Featured Story
May 2005
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Knowledge about the most fearsome dinosaurs
and their relatives is finally measuring up to
the animals’ fame.
By Mark A. Norell and Xu Xing
A HUNDRED YEARS AGO, Henry Fairfield Osborn, a vertebrate paleontologist and curator at the American Museum of Natural History, reintroduced the world to one of the most spectacular animals ever to have trod Earth: Tyrannosaurus rex. The specimen Osborn described had been collected by the legendary fossil collector Barnum Brown in the badlands of eastern Montana in 1902. During his career, Brown collected several other tyrannosaurs—that is, T. rex and its closest relatives. In the October 1915 issue of the American Museum Journal (as Natural History was once known) Brown called T. rex “the very embodiment of dynamic animal force.” Visitors to the museum’s fourth-floor fossil halls can still see what Brown means. The T. rex skeleton reconstructed from Osborn’s and Brown’s efforts has become a New York City icon—an image permanently etched in the mind of many a young visitor to the galleries of the museum.
In the hundred years since the discovery, T. rex has become the most famous dinosaur of them all. Its name has been borrowed to name rock bands and to pitch products such as motorcycles and a new superhighway in Denver, and its body has inspired both Godzilla and Barney—making it, in two senses perhaps, the most terrifying mega-villain in contemporary culture. A frenzy of collection has accompanied Tyrannosaurus’s recent cultural ascendancy. More specimens of the animals have been discovered in the past decade than were found in the preceding nine. Together those finds comprise a sample so good that paleontologists are testing hypotheses about tyrannosaur biology with real data, rather than relying on mere conjecture.
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As a consequence, dinosaur paleontology has changed dramatically since the days of Osborn and Brown, particularly in the past twenty years. Compare a book on dinosaurs published before 1990 with a modern textbook on an existing group, say, mammals or reptiles. The textbook would include chapters on the systematics (evolutionary relationships) and classification of the various groups of animals, their life histories, soft-part anatomy, sociobiology, biomechanics, diet, geographic distribution, and feeding. The older dinosaur book would largely end after chapter one—systematics and classification. But times have changed. Ignorance about so much of a tyrannosaur’s life has given way to ever-improving ideas of how tyrannosaurs grew, how they moved and behaved, and what they looked like—the discovery that some even had feathers is particularly surprising. The book of the tyrannosaurs is not yet etched in stone—unlike the fossils from which it ultimately comes—but it is becoming ever more complete.
Tyrannosaurlike dinosaurs begin appearing in the fossil record about 145 million years ago, near the boundary between the Jurassic and Cretaceous. The group lasted nearly 80 million years, finally disappearing 64.5 million years ago. At least six well-established tyrannosaur species are known, as well as several other species thought to be either closely related to the main group or part of the group itself.
The earliest bona fide tyrannosaur is Dilong paradoxus. It was discovered last year in Liaoning Province in northeastern China, in rocks about 128 million years old. Dilong had features common to many more primitive theropods—the group of dinosaurs most closely related to birds—such as a hand with three fingers (most tyrannosaurs’ hands had just two fingers) and a relatively small body (it was just five feet long). But Dilong also had skeletal features, skull openings, and teeth that were characteristic of tyrannosaurs. In particular, the teeth at the front of its snout had D-shaped cross-sections.
Larger tyrannosaurs, as long as twenty feet, appeared just a few million years later: Alectrosaurus roamed what is now China and Mongolia. Eotyrannus (which was probably, though not certainly, a tyrannosaur) hunted on what is now the Isle of Wight in the English Channel.
Throughout the Late Cretaceous, tyrannosaurs diversified into several species, all with large heads, powerful bodies, and two-fingered hands. Tyrannosaurs lived throughout the Northern Hemisphere, but they are best known from Asia and North America. Two species, Tyrannosaurus, in North America, and Tarbosaurus, in Asia, were so closely related that they constitute strong evidence for a Beringian land bridge joining the two continents in Late Cretaceous times. North America, though, was home to the greatest tyrannosaur diversity; here Albertosaurus, Daspletosaurus, Dryptosaurus, Gorgosaurus, and Tyrannosaurus—as well as the recently discovered Appalachiosaurus—lived at the top of the food chain in Late Cretaceous communities.
