The varieties of Tyrannosaurs: knowledge about the most fearsome dinosaurs and their relatives is finally measuring up to the animals' fame
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 produces 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.
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 [see "Bird's-eye View," by Matthew T. Carrano and Patrick M. O'Connor, page 42]--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 rived 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, feet with three primary toes that all pointed forward, and a wishbone. And in late 2004, when the first specimens of Dilong were described, they corroborated a hypothesis that at least some tyrannosaurs had what might be the most superficially obvious bird trait of them all: feathers.
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 [see "Bringing Up Baby," by David J. Varricchio, page 30]; 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 on page 35]. 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.