Tyrannosaurus rex was one of the largest theropod dinosaurs that ever existed. Its scientific name, Tyrannosaurus (‘tyrant lizard’) rex (‘king’) says it all. 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 so is one of the last dinosaurs to have lived on earth.