Tyrannosaurus means "tyrant lizard" because of its immense size and evil teeth and claws. You can see a real Tyrannosaurus rex tooth on display in the "What If" exhibit. This meat-eater grew up to be forty-six feet long, eighteen feet tall, and weighed six tons. It also had a huge head that measured four feet in length. In its jaws were teeth six inches long! Tyrannosaurus rex was one of the most powerful carnivores ever to walk the planet and also one of the most highly evolved theropods known. Its very small forearms probably were used to help the animal push off the ground to stand up. Tyrannosaurus rex lived during the end of the Cretaceous time period and was found in Western Canada and the Western United States.