Terror Bird—This terror bird (Titanis sp.) skull is almost 50 cm long, and it clearly shows the massive bill, with its hooked tip, that was used to kill and dismember the unfortunate mammals of ancient South America. (Natural History Museum at Tring)
Scientific name: Phorusrhacids Scientific classification: Phylum: Chordata Class: Aves Order: Gruiformes Family: Phorusrhacidae When did it become extinct? Experts disagree on when the last terror bird became extinct. Some scientists argue that it was as little as 15,000 years ago, which is very unlikely. It's far more probable that they became extinct around 1.8 million years ago. Where did it live? The remains of these animals have been found throughout South America, and the fossils of one species have been found in Florida and Texas.
In the right circumstances, birds can evolve into giants. In the vast majority of cases, they have done this on oceanic islands in the absence of any large land predators. Most of the extinct giant birds are decidedly lacking when it comes to predatory ferocity. Birds like the moa and elephant bird were big animals, but they were gentle vegetarians. However, a long-legged bird living in South America several million years ago gave rise to a group of birds collectively known as terror birds. As their name suggests, these animals were not the sort of feathered critter you would be pleased to see at your bird feeder. They were big birds; the smallest were at least 1 m tall, while the biggest stood as high as 3 m. All of them bear the hallmarks of being ferocious predators. Why these nightmarish birds came to evolve in South America is not fully understood as no other place on earth has ever produced a group of predatory giant birds. Gigantism in birds is normally associated with herbivory, yet whatever conditions prevailed in South America many millions of years ago allowed the evolution of a successful and varied group of feathered carnivores.
Following the extinction of the dinosaurs, many niches in earth's ecosystems were left wide open for the vertebrate survivors—the mammals, birds, and remaining reptiles—to evolve into, and for a while, apparently, the terror birds had a power struggle with the mammals for the dominance of the terrestrial ecosystems in South America. Many of them were big and powerful enough to have been the top predators at the time, and many mammals were definitely their prey.
All but one of the terror birds paleontologists know of today have been unearthed in South America. One species (Titanis walleri) managed to reach North America, and it appears to have been quite a success, surviving for more than 3 million years, until it disappeared around 1.8 million years ago—the last of its kind to become extinct. Even though this American species was not the biggest terror bird, it must have still been a terrifying animal. Its vital statistics are impressive: 1.4 to 2.5 m tall and 150 kg in weight. It also had an immense, hooked bill, and with such an impressive beak, it could have probably swallowed a lamb-sized animal in one gulp.
Although we can piece together the skeletons of the terror birds, it's impossible to know what their plumage was like. However, we can look at living birds for clues, and if the other flightless birds are anything to go by, the terror bird's feathers may have been rather hairlike. Like the vast majority of flightless birds, terror birds had stubby little wings, but what they lacked in the wing department they more than made up for with their long, powerful legs, which ended in large feet and fearsome claws. These legs gave these animals a good turn of speed, and it has been estimated that some species of terror bird could reach speeds of 100 km per hour—comparable to a cheetah. The combination of running, big talons, and a monstrous beak made the terror birds very effective predators. It is possible to imagine one of these birds snapping at the hooves of ancient mammals as it pursued them across the grasslands of the Americas. Smaller animals were probably immobilized with the sharp talons before being torn apart by the fearsome hooked bill or even swallowed whole after having their skull crushed in the bird's vicelike grip. Larger prey animals may have been disemboweled with kung fu-style kicks, and it is even possible that crushing kicks may have been used to crack the larger bones of big prey to get at the nutritious marrow within.
Even if the last terror bird became extinct around 1.8 million years ago, these were successful animals that, as a group, survived for more than 50 million years, some of them even taking on the mantle of top land predator in the ecosystems in which they lived. However, around 2.5 million years ago (during the Pliocene epoch), something happened that completely changed the course of life for South America's unique animals—the Great American Interchange (see the "Extinction Insight" in chapter 2). The land bridge that formed between North and South America, what is now known as the Isthmus of Panama, allowed animals from the north to migrate into South America. Among them were lots of predatory cats, and it has been proposed that these animals were so effective as predators that they outcompeted the terror birds. The talons and beaks of the terror birds were no match for the teeth, claws, and hunting prowess of the invaders from the north. This is a very neat answer for the cause of the extinction of the terror birds; however, the extinction of successful animals is very rarely due to one factor, but a combination of events. Perhaps climate change directly affected the terror birds by changing their habitats and the populations of their prey. Although there is a great deal we don't know about the life and times of the terror birds, we do know that one of their number somehow managed to cross into North America and spread through the southern states. For a long time, it was assumed that the North American terror bird spread north via the land bridge, but analysis of its ancient bones paints an alternative picture, as they appear to have reached the southern states of America before the land bridge formed. Perhaps falling sea levels, due to the growth of the polar ice sheets, revealed a path of island stepping-stones across the gap of open ocean that would become the Isthmus of Panama. These stepping-stones allowed the giant birds to colonize the prehistoric North America. Maybe other species of terror bird, the remains of which are as yet undiscovered, also reached North America before following the rest of their amazing kind into the pages of earth history.
♦ The closest living relatives of the terror birds are the seriemas of South America. A bird similar to the living seriemas probably gave rise to the 17 species of terror bird that are known today from fossilized remains. These fossils cover a long period of geologic time, from about 60 million years ago to 1.8 million years ago, which goes to show how successful these birds were.
♦ For many millions of years, large, carnivorous, placental mammals were absent from South America, and in the absence of these predators, the ancestors of the terror birds evolved to fill this niche.
♦ The largest species of terror bird was the gargantuan Brontornis burmeisteri, identified from remains discovered in Argentina. This heavily built bird, with its massive head, rivals the elephant bird of Madagascar for the title of the biggest bird that has ever lived. Remains of this monster are very rare, but it has been estimated that it weighed 350 to 400 kg and was probably around 3 m tall. Like the rest of its kind, it was a meat eater, and in life, it must have been a truly spectacular creature.
♦ In 2003, a high school student in Patagonia unearthed an almost complete skull of a new terror bird species and one that may have been even bigger than B. burmeisteri. This skull was not much less than 1 m long, and it gives a true sense of what imposing creatures the largest terror birds must have been.
Further Reading: Marshall, L. G. "The Terror Birds of South America." Scientific American 270 (1994): 90-95; Alvarenga, H.M.F., and E. Höfling. "A Systematic Revision of the Phorusrhaci-dae (Aves: Ralliformes)." Papéis Avulsos De Zoología 43 (2003): 55-91; MacFadden, B.J., J. LabsHochstein, R. C. Hulbert, and J. A. Baskin. "Revised Age of the Late Neogene Terror Bird (Titanis) in North America during the Great American Interchange." Geology 35 (2007): 123-26.
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