Tyrannosaurus Rex by Safari LtdTyrannosaurus 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 became extinct with the last of the dinosaurs at the end of the Cretaceous Period. Scientific name: Tyrannosaurus rex meaning tyrant lizard and king 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