Nanotyrannus is a genus of tyrannosauroid dinosaur that lived in what is now western North America during the Maastrichtian age of the Late Cretaceous, 67 to 66 million years ago. Its fossils are known from the Hell Creek Formation. It was one of the last-known non-avian dinosaurs and lived until the Cretaceous–Paleogene extinction event. The first named species, N. lancensis, was described as a new species of Gorgosaurus in 1946 by Charles W. Gilmore based on a single skull. Re-examination of the specimen in 1988 by Robert T. Bakker, Michael Williams and Philip J. Currie moved the species to a new genus of tyrannosaurid, named Nanotyrannus in reference to its small body size compared to other tyrannosaurids.