Tyrannosaurus rex is a member of the Family Tyrannosauridae, which includes other fearsome theropod predators such as Albertosaurus, Daspletosaurus, Gorgosaurus, and Tarbosaurus. This group of dinosaurs lived near the very end of the Cretaceous Period (140 to 65 million years ago), and seem to have replaced some of the other large carnivorous dinosaurs as the main predators of the time.
Tyrannosaurus rex was first discovered in Montana in 1902 by Barnum Brown, and was described and named in 1905 by Henry Fairfield Osborn, the director of the American Museum of Natural History, and Brown's boss. The name Tyrannosaurus rex means "tyrant lizard king," and seemed very appropriate for this large (12 meters or 39 feet long, up to 6 meters or 20 feet tall at the shoulder, and weighing over 7,000 kg or 16,000 pounds) carnivorous dinosaur. In fact, before the discovery of the first T. rex skeleton, vicious-looking teeth had been found for years across the badlands of Montana and Wyoming. In the 1850's, Joseph Leidy, a pioneering paleontologist who named and described many early dinosaur finds in North America, described the teeth as Deinodon horridus, meaning "horrifying terrible tooth".
For the first fifty years after its discovery, no-one doubted the fearsome nature of Tyrannosaurus rex; but in the 1960's, scientists began to wonder, just how good of a predator was this large beast? They began to look at the legs of T. rex, how fast could it really run? And the comparatively tiny forearms, were they really useful in catching fast-moving prey?
Paleontologist Jack Horner upset T. rex fans everywhere with his statement that Tyrannosaurus rex was a scavenger. His reasons? Well, T. rex could not run terribly fast, so it would have difficulty in catching prey items, and those tiny forearms did not have much muscle to them, making it difficult to subdue a prey item that it did manage to catch. Horner also examined the skull of T. rex and found that the nasal region contained structures that would have given it an excellent sense of smell. Predatory animaly generally do not need a great sense of smell, often they hunt by sight and sound of their prey, but a scavenger looking for dead animals needs to be able to follow a scent trail to its source.
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