Tyrannosaurus rex was a very large theropod dinosaur that lived in what is now western North America in the Late Cretaceous Period (68-66 million years ago). It went extinct with the last of the dinosaurs at the end of the Cretaceous Period. Characteristics: Tyrannosaurus, at least 40 feet long, and 5-6 metric tons in weight, was top of the food chain in the Late Cretaceous. Its huge jaws contained lots of dagger-like, six inch teeth that it used to slice apart prey.