Nanotyrannus and Tyrannosaurus both were members of a lineage of meat-eating dinosaurs called tyrannosaurs, but were not the same genus, the researchers said. At the twilight of the age of dinosaurs, an agile and vicious predator named Nanotyrannus prowled western North America, resembling a smaller version of Tyrannosaurus - about a tenth the body mass - but with several key anatomical differences.