Rewriting the Book on Dinosaurs

By MICHAEL D. LEMONICK

A cool and misty dawn, circa 78 million B.C. A lone triceratops interrupts a leisurely meal of ferns and twigs to glance around uneasily. Though the 11-ton creature is an intellectual lightweight, it senses the danger lurking in the surrounding forest. Suddenly, out from behind a tree lumbers one of the largest and fiercest carnivores that have ever lived: Tyrannosaurus rex. Although this beast is a mere adolescent, it is 15 ft. tall and armed with dagger-sharp teeth. The triceratops attempts a retreat, but the cold-blooded creature can only move slowly. It is too soon after sunrise, and the dinosaur hasn't had time to absorb the heat it needs to rouse its sluggish metabolism. While T. rex has the same problem, its longer legs enable it to quickly overtake the docile herbivore. And then . . .

Wait! Time out! There is something wrong with this picture. Nearly everything, in fact. Two decades ago, paleontologists might have signed off on such a scenario, but not today. An avalanche of new evidence -- from fossilized bones, dinosaur nests, eggs and even footprints, analyzed with such high-tech equipment as CAT scans and computers -- has completely transformed scientific thinking about dinosaurs. Triceratops and other herbivores were not necessarily dull-witted, nor did they wander around alone; they probably traveled in vast herds and went on annual migrations. They may have cared for their young, and perhaps cooperated with one another to protect them from predators. Predators too were social. All but the oldest and biggest tyrannosaurs traveled in packs and attacked like prowling wolves, as did most of the smaller and nastier predators. (Despite popular belief, Tyrannosaurus was not necessarily the most vicious.)

Dinosaurs probably weren't cold-blooded either. They could move along briskly, even in cool weather; some lived above the Arctic Circle, where the sun never rises in winter. Rather than a uniform dull green, they could easily have been striped, spotted and brilliantly colored. Even the idea that all the dinosaurs died out 65 million years ago is now passe. Many experts believe that one resilient line is still flourishing today. The common name for these modern dinosaurs: birds. Observes Mark Norell, a paleontologist at the American Museum of Natural History in New York City: "Birds are more closely related to Tyrannosaurus rex than Tyrannosaurus is to almost any dinosaur you've ever heard of."

This rewriting of conventional wisdom has accelerated in the past 10 years. New fossil beds have been found and old ones rediscovered in the Gobi Desert, along the ancient Silk Road in the mountains of China, on the margin of the Argentine Andes and in the jungles of Laos and Thailand. Despite the remarkably small number of scientists working in the field -- only about 100 worldwide, splitting a meager $1 million in research funds -- a new dinosaur species is found on average every seven weeks.

Surprises crop up constantly. The latest: a new species from Mongolia, announced last week by Norell and several U.S. and Mongolian scientists. Known as Mononychus (meaning one claw), the turkey-size animal looked like a modern, flightless bird, complete with feathers, but had bone structures characteristic of both birds and dinosaurs. Its discovery cements the bird- dinosaur link even more firmly.

Thanks largely to the explosion of information, dinosaurs are more popular than ever -- if that's possible. In light of the new insights, museums around the world are revamping musty exhibits or installing new ones. They are rearranging the old stilted skeletons on display into new dynamic poses and adding such modern attractions as robotic dinos and interactive computer games. Dinosaur theme parks are booming, while toy stores overflow with stuffed stegosauruses, dinosaur puzzles and models, not to mention the omnipresent videosaurus Barney. And early in June, dino-mania will reach fever pitch with the premiere of Steven Spielberg's long-awaited movie version of the Michael Crichton thriller Jurassic Park. (See following story.)

The rage for dinosaurs is hardly new. The British anatomist Richard Owen first coined the term dinosaur (from the ancient Greek deinos, "terrible," and sauros, "lizard") in 1841 to characterize gigantic fossilized bones found several decades earlier. Dinosaur bones and footprints had actually been known for centuries, but were ascribed to dragons or extinct lizards or even giant ravens. Owen realized that these enormous bones belonged to a previously unknown and long-extinct group of animals related to but different from lizards. Dinosaurs became an immediate rage in London. An 1854 exhibition at Hyde Park's Crystal Palace featured a number of life-size dinosaur models that drew throngs of admirers.