For much of the twentieth century tyrannosaurs were portrayed as lizardlike or crocodilelike, sometimes with hides covered in tubercles and scaly outgrowths like the ones on a large iguana. But new fossil discoveries suggest a more birdlike appearance. Tyrannosaurs shared a number of characteristics with birds, including hollow bones,
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Investigators from the Institute of Vertebrate Paleontology and Paleoanthropology, in Beijing, were digging in the Yixian (pronounced E-she-an) Formation in northeastern China’s Liaoning Province. The rocks of the Yixian, which have yielded many of the exquisite feathered dinosaurs described in the past decade, were formed between 135 million and 128 million years ago. The younger of those sediments, laid down at the bottoms of ponds and lakes, are called paper shales because their layers are paper-thin. Thanks to the oxygen-poor conditions of the lake bottoms, and to the fine grain of the original sediments, the paper shales often preserve soft tissues of plants and animals.
Soft parts are uncommon finds in the fossil record. Among the tissues preserved in the paper shales are delicate feathers, flower parts, hair, insect wings, and scales. The fossils are smashed flat, however, and the anatomical intricacies of the skeleton are often distorted or even destroyed.
Fortunately, the Yixian Formation has given up other treasures. The lower, older rocks are made up of coarsely grained sediments that contain a high percentage of volcanic ash. Those coarse sediments do not preserve soft tissues, but they do preserve specimens in three dimensions, and some of the specimens appear to have been buried alive by the ash. The ashen deposits have yielded spectacular specimens, including groups of baby Psittacosaurus; a mammal called Repenomamus, whose last meal, a young Psittacosaurus, fossilized inside it; and an array of other mammals and theropods, including the most complete specimen of Dilong found to date.
Much of that specimen’s skeleton was preserved, making it ideal for comparison with other fossils. One of us (Xu) noted a similarity between the new specimen and an extremely fragmentary animal collected years earlier from the Yixian’s younger paper shales. Although that specimen was only a few bones spread on several broken slabs of rock, enough was present to confirm a hunch that it too was a Dilong.
What made that realization exciting was what surrounded some of the skull bones and segments of the tail on the fragmentary specimen: the unmistakable traces of a body covering [see photograph below]. The covering looks like a thin film of dark streaks, all running at oblique angles to the skeletal elements. The structures, which are branched and about an inch long, are similar to the coverings of Sinosauropteryx, the first feathered dinosaur that was not a bird to be discovered.
The discovery of the first feathered dinosaurs in the late 1990s caused quite a stir. By now paleontologists have firmly established that many theropod dinosaurs had feathers. They range from simple structures, such as the ones on Sinosauropteryx and Dilong, to feathers just like those of a modern bird, which occur on Caudipteryx and several other species.
Feathers likely evolved in multiple stages, beginning as hollow, hairlike structures that may have served as insulation. Indeed, feathers insulate birds to this day, and a covering of feathers may have been a factor in the origin of endothermy (warm-bloodedness) in dinosaurs—especially the ones closely related to birds. Subsequently those primitive feathers specialized and diversified into a range of types. Some dinosaurs are known to have had long tail plumes and large feathers on the backs of their hands—not for flight, but perhaps for display of some kind. Eventually, though, evolution co-opted the feathers for flight.
Although it was a little surprising to discover that Dilong had feathers, it was far less surprising that the feathers were of the primitive type. After all, the theropod dinosaurs that have feathers similar to those of modern birds are much more closely related to birds than tyrannosaurs are.
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As the tyrannosaurs’ physical appearance has become clearer, so have the growth patterns and life histories of the animals. Dilong, the smallest form, was also the most primitive. Alectrosaurus, another early tyrannosaur, was bigger but still not gigantic. Albertosaurus, Daspletosaurus, and Tarbosaurus, which appeared in the fossil record at the end of the Cretaceous, all weighed in excess of 3,000 pounds and measured more than twenty-five feet long. T. rex, at more than 12,000 pounds, forty feet long, and fifteen feet high, was, of course, the biggest of them all.
What kind of growth pattern led to T. rex’s massive size? One possibility is that it grew at the same rate as its smaller ancestors, but just kept growing for a longer time. Or perhaps it grew for the same length of time as its relatives, but faster. A third possibility is that it began life in a large egg—after all, an ostrich egg is twenty-four times larger than a chicken egg. There are physical limits to the size of eggs, however, which make it highly unlikely that T. rex’s eggs were much larger than those of Albertosaurus.
Fortunately, fossils record dinosaur growth. Many vertebrates, including dinosaurs, leave annual rings in their bones as they grow. By examining the cross sections of bones microscopically, paleontologists can determine how much the bones grew each year, as well as the age of an animal at death. But applying the technique is complicated somewhat by the fact that most of the weight-bearing bones in tyrannosaurs are hollow. The hollow cavities were formed as the animal matured and grew to adulthood, erasing the early growth rings. Some bones, however, remained solid for life. One is the fibula, a small bone on the outside of the leg. That bone grows through accretion, and is not extensively remodeled as the animal matures. The adult fibula even contains the embryonic fibula—so the bone captures the entire life history of the animal.
