THE dinosaurs are long dead. This is the age of the mammals. And this is Australia. So what鈥檚 big enough and fearsome enough to chase a hungry marsupial lion away from its kill? Wait for it-a duck.
Clearly this is no ordinary duck. It is almost 3 metres tall, with legs like tree trunks, a body like a tank and a head the size of a horse鈥檚-with a terrible beak. Meet the demon duck of doom.
Strictly speaking, the monstrous bird isn鈥檛 exactly a duck, but it is close family. As for the rest of its nickname-Steve Wroe, a palaeontologist at the University of Sydney and the Australian Museum, thinks it fits the bill. He is in no doubt that this bird ate flesh. 鈥淭he beak was a huge pair of secateurs that could scissor out chunks of meat,鈥� he says. 鈥淚t could hook into your thigh and rip out a nice slab of meat quite easily.鈥�
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This awesome bird is Bullockornis planei, a 300-kilogram giant that terrorised the rainforests of Australia鈥檚 Northern Territory 15 million years ago. Bullockornis belonged to a uniquely Australian family of extinct flightless birds called the dromornithids, or thunder birds. These huge birds were once some of the most conspicuous members of the Australian fauna. They lived on the continent from at least 24 million years ago until 50 000 years ago and perhaps more recently still. The last of the family, Genyornis newtoni, almost certainly overlapped with the first humans.
Since the discovery of the first huge leg bone in the 1830s, fossil hunters have unearthed eight species. Some were not much bigger than an emu. Genyornis was almost as big as Bullockornis-but there was one species more monstrous still. Dromornis stirtoni, which lived around 8 million years ago, was half a metre taller than Bullockornisand weighed in at more than half a tonne. It may have been the biggest bird that ever lived.
For birds of such spectacular size, the dromornithids have remained remarkably mysterious. Over the past 150 years, fossilised bits of the giant birds have turned up often enough to suggest they were quite common. There are fossilised vertebrae, some the size of a fist, others as big as a large grapefruit, and monumental chunks of leg bone. Clearly, these were big, flightless birds with long necks: they had stubby little wings and big, strong legs for running. But to the immense frustration of palaeontologists, the parts that would help them to fix the birds鈥� position on the avian family tree-the skulls, and in particular the palates-were always hopelessly damaged or missing. In their absence, palaeontologists assumed that Australia鈥檚 giant birds were related to ratites-the group of flightless birds that includes emus, cassowaries and ostriches, together with the extinct moas and elephant birds. And because these birds are primarily plant-eaters, everyone assumed thunder birds were too.
Then, towards the end of 1998, Peter Murray and Dirk Megirian, fossil experts from the Central Australian Museum in Alice Springs, provided the first glimpse of the face of one of Australia鈥檚 long-dead giants. They had finally pieced together enough skull fragments to reconstruct Bullockornis鈥檚 head and beak. Unlike emus or ostriches, which have tiny heads perched on long, sinuous necks, this bird鈥檚 head was huge-almost half a metre long-and ended in a deep, curved beak. The reconstruction shattered any illusions about thunder birds being related to the ratites. It also raised serious doubts about their image as benign herbivores.
After painstaking comparison with skulls of birds from other families, Murray and Megirian concluded that dromornithids were not even distantly related to the ostrich family. Instead they shared a surprising number of features with a group of birds called the anseriforms-ducks, geese and screamers. Any bodily resemblance to an emu was the result of adaptation to fit a life running on land-a case of convergent evolution, says Walter Boles from the Australian Museum.
What excited Wroe more than the fine detail of the bird鈥檚 palate bones was the sheer size of its head and formidable beak. Enormous circular pits in the scimitar-like lower jaw provided anchorage for immense jaw muscles. This beak was a powerful tool worked by strong muscles, a pair of shears that could snap shut with a fearsome bite. What, asked Wroe, did a bird with a head as big as a horse鈥檚 do with such a lethal-looking beak?