The early dinosaur experts were hampered, however, by a shortage of fossils, and they made egregious mistakes about what the creatures looked like. Owen believed, for example, that Iguanodon, a grazing beast some 30 ft. in length, was built something like a hippopotamus, with a small, sharp horn on its nose. Half a century later, scientists decided the creature was shaped more like a kangaroo and the horn was really a misplaced claw that belonged on its forefoot. Now they think it was probably four-footed after all.

Despite all the fossils unearthed since then, scientists are still working with spotty information. "We probably don't even know 1% of all the species," admits Jack Horner, curator of paleontology at the Museum of the Rockies in Bozeman, Montana. Yet they have made tremendous progress in understanding how dinosaurs evolved, how they came to dominate the world for an incomprehensibly long 165 million years (humans, by contrast, have been around fewer than 4 million), how they lived and behaved, and how they finally passed into history.

THE RISE OF DINOSAURS

During the Triassic period -- say, 225 million years ago -- it would have seemed absurd to suggest that dinosaurs would soon inherit the earth. At the time, they were inconsequential creatures, perhaps the size of dogs, living among far more imposing giant crocodiles and other reptiles. During Triassic times, the continents were stuck together in a single mass that scientists call Pangaea. The planet was warmer and rainier than it is today -- ideal conditions for the growth of vast forests along coastlines and adjacent to rivers. Conifers, horsetails, tree ferns and ginkgos were the dominant vegetation. Giant 3-ft. dragonflies whirred through the air, and 18-in. cockroaches scuttled along the forest floor. The seas teemed with mollusks, algae and large marine reptiles.

No one knows what the very first true dinosaur looked like, but a young paleontologist named Paul Sereno of the University of Chicago has come closer than anyone else to finding out. In 1991, working with Argentine scientists in Ischigualasto Provincial Park at the edge of the Andes, he unearthed one of the oldest dinosaur fossils ever found. The animal, now known as Eoraptor, was a carnivore that dates from 230 million years ago. Like the much later Tyrannosaurus, the Eoraptor belonged to the saurischian, or lizard-hipped, category of dinosaurs. (The name refers to the arrangement of its pelvic bones; the other category of dinosaurs, which includes Stegosaurus and other herbivores, is labeled ornithischian, or bird-hipped. Ironically, birds are descended from the lizard-hipped class.)

Eoraptor has so many primitive features, including an exceptionally simple jaw, that Sereno thinks it probably originated just a short time after the ornithischians and saurischians diverged. Says Sereno: "Fifteen years ago, it was a radical idea to think that dinosaurs came from a common stem. Now we are just inches away from finding that stem."

Sereno is even more interested in the question of how dinosaurs managed to take over the world. One thing is clear from his Argentine excavations: it happened quickly. In Eoraptor's day, dinosaurs were rare. Ten million years later, however -- the blink of an eye in geologic terms -- many reptiles and crocodilians were in steep decline, while dinosaurs were headed toward dominance.

The reason, he and many colleagues believe, may have been a mass extinction of many of the planet's species late in the Triassic period. It could have been caused by the impact of a massive asteroid or comet, perhaps, or by dramatic climate changes triggered as Pangaea separated to form distinct continents. As other animals disappeared wholesale, the dinosaurs evolved rapidly to fill vacant ecological niches. Says Sereno: "It's very difficult to argue that the dinosaurs had something the others didn't. Instead of evolving because they were better, maybe they evolved because there was a sudden vacuum." For whatever reason, the early mammals, although they arose at about the same period, remained bit players for the next 150 million years. "Mammals during this time," says Hans-Dieter Sues of the Royal Ontario Museum in Toronto, "were nothing more than small, insect-eating organisms."

