Terror, Take Two

After the swift bipedal dinosaurs went extinct, the world never saw their like again--until a giant killer bird, sporting arms instead of wings, raced onto the stage.

By Carl Zimmer|Sunday, June 01, 1997
RELATED TAGS: DINOSAURS
At the moment a hungry public is gorging on yet another chunk of dinosaur flesh, unofficially known as Jurassic Park II. What can explain the success of its multibillion-dollar, multimedia franchise? In part, our fascination with death and resurrection: though dinosaurs vanished 65 million years ago, people are entranced by the impossible dream of the great beasts’ return. And in part, the fascination lies in the dinosaurs’ being truly great beasts, in their numbering among them fearful predators of a kind now absent from the planet. Of course, paleontologists like to remind us that strictly speaking, dinosaurs aren’t dead. The pigeon perched on the gutter, the starling yakking in the bushes, the finch pecking at the feeder--they are all dinosaurs as much as humans are mammals.

Somehow, though, that message doesn’t really take. When we think of dinosaurs, we don’t think of them as a clan of species all related by evolutionary descent. We think of them in the etymological sense, as the terrible lizards. Some, like Tyrannosaurus, were terrible in size; others, like Velociraptor--the swift marauding costar of the first Jurassic Park--made up for a relatively small stature with an alien predatory grace. Velociraptor in fact was terror incarnate, a prime example of the general dinosaur type whose disappearance we so mourn: a fast-running biped elegantly fitted for carnivory, one that could run down prey, disable it with slashing, clawed feet, tear at it with razor-sharp teeth, and hold it down with arms to finish it off. A crow hardly makes up for its loss.

Small wonder, then, the broad appeal of dinosaur resurrection tales. Yet to tell such a story, you don’t need a lab of special-effects tech-heads or a coffee bar of script doctors. All you need are a few bones sitting at the bottom of a Florida river: they suggest that a strikingly dinosaurian mode of existence really was resurrected once, by a group of birds, no less. And like the original version, the sequel succeeded admirably. Indeed, it continued playing until quite recently.

Many paleontologists now accept that birds originated at some point before 145 million years ago from a small dinosaur that looked something like Velociraptor. Eventually arms changed to wings, and birds became primarily creatures of the air. From time to time a lineage has settled back on the ground and given up the metabolically expensive luxury of flight. Some flightless species--dodos and kiwis, for example--arose on secluded islands where a waddling bird could peck for insects without worrying about the bothersome attentions of a predator. Others took on impressive proportions. New Zealand’s moas, which went extinct only 200 years ago, stood as high as 15 feet. Africa’s modern-day ostriches aren’t nearly as tall, but they still reach up to 8 feet; and thanks to narrow hips and long legs, they can run 50 miles an hour. Their diet, however, tends toward seeds and leaves, so they use their formidable bodies to run from predators, not toward prey. They are content with the pecking life.

In the 1880s paleontologists in Argentina began unearthing fossils of a very different kind of land-dwelling bird. These creatures were like avian linebackers. They stood six feet high and weighed up to 350 pounds. They had necks as thick as tree stumps, two-foot-long heads that were practically all beak, and large feet tipped with four-inch-long claws. Wing fossils were never found, but paleontologists assumed that, as is the case with most flightless birds, the wings were vestigial. These animals were certainly not grazers. They could run nearly as fast as ostriches, and paleontologists have envisioned them bursting out of tall grass to ambush prey, slashing and kicking their victims with their feet and ripping them apart with their massive beaks. Understandably, these flightless hunters came to be known as terror birds.

The oldest indisputable fossils of this predatory lineage are 24 to 29 million years old and come from Patagonia, at the southern tip of South America. That continent offered an especially hospitable environment for terror bird development. For tens of millions of years it was completely isolated, until 3 to 4 million years ago, when the Isthmus of Panama emerged and joined it to North America. During its long years of solitude, South America was home to an odd collection of mammals, including little horses and giant ground sloths. There were none of the familiar placental-mammal predators, like panthers or wolves; instead, top mammalian-predator slots were filled by marsupials that had evolved into species strikingly similar in appearance to hyenas and saber-toothed tigers, though not as fast or efficient. Once the isthmus arose, these efficient killers did finally arrive in South America; not long after, terror birds stop showing up in the South American fossil record. From these clues, researchers envisioned a continent ruled by an accidental monarch. Terror birds seemed to be able to more than hold their own against competing marsupial predators, but placentals proved to be more cunning and swift and drove the birds into oblivion.

This imagined scenario lasted until a scuba diver named Benjamin Waller began puttering around in the Santa Fe River near Gainesville, Florida, in the 1960s. A 2-million-year-old layer of loose sandstone was weathering out of the river bottom, unloading its fossils into the water. Among them Waller discovered bits of bone that were eventually recognized as parts of the toe and foot of a terror bird, from a species subsequently dubbed Titanis walleri. Waller’s discovery showed that terror birds not only survived the docking of continents but colonized North America, where their supposed exterminators had originated.

