Tyrannosaurus rex (2024)

T. rex was probably an active predator.

Besides fossilized stomach contents, teeth are our best clues to the diets of dinosaurs. The teeth of carnivorous animals are usually sharp, serrated dagger-like blades. These teeth help the animal both to kill the prey and to slice the meat before swallowing. T. rexdidn’t chop or grind its food; it swallowed chunks whole. In addition to its sharp, serrated teeth, the lower jaw of T. rexhad a joint midway through its length that may have helped absorb some of the shock generated by struggling prey.

In western North America.

Paleontologists have found most T. rexfossils in the Northwest, in states such as Montana and South Dakota. T. rexfossils have also been found in Alberta, Canada.

T.rexlived at the very end of the Late Cretaceous, which was about 90 to 66 million years ago.

How do we know? One way of dating fossils relies on their relative positions in the ground. When paleontologists dig deeper in sedimentary rock they are, in effect, looking back in time. As sediments carried by wind and water accumulate, they bury older layers—so the bottom layers in a geological sequence are usually the oldest and the top layers the youngest. To date rocks and fossils precisely, scientists can measure the amounts of particular radioactive elements—often radiocarbon or potassium—present to determine when a rock was formed, or when the animal died.

Probably not more than about 28 years.

Sometimes animal bones, like the trunks of trees, accumulate yearly growth rings. The number of rings tells scientists the age of a specimen; the width reveals whether the animal was still growing. But only certain bones preserve a complete set of growth rings; unfortunately, the large limb bones that survive best as fossils are not among them.

Growth rings indicate the T. rexgrew quickly, reaching adult size as a teenager—and that the animals died young. The oldest specimen analyzed was only 28 years old. The image below shows a thin section of a T. rexrib with annual growth lines 12 through 19.

Learn about how scientists study growth rings in bone.

Up to 40 feet in length and 12 feet in height.

Based on fossil specimens, scientists have determined that a Tyrannosaurus rexcould be up to 40 feet long and 12 feet high.

T. rex is estimated to have weighed between 11,000 and 15,500 pounds (5,000 and 7,000 kilograms) with skin and flesh on its huge bones. That's about as much as the largest African elephant.

Scientists continue to disagree about its speed —fast or slow?

Estimates of dinosaurs' speeds are usually based on fossil trackways, like the one shown here. The most reasonable interpretations, based on the best-preserved trackways, indicate a walking speed of 3 to 6 miles per hour. Running speeds are often based on the study of biomechanics. Researchers study the muscles and body design of large living animals—and the stress on the body caused by running. Applying this research to extinct animals, they think that T. rexwas a slow runner, achieving perhaps only about 10 miles (16 kilometers) per hour—about as fast as an average human runner. These experts say size slowed T. rexdown. But why is it so hard to be big and fast? Bones, muscles, and posture all play a part.

Learn more about the size and speed of T. rex.