BONES THAT SPEAK VOLUMES

The traditional way to understand dinosaurs is through their bones, the only body parts that are preserved and converted into rock by the process of fossilization. The way bones fit together can reveal how an animal's joints worked, how its limbs moved, what kind of food it ate and how agile it was. Comparisons with living animals are also invaluable. "To understand dinosaur bones, you must take apart living animals," asserts paleontologist David Weishampel, who teaches anatomy at the Johns Hopkins University School of Medicine. "Fossils don't come with instruction kits."

If the earliest dinosaurs were meat eaters, how did they evolve into herbivores -- a key to their ability to survive in a variety of environments? The arrangement of teeth and jaws was probably a major factor, and that may explain in part why dinosaurs were so successful overall. Weishampel is trying to correlate tooth design, patterns of tooth wear, the size of the mouth and other aspects of skull mechanics with the types of plants the dinosaurs might have munched. "You can get a rough feeling for how fibrous the material was that they ate, and whether they sheared, ground or pulped their food."

Take sauropods, for example, the four-legged, long-necked giants that flourished in the Jurassic, the middle period of the dinosaurs' reign, which lasted from 208 million to 144 million years ago. These largest of all dinosaurs include Brontosaurus (an out-of-favor name these days: call them Apatosaurus, or risk correction by a knowledgeable six-year-old). They evidently used their spoon-shaped and pencil-shaped teeth to bite off leaves and twigs, relying, like many modern birds, on gizzard stones to do the actual chewing. Horned dinosaurs like Triceratops, which lived toward the end of the dinosaur era, in the late Cretaceous, had very inefficient jaws. "Their teeth were arranged in a vertical plane, which is very unusual," explains University of Pennsylvania paleontologist Peter Dodson. "That essentially means they were eating salad with a pair of scissors."

Weishampel is also using his dental analyses to determine how the advent and proliferation of flowering plants during the early Cretaceous might have influenced population levels of large, plant-eating dinosaurs. There is some evidence, he says, that the spread of flowering plants hurt large-bodied dinosaurs like sauropods and helped the somewhat smaller duck-billed and horned dinosaurs. When flowering plants began to dominate the landscape in the mid-Cretaceous, they edged out the conifers, tree ferns and other plants that the long-established sauropods depended on. The smaller vegetarians, which evolved much later, had not become so set in their eating habits.

Another idea, posited by David Norman, director of the Sedgwick Museum at the University of Cambridge: the giant, established herbivores may have overgrazed their customary food plants, giving the newly evolving flowering plants a chance to compete. Says Norman: "It's rather an exaggeration, but you could say that in a sense dinosaurs might have invented flowering plants."

Dinosaur bones also hold clues to parts of the body that have disintegrated over the eons. By assessing the relationship in living animals between the vertebrae and the delicate nerves they protect, Emily Giffin, a paleontologist at Wellesley College, attempts to make inferences about the neuroanatomy of dinosaurs. Vertebrae are especially revealing because the canal running through them varies in size according to the number of nerve fibers it contains, and that in turn depends on how much the muscles controlled by these nerves are used. Giffin is trying to determine whether theropods -- the dinosaurian suborder that includes fierce predators like Oviraptor, Deinonychus, Velociraptor and Tyrannosaurus rex -- could have used their undersize forelimbs for grasping or whether the arms were purely vestigial.

WARM BLOOD OR COLD?

The assumption that dinosaurs were ectothermic -- cold-blooded -- was originally based on a simple argument. Reptiles are ectothermic -- they can't regulate their body heat. If they get too hot, they die. If they get too cold, they get sluggish. Dinosaurs were closely related to reptiles. End of argument.

As early as the 1950s, though, some researchers claimed that the rich blood supplies within dinosaurs' bones, as evidenced by the channels left behind in fossils, were more like those of fast-growing (and warm-blooded, or endothermic) birds and mammals than like those of reptiles. Maybe dinosaurs were warm-blooded after all.

