Tyrannosaurus rex was one of the largest known theropod dinosaurs. It lived in what is now western North America in the latest Cretaceous Period (68-66 million years ago). Tyrannosaurus rex, often called T. rex because it is shorter, lived right up to the end of the Cretaceous Period, so was one of the last dinosaurs to have lived on earth. History: The very first Tyrannosaurus was found by the famous fossil collector, Barnum Brown in 1900 in Wyoming. Over the next five years, he collected four more skeletons, and Henry Osborn, from the American Museum of Natural History in New York, studied these skeletons and named Tyrannosaurus rex in 1905. The largest of Brown's skeletons can be seen on display At the American Museum, and another is on display at the Cleveland Museum of Natural History. Scientific name: Tyrannosaurus rex meaning tyrant lizard and king Characteristics: In real life, Tyrannosaurus, was large (at least 40 feet long).