tyrannosaurus, meaning 'tyrant lizard') is a genus of theropod dinosaur. the famous species tyrannosaurus rex ('rex' meaning 'king' in latin), commonly abbreviated to t. rex, is a fixture in popular culture around the world. it lived throughout what is now western north america. fossils of t. rex date to the last three million years of the cretaceous period, approximately 68 to 65 million years ago; it was among the last non-avian dinosaurs to exist prior to the cretaceousäìtertiary extinction