There are no maybes about it as far as Robert Bakker is concerned. Long- haired, bearded and strongly opinionated, free-lance paleontologist Bakker has been the bad boy of the field for years, and does not suffer fools gladly. "There are still a few of my colleagues who think, 'If it walks like a duck, breathes like a duck and grows like a duck, it must be a turtle.' "

The fact that dinosaurs were warm-blooded should be especially obvious, says Bakker, because they were known to have had chest cavities large enough to hold huge hearts, like birds. Additional evidence is found in their migratory patterns. "There's no question that dinosaurs got as far north and as far south as there was land," says Bakker. "What should have been the tip-off is that the ones you find in the far north are the same ones you find in the south, so they could live in a wide range of climates. Also, I don't see any way dinosaurs could have survived up there unless they migrated, and migration takes energy. They would have to have been warm-blooded."

Scientists now recognize that there are, in fact, five or six different kinds of warm- and cold-bloodedness, and they are sometimes hard to distinguish, even in living animals. Moreover, making generalizations about the relationship between an animal's activity level and its metabolism can be misleading. "We tend to think that cold-blooded animals are sluggish, but that's not very accurate," says Yale paleontologist John Ostrom. "Some snakes, lizards and crocodiles can move faster than humans can. At the same time, we tend to think that warm-blooded animals are fast and very active, but the average house cat spends a lot of time snoozing."

The current consensus is that dinosaurs were not strictly ectothermic but fell short of full-fledged endothermy. "The problem," notes Michael Brett- Surman of the Smithsonian Institution, "is that there is no such thing as 'the dinosaur.' There were seven groups living 150 million years ago that started out as one thing and perhaps evolved into something else." Although Deinonychus, Velociraptor and other small, meat-eating bipeds may have been warm-blooded, Brett-Surman believes large predators like Tyrannosaurus rex, which went through three vastly different growth stages, may have been equipped with a variable metabolism.

A 7-ft.-tall juvenile T. rex, he speculates, was probably very active, capable of scampering like a groundbird. By contrast, mid-size individuals, averaging 12 ft. to 15 ft. in height, were probably somewhat less agile and may have traveled in packs. A full-grown, 40-ft.-long, eight-ton tyrannosaur must have slowed down even more, and may even have reverted to a solitary life-style. Says Brett-Surman: "They certainly wouldn't have turned somersaults across the landscape." As for the giant herbivores, which would have required hundreds of pounds of vegetation a day to sustain their enormous bulk, they might have had their own unique metabolism fueled by the heat given off by nonstop digestion.

THE GOOD MOTHER AND OTHER LIKABLE MONSTERS

In 1978, when Jack Horner happened upon 14 rocky nests in an eastern Montana excavation that was later dubbed Egg Mountain, another dinosaur myth bit the dust. The egg-filled nests belonged to hadrosaurs -- duck-billed dinosaurs -- which had apparently built vast rookeries much the way social birds like penguins do. Though dinosaurs were never thought to be especially cuddly or caring, these creatures clearly nurtured their young, probably feeding them by mouth like baby birds until they were strong enough to leave the nest. Horner and his colleagues named the species Maiasaura -- Greek for "Good Mother Lizard."

The evidence for communal living was the fact that groups of nests were found in a single layer of sediment, implying that they were all built in the same year. Beyond that, the nests were spaced an average of 23 ft. apart -- about the size of an adult maiasaur. Birds often do the same thing, laying their eggs close enough together for maximum mutual protection, yet far enough apart so that they can move easily past their neighbors. Inside the nests, Horner found lots of tiny eggshell fragments. If the baby maiasaurs had simply hatched and wandered off to fend for themselves, he reasoned, the shells would simply be broken; the fact that they were thoroughly smashed convinced him that the babies stayed around to be cared for and fed. He also believes -- somewhat controversially -- that the babies' oversize eyes and snub noses would have appeared "cute" to their parents, the way the same characteristics do in humans, and thus inspired caring behavior.

