A pair of legs belonging to what could be the largest species of bird to ever stalk our planet has been unearthed from a fossil site in outback central Australia. Excitingly, other remains may still lie nearby, waiting to be unearthed.
Described by a paleontologist as a “extreme evolutionary experience“, Stirton’s Thunderbird (Stirtoni of Dromornis) is a patchwork of strange anatomical features. Its oversized beak protrudes from an undersized skull, all perched on a body that peaks at 3 meters (10 feet) and weighs up to half a ton.
Just to make the animal even more absurd, these 8 million year old forest giants are actually related to modern day poultrylike chickens and ducks.
While oversized “demon ducks” are undoubtedly heavy hitters, getting an accurate measurement of their size from a jumble of bones is easier said than done. This latest finding could take some guesswork out of models attempting to describe the true size ranges of Dromornis species.
For the first time, remains of these massive flightless birds have been found articulated, arranged more or less as they existed inside the once living animal.
A great time to #alcoota2022 with the very first jointed leg -Dromornis stirtoni- excavated from Classy Corner. Thanks @Phoebyornis for the scale! #fossilfriday @fupalaeosoc @FlindersPalaeo pic.twitter.com/mJaKvacOtg
— Museum and Art Gallery of the Northern Territory (@MAG_NT) July 22, 2022
“What that means is that the carcass was whole when it was buried,” Adam Yates, paleontologist and curator of earth sciences at the Museum and Art Gallery of the Northern Territory, told ScienceAlert.
“We only got the lower legs because that’s as far as we dug. There is every reason to expect that much of the rest of the skeleton – if not the entire skeleton – could be find in the next dig as we dig further into the bank which legs come from.”
The fossilized bones were discovered in Alcoota Reserve, a dense fossil site 190km northeast of Alice Springs with one of the largest concentrations of terrestrial vertebrate remains in Australia. While this location has produced thousands of fossilized specimens since the beginning of the excavations in 1986, most of them are mixed fragments of different species thanks to the historical floods which mixed the remains.
Thus, most Alcoota fossils have required careful sorting into species and reconstructions involving parts of several individual animals. Such composite reconstructions necessarily involve a degree of creative thinking that introduces occasional errors.
“Even if you get all the species right – you match the right bones with the right species – you’re still going to have proportional errors because of course there is natural variation between individuals,” Yates says.
The new legs are an exciting discovery because they can provide researchers with a much more accurate idea of the true proportions of these animals. It will also help paleontologists better identify more D.Stirtoni bones of other fossils mixed in Alcoota.
Deb’s wand now in one piece. #FossilsFriday pic.twitter.com/lrtzobn0oP
— Sam Arman (@Samosthenurus) September 2, 2022
Flinders University paleontologist Warren Handley, Yates and his colleagues had previously compared an assortment jumble D.Stirtoni bones discovered in the area and were able to identify a difference in size between males and females.
They took bone samples and identified a type of tissue called medullary bone in smaller specimens. It’s a temporary store of calcium that females draw on to shell their eggs, a feature that males lack, Yates says.
Judging by the size of the newly discovered leg bones, researchers suspect the remains belong to a woman D.Stirtoni, whom the team dubbed Deb. They plan to do a histological test to confirm their suspicions.
Meanwhile, Deb’s fossils are being prepared for a temporary display at the museum later this year. Carefully cleaned and hardened with plastic acetate fill in any gaps, the bones will be preserved for future study.
Traces of thunderbirds have only ever been found in Australia, dating to the late Miocene. These absurdly bloated chickens with small stubby wings lacked the specialized keeled sternum that other birds rely on for their large flight muscle attachments. They stalked dry forests and probably used their huge beaks to gobble up fruit and other vegetation.
Other herbivores found in Alcoota dated to the same period include marsupials such as wallabies and ancient relatives of cow-sized wombats.
These findings suggest D.Stirtoni was the tall navigator of this dry ecosystem, similar to today’s camels – using its height to reach vegetation out of reach of its smaller herbivorous companions, Yates explains.
At the time, “it wasn’t a mammal that played that role, it was a bird,” Yates says.
Fossil records suggest that these epic birds and their relatives existed for an incredible 25 million years. But at the end of the Miocene epoch, Australia dries up, perhaps too fast to D.Stirtoni adapt.
Yates notes that young thunderbird fossils are extremely rare to find, suggesting that these animals did not have a rapid rate of reproduction, perhaps only producing one or two chicks per year. Moreover, “it took an extraordinarily long time to mature for a bird. Dromornis took 15 years to reach adult size and sexual maturity.
These characters are well known to leave animals vulnerable to changing environmental conditions.
The corner of the fossil bed where the paleontologist found Deb also contained an articulated wallaby, so Yates is looking forward to returning to the field next year. He is convinced that more and more Deb is waiting to be discovered in the earth bank, and there is an enticing possibility that this site also contains articulated fossils of unknown species.