Gregory M. Erickson of Florida State University in Tallahassee and a group of colleagues, which included one of us (Norell), determined that advanced tyrannosaurs all reached maturity in about twenty years. The oldest of the seven T. rex specimens the team surveyed was twenty-eight
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To reach even the half-ton sizes of the smaller tyrannosaurs, an animal would have to consume a great deal of food. Little is known, though, of how or what tyrannosaurs ate. Everything from scavenging to aggressive predation has been proposed. Yet some clues to their diet do exist. A few bones of plant-eating dinosaurs such as Edmontosaurus and Triceratops have bite marks that seem to correspond to the serration pattern of tyrannosaur teeth. It cannot be determined, however, whether those animals were alive or dead at the time of the bite.
Other evidence comes from a coprolite—fossilized dung—from southern Saskatchewan that seems to have come out of a tyrannosaur. An analysis of the dung by Karen Chin, now at the University of Colorado Museum in Boulder, and her colleagues initially found small digested bones, confirming little more than what all schoolchildren already know: T. rex ate meat. More careful analysis indicated that the bones came from juvenile herbivorous dinosaurs. Certainly juvenile animals are a common prey of large carnivores today, and it is no surprise that similar patterns should have played out in the past.
Skeletons reveal more than just how fast tyrannosaurs grew, however, or even what they ate. Many tyrannosaur skeletons display bones that have broken and healed; that is certainly the case with several ribs of the specimen at the American Museum. "Sue," a T. rex at the Field Museum in Chicago, shows similar rib injuries, and it has leg injuries as well. Many T. rex specimens, as well as other tyrannosaurs such as Albertosaurus, also have pathologies of the skull and vertebral column.
Perhaps the most illuminating injuries occur on the teeth and snout. Many tyrannosaurs have multiple bite marks on the muzzle and nicks on the sides of the teeth. Such wounds might have come from nuzzling or other face-to-face contact, perhaps in battles for territory or mates. It is impossible to say whether some of the scars and broken bones resulted from hunting or from roughness associated with mating. But there is no doubt that the injuries were painful, and that the animals lived a hard-knock life—and that the pain could have made a six-ton, forty-foot T. rex extremely cranky.
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Other evidence, though, suggests tyrannosaurs were gregarious. For example, some tyrannosaur excavations have yielded multiple individuals. One of those is a quarry that Barnum Brown excavated in the Red Deer River area of what is now Dinosaur Provincial Park in Alberta, Canada. The quarry was re-excavated by Philip J. Currie of the Royal Tyrrell Museum of Paleontology in Drumheller, Alberta. Currie’s analysis of collections both old and new showed that several Albertosaurus individuals of various ages and sizes were preserved together. Because no other dinosaur species were preserved with those animals, Currie surmised that they died at the same time, perhaps while crossing a dangerous river. Although the find is not definitive evidence for pack behavior, it and other similar depositions of multiple tyrannosaurs are at least highly suggestive that such behavior took place.
As for the speed of tyrannosaurs, some fantastic claims have suggested the huge animals could reach sprinterlike speeds. But those claims fail to take account of some basic issues in the physics of movement of large animals. John R. Hutchinson, now at the University of London’s Royal Veterinary College, and his colleagues digitally modeled the hind limb and hips of a T. rex [see “A Weighty Matter,” by Adam Summers]. By varying the controllable factors in the model such as posture and the total weight of the animal, Hutchinson was able to calculate how big the muscles of the hind limb must have been for the animal to move at various speeds.
His simulations clearly showed that T. rex adults could never have run much faster than twenty-five miles an hour. Going faster would have tied up such a high percentage of the total body mass in the hind-limb muscles that the rest of the animal would have been emaciated.
In fact, the fastest runners were probably juvenile T. rex’s and other smaller tyrannosaurs. That suggests the various tyrannosaur species would have exploited different prey in areas where they lived together—just as cheetahs, leopards, and lions do in Africa today. The speed analysis also suggests that T. rex and other big species would have gone from speedy youth to lumbering adulthood.
The book of the tyrannosaurs is still, slowly, being written. The tyrannosaurs of 2005 are quite different from the ones familiar to Osborn and Brown. No longer just large, imperious lizards, they represent an evolutionary explosion of diversity—some feathered, some not, some faster than others, but all capable of wreaking havoc—culminating in some of the most magnificent animals ever to walk the Earth. Both of us look forward eagerly to adding more to every chapter.
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Copyright © Natural History Magazine, Inc., 2005