Whatever the reason, says Horner, "we have pretty good evidence that all duck-billed dinosaurs were nest-bound and nurturing. We also see a lot of herding behavior among hadrosaurs as well as ceratopsians," a group that includes Triceratops. In fact, claims Horner, "most of the herbivores cared for their young."

$ China's leading paleontologist, Dong Zhiming, believes some meat eaters too may have been caring parents. In fact, he takes the contrarian view that Oviraptor, a toothless predator whose very name means egg stealer, is the victim of a bum rap. The sharp-clawed creature has been found in close proximity to nests not because it was poised to devour unhatched babies but because "it was the mother and was protecting them," says Dong.

FOOTPRINTS

A second line of argument that supports the idea of dinosaurs as social creatures is the vast trackways that have been uncovered in both North America and Asia. Hundreds of sauropods -- Apatosaurus and its kin -- would evidently travel in herds across the late Jurassic landscape, leaving footprints as they went; similar trackways have been discovered for Triceratops and Maiasaura. The tracks of the theropods, the aggressive predatory group that includes T. rex, are often found in multiple sets, a strong clue that they traveled, and presumably hunted, in packs.

Footprints can tell scientists more than that, though. Their depth and spacing also give testimony about dinosaurs' size, weight and speed. All the evidence suggests that dinosaurs in general were strong and efficient walkers, capable of maintaining a brisk pace. Theropods, in particular, observes paleontologist James Farlow of Indiana University-Purdue University at Fort Wayne, "put their feet almost one in front of another. They had a gait very similar to a human being's." They almost never dragged their tail, as out-of- date museum exhibits would have people believe; instead they probably used them for balance. "Hardly ever do you see tail marks," explains Farlow. "I sometimes envision theropods as big animated seesaws with one end that can bite you."

Theropod tracks he has studied in Texas convince Farlow that the predators moved along briskly. "Their walking pace was somewhere between three and six miles an hour," says Farlow. He has also studied trackways that were probably made by running theropods. Top speed: between 15 and 20 m.p.h. "That's not as fast as an ostrich or a good racehorse," he says, "but it's faster than anything a human can do."

The most significant thing about dinosaur tracks, says Martin Lockley, a geologist at the University of Colorado, is that "they're so abundant relative to bones. Every animal has only one skeleton, but it can leave thousands of prints." Example: 80 years of quarrying at Dinosaur National Monument in Utah and Colorado have turned up evidence of only 80 individual dinosaurs. "We went in for three years to look for tracks," recalls Lockley, "and found footprints from 240 individual dinosaurs."

Lockley has found a similar wealth of tracks in South Korea and, to his surprise, discovered many prints belonging to baby apatosaurs (a.k.a. brontosaurs). Boasts Lockley: "The conventional wisdom was that baby brontosauruses were hard to find." Now he is tackling the question of why some dinosaurs limped, alternating short steps with long ones. "We're finding that those are quite common among both quadrupedal and bipedal dinosaurs," Lockley reports. It could be because of injury -- yet why do so many different species show the same limp? Is it a trotting gait? Is the animal carrying a burden, like its young? Is it staggering away from a fight with a predator hanging onto (or biting into) its side? The answer, he hopes, will eventually be found in the tracks.

THE BIRD CONNECTION

The notion that dinosaurs and birds are related dates back over a century. In 1861, quarry workers near Solnhofen, Germany, uncovered the fossil of a pigeon-size creature. Its bone structure and teeth were similar to those of dinosaurs. Yet along with the bones, the 150 million-year-old limestone in which it was trapped had also preserved the unmistakable impressions of feathers and wings. It was ultimately decided that Archaeopteryx, as it was named, was a transitional animal, related to dinosaurs but well along the evolutionary pathway to modern birds.

As happens so often in paleontology, though, the story has become much more muddled. The confusion began in 1964 with the discovery of a 13-ft.-long theropod called Deinonychus that was remarkably similar to Archaeopteryx, perhaps 50 million years more recent, but lacked wings and feathers. Apparently, the evolution from theropod to bird took many turns and detours.

