and grasslands of Australia. They were larger than humans and weighed hundreds of pounds.
The last mihirungs became extinct about 45,000 years ago. The lone skull, found in 1913, was incomplete and badly damaged, raising questions about the giant bird’s face, habits and origins.
Now the discovery of a complete skull of G. newtoni has solved this long-standing mystery, bringing scientists face to face with the enormous mihirung for the first time.
And it has the face of a very strange goose.
G. newtoni was about 2 meters long and weighed up to 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 excavated a fossil jackpot of G. newtoni from Lake Callabonna in South Australia, discovering multiple skull fragments, a skeleton and an articulated skull that provided the first evidence of the upper beak of the bird. 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.
Although scientists have known about Genyornis for more than a century, the new fossils and reconstruction provide crucial missing details, says Larry Witmer, professor of anatomy and paleontology at Ohio University, who was not involved in the study.
“The skull is always the prize, simply because there is so much important information 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 usually where the display organs (horns, buds, wattles and combs, etc.) are located,” he said. “In addition, skulls often bear structural features that give us clues about their genealogy.”
In the new study, the authors “milked these new fossils for all they had,” Witmer said. The researchers didn’t just model the bones in the skull; they also analyzed the arrangement of jaw muscles, ligaments and other soft tissues that indicated the bird’s biology.
“This latest discovery of new Genyornis skulls has really helped fill in the blanks,” Witmer said.
‘Very goose-like’
The newly discovered skull is the focus of a digital reconstruction, supplemented with other skull fossils and data from modern birds, and provides previously unknown clues about the appearance of G. newtoni, said lead study author Phoebe McInerney, a vertebrate palaeontologist and researcher at Flinders University in South Australia.
“Only now, 128 years after its discovery, can we say what it really looked like,” McInerney said in an email. “Genyornis has a very special beak that looks very similar to a goose.”
Compared to the skulls of most other birds, the skull of G. newtoni is quite short. But the jaws are enormous, supported by powerful muscles.
“They would have had a very wide gap,” McInerney said.
The skull also indicated the diet of G. newtoni. A flat grasping zone in the beak was suitable for tearing soft fruits and soft shoots and leaves, and a flattened palate on the underside of the upper beak may have been used to grind fruit into a pulp.
“We knew from other evidence that they were probably eating soft foods, and the new beak supported that,” McInerney said. “The skull also showed some evidence of adaptations for feeding on water, perhaps on freshwater plants.”
This suggestion of underwater feeding is unexpected, given the enormous size of G. newtoni, Witmer said.
“Perhaps that shouldn’t be too surprising, since dromonithids like Genyornis are related to the group that includes ducks and geese, but Genyornis was 6 feet to 6 feet tall and weighed perhaps 500 pounds,” Witmer said. Additional fossil discoveries could help determine whether such adaptations are unused features inherited from aquatic ancestors, “or whether these giant birds waded into the shallows in search of tender plants and leaves.”
‘A strange amalgamation’
The reconstruction helped scientists resolve the conflicting lineage of dromornithids, placing them within the waterfowl order Anseriformes, the study authors reported. Based on the bone structures and associated muscles, dromornithids were likely close relatives of the ancestors of modern South American screamers, duck-like birds that inhabited the wetlands of southern South America.
Although G. newtoni had a goose beak, its face did not perfectly match those of modern geese, said co-author and bird paleontologist Jacob Blokland. Blokland, a researcher with the Flinders Paleontology Group at Flinders University, illustrated reconstructions of the skull and of G. newtoni in life.
“I was surprised at how superficially goosey it looked, with its large spatula-shaped beak, but absolutely different from any other geese we have today,” Blokland said in an email. “It has some aspects reminiscent of parrots, to which it is not closely related, but also to land birds, which are much closer relatives. In some ways it seems like a strange amalgamation of very different-looking birds.”
For the new reconstruction, Blokland started with the bony external ear area, “as there were several specimens that had preserved this part,” he said. From there he constructed a scaffold that was consistent across multiple skull fossils. Some parts of the reconstruction were based on skulls from other dromonithids or from modern waterfowl, and anatomical studies of modern birds hinted at how muscles and ligaments might move the bones.
A previously unknown detail was a broad triangular bony shield called a helmet on the upper beak, which may have been used for sexual displays, the study authors reported.
Large, flightless emus and cassowaries (which are not close relatives of thunderbirds) currently roam Australia, but cast a much 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 remains to be discovered, she added, such as how inner ear structures related to head stabilization and locomotion may have been affected by gigantism and flightlessness.
And while the new perspective on G. newtoni is the most accurate yet, additional fossils will bring into sharper focus the portrait of this unusual giant goose — the last of the mighty thunderbirds — and of its vanished habitat, Blockland said.
“Such a gigantic and unique bird undoubtedly had an impact on the environment and other animals it came into contact with – large or small,” he said. “Only through research can we build a bigger picture and discover what we are missing.”
Mindy Weisberger is a science writer and media producer whose work has appeared in the magazines LiveScience, Scientific American, and How It Works.
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