Tyrannosaurus rex was one of the largest theropod dinosaurs that ever existed. Its scientific name, Tyrannosaurus (‘tyrant lizard’) rex (‘king’) says it all- it was one mean, lean biting machine! It lived in what is now western North America in the latest Cretaceous (68-66 million years ago). T. rex went extinct at the very end of the Cretaceous, and was one of the last dinosaurs to have lived on earth.