Murray and Megirian had pondered this question too. They decided that the bird鈥檚 gigantic beak was a specialised cutting tool for shearing through the toughest parts of plants-twigs, leathery-coated fruits or seed pods or large nuts with hard shells. With its long legs and neck, they suggested, Bullockornis browsed in the lower branches of trees, 2 or 3 metres off the ground. Wroe has other ideas. He believes that the massive head and beak with industrial-strength muscles were adaptations for slicing through meat. 鈥淔rom the power generated in the bite, it鈥檚 difficult to rationalise as something that cracked open seed pods or munched on leaves and suchlike,鈥� he says.
Wroe argues that if Bullockornis was eating plants, then it was vastly over-equipped for the job. Natural selection generally provides an animal with the tools it needs to make a living-nothing less and certainly nothing more. Unlike mammals, birds don鈥檛 process their food in their mouths-although in some birds the gizzard stones in the crop grind it up before it passes into the stomach. For a bird that eats plants there is no advantage in having a head bigger than that needed to strip leaves or open a nut. 鈥淚t only needs to be big enough to procure the food item of its choice,鈥� says Wroe. 鈥淚n the gigantic birds we know were primarily vegetarian, such as the moas and elephant birds, the heads are relatively small.鈥� Even the largest of the moas- 3.5 metres tall and 300 kilograms-ate plants, says Boles. 鈥淎nd it had a head a mere fraction of the size of these.鈥� If Bullockornis ate plants, evolution had flunked on the design front: this bird was walking about with a giant head it didn鈥檛 need.
For a meat-eater, a larger head does bring advantages: it can tackle larger prey. This explains why these giants had such huge heads, says Wroe. 鈥淚 say they included significant amounts of meat in their diet.鈥� The only other groups of birds with heads anything like this size were the carnivorous terror birds, or diatrymids, that lived in North America and Europe some 60 million years ago, and the terror cranes, or phorusrhacoids, equally vicious predators that may have lingered on in North America until as recently as 12 000 years ago.
In fact, Bullockornis鈥檚 skull has much in common with that of the terror bird Diatryma. The edges of the beak at the rear are designed for crushing, while the front was designed for tearing, says Boles. 鈥淟ike Diatryma, Bullockornis could both cut and crush.鈥� Strong crushing surfaces could prevent damage if the bird inadvertently bit on a bone-or might even be an adaptation for bone crushing, says Boles.
If Bullockornis was a carnivore, what might it have been eating? 鈥淧retty much whatever it wanted,鈥� reckons Wroe. The rainforests that covered the region around Bullock Creek in the Northern Territory where the great bird was found were inhabited by plenty of ground-dwelling marsupials, from smallish bandicoots and kangaroos to larger diprotodontids-stout, wombat-like animals-and an assortment of reptiles.
As yet, there is no hard evidence that the demon duck was a killer. The pieces of skull excavated so far show no sign that the beak ended in a lethal hook like those of modern birds of prey. But Bullockornis might have had a hook sculpted from the sheath of keratin that coats the beak but which hasn鈥檛 been preserved. Or maybe it didn鈥檛 need one. Diatryma managed perfectly well without. 鈥淎 big hook indicates a carnivorous diet, but it isn鈥檛 necessary to have a hook to eat meat,鈥� says Boles.
The jury is also out on whether Bullockornis was fast enough to hunt warm-blooded prey. It lacks the elongated foot bones that make emus and ostriches so swift. But speed might not even have been necessary: many of the marsupials of the Miocene forests were large, plodding animals incapable of moving fast. Besides, as Wroe points out, carnivores don鈥檛 have to kill to eat. The demon duck could have been a scavenger, stealing meat from more proficient predators. 鈥淎t half a tonne, it was big enough to drive any other animal off its kill-even acting singly,鈥� he says. Fifteen million years ago, the fiercest mammalian predator was the marsupial lion, Wakaleo vanderleueri, a beast the size of a Rottweiler with dreadful meat-slicing molars-but no match for the huge Bullockornis. 鈥淎 scavenger can be pretty scary. Few animals are purely predators or purely carnivores,鈥� says Wroe.