Inspired by Waller’s discovery, paleontologists from the Florida Museum of Natural History have been diving into local rivers for 30 years and bringing up bones that document the past few million years of mammalian evolution. Yet for the most part Waller’s original terror bird site in the Santa Fe went practically untouched until the early 1990s. It really became a sleeper--maybe a few amateurs floated by, says paleontologist Robert Chandler. Chandler came to the museum in 1991 to organize and catalog its collection of fossil birds, and as part of his work he looked through the inevitable drawers of bones marked indeterminate vertebrate. Before long he was discovering new bones of Titanis--fragments of its leg bones, pieces of its spine, skull, and toes. Very few people--even well- trained paleontologists--have a search image for a bird that’s six feet tall, says Chandler. Some of the elements were so big they had been identified as horse.

Chandler decided that the Santa Fe must be hiding more terror bird bones. After securing enough funding (and getting himself recertified for scuba diving), he went to the river. During his first two dive seasons, he was content to pick up fossils that had tumbled out of the sediment and into the catch basin. But in 1993 he discovered the layer of stone out of which they had come and began a more industrial operation. Today, with a team of students from Georgia College and State University, where he now teaches, Chandler operates a 25-foot-long dredge--basically a giant vacuum cleaner--that sucks in the sediments and any tiny fossils they contain. The sand is pumped up to the surface and dumped through a screen, where volunteers can sort and bag any fossils they find. In a typical day Chandler can catch most of the fossils in 700 pounds of sediment.

Chandler’s paleontological housekeeping has brought him a vastly improved picture of the ecosystem in which Titanis lived. Two million years ago, Florida was an open savanna dotted with oak trees, where little rodents sneaked through the grass while ten-foot-tall ground sloths fed on leaves alongside mastodons, tortoises, three-toed horses, goat-size antelopes, giant armadillos, and jaguars. Chandler has also managed to flesh out our picture of North America’s terror birds. It has not yet been his luck to find a full skeleton of Titanis laid out from beak to tail, but he has unearthed shoulder bones, parts of its skull, snapped foot bones, vertebrae, claws, and thighbones--enough so he can safely say Titanis was among the biggest terror birds that ever lived. And Chandler has identified the first bones of a terror bird’s wing. Like other paleontologists, he’d assumed Titanis’s wings were vestigial, but the fossils have proved this assumption deliciously wrong.

Among those fossils is a three-inch fragment of the shoulder end of the bird’s humerus, the long bone of the upper arm. The bone was far bigger than Chandler expected and indicated that Titanis had a three-foot- long wing. That was still far too short to let the bird fly (condors, which weigh only 22 pounds, need wings five feet long), but it was significantly bigger than an ostrich wing. More important, the humerus of Titanis was a solid mass of hard tissue; most birds have thin-walled, hollow wing bones, which can remain stiff during flight without weighing the animal down. It doesn’t look like a bone of a bird, says Chandler. It’s got tremendous strength; it could be twisted and it wouldn’t break the way a normal bird bone would.

Another bone in Chandler’s collection came from Titanis’s hand. The dinosaurian ancestors of birds had three long fingers, which fused with the wrist to transform the arm into a long, jointed shaft that could support a sheet of feathers. Much of the wrist and portions of the fingers united into a single bone called the carpometacarpus, and the second digit enlarged to act as the tip of the feather-bearing shaft. Meanwhile the first digit, stiff and shrunken, became covered by a tuft of feathers known as the bastard wing, which helps keep a bird stable as it lands. The third digit became useless.

The joints of normal bird wings are so well-sculpted that the wings can fold up snugly against the body. Where the carpometacarpus of Titanis made contact with the forearm, however, the surfaces were flat. That, Chandler realized, meant the wrist couldn’t bend--the bird had to hold its hand out in front of its body. Equally bizarre was its first finger. On most birds, the joint of this finger is stiff, since the bastard wing needs only to be raised and lowered slightly to change the airflow around the wing. But on Titanis, Chandler found, the digit contacted the carpometacarpus at a ball joint shaped like the base of our thumb. It would have a ball joint only if it were doing something with it, explains Chandler.

These two bones and their wayward shapes have forced Chandler to conclude that Titanis had wings that would be better called arms. It held them out in front of its body, palms facing inward, and on each hand was a giant movable claw and two smaller fixed claws. Titanis would stalk mammals in the tall grass, then attack at high speed, and strike with its giant beak, possibly taking a quick swipe at a victim’s spine to paralyze the prey as lions do. And they would use their arms, Chandler suggests, to keep the prey from goring them with their horns or kicking them with their feet. As much as an antelope might struggle, the terror bird’s sturdy arm bones could resist its force. They could manipulate prey with their hands and impale them with their claws. Chandler further speculates that Titanis’s arms were probably bare--feathers would get matted down with blood and be a likely haven for infection.

