Nobody saw T. rex alive, so its age comes from the rocks around its bones. Hell Creek ages come from volcanic ash: researchers recalibrated the argon-argon dating method and then dated ash collected in the Hell Creek area of Montana. In 2008 the recalibrated method gave 65.95 million years ago as the best date for the end of the Cretaceous, and with recalibrated dating, the date of the impact came out the same, within error limits, as the date of the extinction, precise to within 11,000 years. A giant asteroid hit Earth about 66 million years ago, and T. rex and the other big dinosaurs died out, so T. rex lived near the end of the time of the dinosaurs. Sue lived about 67 million years ago. T. rex lived in a warm, humid, almost subtropical world with lush plants.

T. rex did not come from nowhere: Moros, an early tyrannosaur, lived about 96 million years ago in what is now Utah and stood only three or four feet tall at the hip, about the size of a modern mule deer. The discovery of Moros narrowed a 70-million-year gap in the fossil record of tyrant dinosaurs in North America. Henry Fairfield Osborn was the scientist who first named and described T. rex, and Barnum Brown found two early skeletons, in 1900 and 1902; as of August 2004, more than 30 T. rex skeletons had been found, in both the United States and Canada. Birds are modern-day dinosaurs, and a pigeon is a distant relative of T. rex. Yutyrannus, a feathered tyrannosaur relative, was about 9 metres long and 1,400 kilograms and lived in a colder climate than T. rex, which may be why it had a feathery coat.

Records are slippery. Estimates for the length of the biggest T. rex run from about 40 to 46 feet, and a typical adult probably weighed between 5,000 and 7,000 kilograms. A grown T. rex could weigh about six tons, like an elephant, and it stood about 13 feet tall at the hips. Most T. rex were probably smaller than the record sizes. As of March 2019, a T. rex nicknamed Scotty, found in Canada, is the largest T. rex known so far, and it is also the oldest, having seemingly lived to its early 30s. Scotty weighed an estimated 19,400 to 19,600 pounds, but it beat Sue's weight estimate by only about 5 percent, which lies within the error of weight guesses for extinct animals. Scotty was found in 1991 in Saskatchewan, and its bones took about a decade to cut out of hard sandstone.

How many kinds of T. rex were there? In February 2022 a study argued that T. rex fossils form three species and named two new ones, T. regina and T. imperator. Many leading T. rex experts were not convinced by the three-species study. T. rex bodies varied from one animal to the next, with some lankier and some more robust, and T. rex teeth were thick and blunt, not thin flat blades like those of most other meat-eating dinosaurs. Over millions of years the T. rex snout grew narrower and its eyeballs bigger, which cleared its view straight ahead. Preserved skin patches from T. rex and relatives such as Albertosaurus, Daspletosaurus, Gorgosaurus and Tarbosaurus show small pebbly scales, so as of November 2026 most of T. rex was probably not feathered.

Scientists have argued over whether T. rex hunted its food or mostly ate animals that were already dead; Jack Horner argued that T. rex could have been a scavenger, pointing to the large smell centres in its brain. Against that view, one study suggests the vision of T. rex was too good for a pure scavenger, since a 2006 study found a binocular range of 55 degrees, wider than a modern hawk's. A healed bite wound in a plant-eater is read as a probable sign of predatory behavior, because the animal lived long enough after the attack for the bone to heal, and prey that got away with a wound that later healed show that tyrannosaurs hunted live animals, not only dead ones. T. rex had a large part of its brain for smell, which helped it find food from far away.

The bones of single animals add detail. T. rex is unique among reptiles in crushing bone like wolves and hyenas do, but without teeth that fit together tightly, and as of May 2017 its tooth tips reached 431,000 pounds per square inch. Holes in the lower jaw of Sue were once thought to be bite marks from another T. rex; a scientific paper argued that jaw holes like Sue's match a parasite infection that today harms birds, called trichomonosis. As of November 2026, a bone growth-ring study of the Jane specimen put her at 11 to 13 years old and about 21 feet long, which is why she is treated as a young animal. As of November 2026, Sue is one of the most complete T. rex skeletons ever found, about 90 percent complete. Sue's bones show that she was sick and hurt during her life, and scientists do not know if Sue was a girl or a boy.

Words to know

• scavenger: eats dead animals
• species: animal kind
• Cretaceous: final dinosaur age