If Bullockornis deserves its new reputation, does that make the other dromornithids demon ducks too? 鈥淭hey were a varied group of birds and maybe they had different habits,鈥� says Boles. The immense Dromornis was certainly big-headed. Enough pieces of the skull remain to show that it was even bigger than Bullockornis, with the same sort of beak. Dromornis also had powerful muscles that could have pumped the legs by sheer brute force. 鈥淚t could have got up a bit of steam,鈥� says Boles. But a piece of skull from one of the oldest birds, Barawertornis, which was about the size of a big emu, suggests it lacked massive jaw muscles. Genyornis also had a small head by dromornithid standards-but its jaws had sharp cutting edges at the front and thick, crushing parts at the back. 鈥淚t was capable of really crunching something,鈥� says Boles.
The hunt continues
Piecing together the lives of these long-dead giants isn鈥檛 going to be easy. 鈥淲hat would be really nice is to discover some old bog the birds fell into which preserved their stomach contents,鈥� says Boles. 鈥淏ut that`s probably asking a bit much.鈥� In the meantime, the hunt for clues continues at Riversleigh, Australia鈥檚 most famous fossil site in northwest Queensland. Two species of thunder bird have emerged from the limestone rocks there, both dating from between 24 and 15 million years ago. One is Barawertornis. The other, so far known only as Big Bird, could be another demon duck. Big Bird was as big as Bullockornis, and the one small fragment of lower jaw is of a similar size and shape.
Last June, a lucky strike of the hammer produced a new source of clues: a perfect cast of a brain. 鈥淚t just popped out of the rock,鈥� says Mike Archer, director of the Australian Museum. 鈥淎nd there was only one thing it could have belonged to-a bird, and a big one.鈥� The brain, which fits comfortably into the palm of a hand, was too big for an emu. 鈥淚t had to be the Big Bird,鈥� says Archer.
The brain cast is a bonus, revealing aspects of the bird鈥檚 biology no bones can. It provides some hints about how good the bird鈥檚 senses were-and how smart it was. The olfactory bulbs are missing, but the size of the attachments suggest that its sense of smell wasn鈥檛 bad, says Boles. Smell is important to a scavenger. 鈥淎nd from what we can see of the optic lobes it might have had reasonably good eyesight-although it wasn鈥檛 eagle-eyed.鈥� But did it have the brainpower to outwit prey? Possibly. The area at the top of the brain is quite prominent. 鈥淚n intelligent birds such as ravens, this part is big and well developed. In `birdbrains鈥� it鈥檚 small. This is in between,鈥� says Boles. 鈥淪o it wasn鈥檛 tripping over its own feet, but it wasn鈥檛 doing a lot of high-level reasoning.鈥�
Next month, the team heads back to Riversleigh to hunt for more bits of Big Bird. The brain came from a new search site on the final day of last year鈥檚 excavations, and everyone鈥檚 hopes are high. 鈥淲e may find the rest of the skeleton in that patch,鈥� says Boles. 鈥淚 guarantee that Riversleigh will produce a whole head- it鈥檚 only a matter of time,鈥� insists Archer. Riversleigh is famous for preserving not just bones but soft tissue and prints, so the team will also be looking for more subtle clues, such as an imprint of a beak.
There are other ways of approaching the problem. One would be to take an engineer鈥檚 view, and calculate the force of the bird鈥檚 bite, the mass of its jaw muscles and the power generated at different parts of the beak. Even this is unlikely to provide cast-iron evidence that the demon ducks devoured flesh.
But Wroe has one simple question for those who doubt the bird鈥檚 carnivorous habits. 鈥淚f Bullockornis was not butchering carcasses with that massive head and massive muscles and massive beak, then what was it doing?鈥� And if it was killing bandicoots or possums or even stealing food from marsupial lions, that would make it the biggest carnivore on two legs since the dinosaurs disappeared.