The picture Chandler draws of Titanis is surprisingly similar to that of predatory dinosaurs like Velociraptor, which lived many millions of years earlier. Given that birds are the descendants of Velociraptor-like dinosaurs, it might seem that the arms and armaments of Titanis were the result of some drastic atavism--the sudden resurgence of a genetic program hidden for millions of years. It is true that silenced genes can find their voice again after thousands of generations. On rare occasions they can produce whales with the crude cartilage remnants of hind legs or horses with separate toes. If the mutations that produce them crop up often enough in a population of animals, atavisms can even help establish entirely new species. But this was not the case with Titanis.

The fusion of hand bones is a striking anatomic change, but it was not the only one undergone by birds as they evolved from their flightless ancestors. They also reoriented their shoulder joints to let their wings flap. They developed huge breastbones and wishbones to anchor the muscles needed to power flight. Dinosaurs like Velociraptor had long tails that helped them balance as they made quick predatory moves. Birds shrank this tail down to a plate of bone arrayed with muscles that could control a flight-stabilizing fan of feathers.

Terror birds moved toward a dinosaur body, but not by going backward in evolutionary time. They retained a muscle-attaching breastbone, and they did not re-evolve the sharp teeth of their ancestors. Their tails remained short, suggesting perhaps that they couldn’t turn as agilely as Velociraptor. And rather than the free-fingered hand of a dinosaur, Titanis had a fused bird’s hand resculpted to function like a dinosaur’s. While Titanis thus falls short of the dinosaur model, it was actually superior to its predecessors in one important respect: birds evolved huge flight muscles extending from their breast out to their shoulders, and Titanis was able to apply this strength to clutching prey. Certainly inheriting the basic features of dinosaurs--such as their bipedalism and enlarged brains-- made it far easier for Titanis to become a dinosaur in essence long after the death of dinosaurs in fact. But it moved forward on its own evolutionary path rather than retracing an old one.

Chandler has been studying the skeletons of Titanis’s closest South American relatives, and while their wing bones haven’t been found, the bones that have been discovered show Titanis proportions--hinting that they too might have had arms, from their very beginnings more than 24 million years ago. But this new vision of terror birds only renders them more puzzling. The dinosaurs were one of the great triumphs of evolution-- only the impact of a comet or some other global climatic disaster could bring their 160-million-year tenure to an end. Terror birds in South America apparently rediscovered the dinosaur way of life some 40 million years after the dinos had gone, then rampaged all the way up the Gulf Coast to Florida, where they held their own against jaguars and other supposedly superior predators. Perhaps it was another climatic catastrophe that claimed them. At the same time that Panama was being born, the Andes were leaping from the earth. These new mountains, by turning flat savannas to rugged foothills, may have robbed the terror birds of their habitat. Perhaps they found refuge in the Gulf Coast grasslands only until the global climate cooled about 1.8 million years ago, transforming these parklands into forests of pine.

Yet even this scenario may not do the great birds justice. Recently Jon Baskin, a paleontologist at Texas A&M; University in Kingsville, discovered a four-inch-long Titanis toe in a gravel pit along the Nueces River near Corpus Christi. The find is not at all surprising in terms of geography--if Titanis’s ancestors came north to Florida, they had to go through Texas first. But it may be a real shock in terms of chronology. The other fossils in the gravel pit can be assigned to two separate sets, from two distinct times. One set consists of bones from 5- million-year-old horses, which were carried to the pit from beds about ten miles upstream and have turned ebony with mineralization. The other set of bones was probably laid down at the gravel pit itself; it includes mammoths and bison that died no more than 15,000 years ago, leaving behind fossils that are still white. They look as if they were lying around since yesterday, says Baskin.

It is unlikely that Titanis lived alongside the old horses, since 5 million years ago North America and South America hadn’t yet joined. Moreover, the Titanis toe is as fresh-looking as Baskin’s bison fossils. It’s rather tentative, Baskin admits. I can’t say for sure that it is 15,000 years old rather than 2 million years old, like what Bob has. But based on what I can see, it doesn’t seem to be any differently preserved than the bison. There’s nothing else from the fauna--there’s a 5-million- year-old component and a 15,000-year-old component. And of those two, it’s most like the 15,000-year-old component.

A stray toe isn’t much to work with, yet it’s enough to set an imaginative mind spinning. Baskin’s youngest date is tantalizingly close to when humans arrived in the New World. By 12,400 years ago, archeologists have recently decided, humans had already reached southern Chile. With that in mind, it becomes easy to conjure a vision of humans meeting up with a resurrected dinosaur--and not in a darkened movie theater but face-to-face on a bright Texas prairie.
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