Scientific name: Dromornithids Scientific classification:
Where did it live? The bones of these birds are known only from Australia.
Today, Australia is home to two species of giant flightless bird: the emu of the bush and plains and the cassowary of the northern forests. These two species are closely related to the other ratites, the giant flightless birds that evolved on the immense southern landmass of Gondwanaland: the ostrich of Africa, the rhea of South America, the kiwis of New Zealand, and the extinct moa and elephant birds of New Zealand and Madagascar, respectively.
Up until 30,000 years ago, Australia supported even more types of giant flightless bird, which were very distinct from the ratites. Collectively, these feathered brutes are known as the dromornithids, or thunderbirds, and they appear to have been diverse, common animals of prehistoric Australia. Seven species of Australian thunderbird have been identified from remains found throughout the continent, and they range in size from animals the size of the cassowary to Stirtons thunderbird (Dromornis stirtoni), a 3-m-tall, 400-kg whopper that may challenge the elephant bird, Aepyornis maxiumus, for the mantle of the largest bird ever.
The Australian thunderbirds share certain characteristics with the ratites, such as an absent keel bone (the anchor for the attachment of large flight muscles); tiny wings, useless for flying; long legs; and powerful feet. Outward similarities in nature can be misleading, and the parallelism between the thunderbirds and the ratites is simply due to the phenomenon of convergent evolution. The origins of the thunderbirds are very different from the origins of the ratites. Essentially, they were ducks that grew to enormous proportions in the isolated refuge of Australia.
The first bone of a thunderbird was encountered in the late 1820s in the Wellington Caves, New South Wales, by a team led by Thomas Mitchell, but almost 50 more years went by until the first species of thunderbird was formally identified by Richard Owen. Since then, many thunderbird bones have been found throughout Australia. The most common finds have been vertebrae, the long bones of the hind limbs, and toe bones. Bird skulls are particularly fragile, and until very recently, no one had much of an idea how the head of a thunderbird looked. Recent discoveries show that these birds had enormous heads and very impressive beaks. The beaks are very deep, but quite narrow, and some of the species appear to have been equipped with powerful jaw muscles. Naturally, the impressive biting apparatus of these extinct birds has led paleontologists to speculate about what they ate. Some paleontologists believe that they were carnivores, or perhaps even scavengers capable of breaking the bones they found at carcasses. Others believe that the thunderbirds were herbivores fond of nibbling vegetation and using their terrific bill to crack open seeds and nuts. The image of a giant, carnivorous duck is an enticing one, especially for the media, but it is highly unlikely that these huge birds were meat eaters, or even scavengers. They lack the equipment of true predatory animals. The bill may be big, but it certainly isn't hooked, a necessary tool for any animal hoping to tear chunks of flesh from a carcass. Also, the feet of the thunderbirds lack the talons we see in all predatory birds, regardless of their size. Last, the eyes of the thunderbird are not positioned in a way to provide binocular vision: they are situated on either side of the head and give good all-around vision but leave blind spots directly in front of and behind the animal. This is the vision of an animal that is hunted, not a hunter. Chemical analysis of numerous egg shell fragments from one type of thunderbird shows that this species was undoubtedly a herbivore with a penchant for eating grass. Other common thunderbird fossils also point to herbivory. Along with the bones of thunderbirds, paleontologists have unearthed numerous polished stones, known as gastroliths. These were swallowed by the bird and ended up in the gizzard, where they helped break up fibrous plant matter.
As it's very probable the thunderbirds were herbivorous, the numerous predators that once stalked Australia must have hunted some of these birds, especially before they reached adulthood. This is one reason why some of the thunderbirds grew so huge, as large size is an excellent defense against predators. Their other defense was powerful legs, which probably endowed some of the species with a powerful kick and a good turn of speed to get them out of harm's way.
As well equipped as they were to deal with the rigors of prehistoric Australian life, these giant birds lacked the adaptability to deal with the combination of humans and the devastation they bring and climate change. Exactly when the thunderbirds became extinct is a cause of dispute among paleontologists, but the last species is widely thought to have clung to existence until around 30,000 years ago. Scientists have used ancient egg shells of one species of thunderbird (Genyornis newtoni) to assess the impact of human activity on these birds, and the Australian landscape in general. It seems that before 50,000 years ago (before the widespread human colonization of Australia), this particular thunderbird pecked at nutritious grasses. However, only 5,000 years later, the diet of this species had completely switched to the leaves of bushes and trees. The scientists' theory is that around 45,000 years ago, humans began to have a drastic effect on the fragile Australian landscape by starting bushfires, which may have burned out of control. With their preferred food up in smoke, the thunderbirds were forced to eat other plant matter, and it seems that they may not have been able to adapt to this change. In the centuries that followed the human colonization of Australia, the thunderbirds dwindled away to extinction.
♦ In some Australian Aboriginal rock paintings, there are birds that appear to represent the thunderbirds. The depictions are certainly too large for emus and cassowaries and are probably the artist's attempt at painting one of the larger thunderbird species. Engraved trackways depicting the footprints of large flightless birds have also been found in Australian rocks, and these, too, are thought to represent the thunderbirds.
♦ Footprints, thought to be made by the thunderbirds themselves, have also been found in the Pleistocene Dune Sands of southern Victoria, Australia.
♦ Apart from the effects of deliberate bushfires, it is very likely that thunderbirds were hunted for food by the first Australians.
Further Reading: Murray, P., and P. V. Rich. Magnificent Mihirungs: The Colossal Flightless Birds of the
Australian Dreamtime. Bloomington: Indiana University Press, 2003.
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