Now comes Mononychus, one of the fruits of the first Western expeditions into the Mongolian Gobi in 60 years. "Central Asia probably has the greatest dinosaur-yielding potential of any area in the world," says Michael Novacek, dean of science at the American Museum of Natural History, who went to the Gobi in 1990 and has returned every year since. "There are areas the size of Montana that haven't even been prospected. You could spend a whole lifetime there."

Mononychus may be the discovery of a lifetime. The turkey-size predator, ! with its mouthful of sharp teeth and long tail, looked quite similar to the theropods. Even so, says paleontologist Mark Norell, it shares a number of features with modern birds. "In Archaeopteryx, for example," he explains, "the fibula ((the thin bone in the leg)) touches the ankle. In birds that doesn't happen, and the same is true of Mononychus. Birds have a keeled sternum ((or breastbone)), where the flight muscles attach. Mononychus also had a keeled sternum." Some of Mononychus' wristbones were fused together, which is another hallmark of adaptation for flight, suggesting that Mononychus may have evolved from a flying animal, just as ostriches and emus are descended from flying birds. That being the case, it was probably covered with feathers.

There are researchers skeptical, of course, about how Mononychus is labeled and about the larger question of how dinosaurs are related to birds. But since scientists cannot really decide for sure whether Mononychus should be considered a primitive flightless bird or a dinosaur, it seems plausible that there is really no essential distinction: it was both.

EXTINCTION BY COSMIC CATASTROPHE

The leading theory about what wiped out the dinosaurs used to be planetwide climate change; now it's something completely out of this world. Sixty-five million years ago, goes the story, at the very end of the Cretaceous period, an asteroid or comet smashed into the earth, throwing up a planetwide pall of dust. The sun was blotted out for months, killing most vegetation and starving the dinosaurs. The mammals, which had blown a chance during the last mass extinction, 150 million years earlier, rushed in to take over the suddenly vacant ecological slots.

The evidence for this theory is a thin layer of iridium, an element rare on the earth's surface but relatively abundant in comets and asteroids, found at about the right level in ancient sediment. The iridium layer was discovered in Italy by the late physicist Luis Alvarez and his geologist son Walter, and has since been found all over the world. An impact crater that may be the right age and size was identified two years ago on Mexico's Yucatan Peninsula.

The comet-asteroid theory is much beloved by physicists, astronomers and the general public; but while many paleontologists accept it, others have their doubts -- and some don't even care. "Everyone wants to know why dinosaurs went extinct except me," says Horner, who is far more interested in how they / lived. And Bakker states flat out that the meteorite theory is a crock. "It just doesn't wash ecologically," he says. "The wrong animals die." His point is that a worldwide catastrophe should have wiped out late Cretaceous creatures such as frogs and turtles, which were vulnerable because they cannot adjust easily to environmental changes. Yet those animals survived, while the presumably more adaptable dinosaurs disappeared.

There is evidence that dinosaurs were already on the way out, even if an asteroid did deliver the final blow. The fossil record shows that the number of dinosaur types dropped 70% between 73 million and 65 million years ago. Flying and swimming reptiles, which weren't true dinosaurs, declined too: pterosaurs, ichthyosaurs, plesiosaurs and mosasaurs all died out before the dinosaurs did. "What caused the decline?" the Smithsonian's Brett-Surman asks. "Was it a change in climate? A change in ocean currents? The changing distribution of plants?"

Whatever the reason, Horner insists that the more interesting and surprising question is how the dinosaurs managed to hang on for so long. Humans should be as lucky. "It was only 80 years from the time that Darwin published On the Origin of Species until we detonated the first nuclear bomb," he says. "In the lifetime of one person, we went from figuring out where we came from to figuring out how to get rid of ourselves." When the history of life on earth is complete, Horner suspects, the world's most beloved extinct creatures may have outlived their admirers by some 100 million years.

With reporting by Andrea Dorfman/New York, David S. Jackson/Bozeman and J. Madeleine Nash/Chicago, with other bureaus