Nanotyrannus

Nanotyrannus (/ˈnænoʊˌtaɪˌrænəs, -tɪ- /, ) is a genus of tyrannosauroid dinosaur that lived in what is now western North America during the Maastrichtian age of the Late Cretaceous, . 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.

Subsequent research indicated that the skull belonged to an immature animal, leading many researchers to favor its identification as a juvenile Tyrannosaurus rex. Its taxonomic status has since been a subject of intense scientific debate. In 2025, Lindsay Zanno and James Napoli published an exhaustive revision of Nanotyrannus. Therein, they described a complete tyrannosauroid skeleton from the Hell Creek Formation, also known by its nicknames "Bloody Mary" and "Manteo" (part of the Dueling Dinosaurs specimen), which they demonstrated to be a mature N. lancensis individual. These researchers named a second species, N. lethaeus, based on the "Jane" specimen. An independent study by Griffin and colleagues published later that year reached similar conclusions regarding the validity of Nanotyrannus, as histology of the hyoids indicates the N. lancensis holotype belongs to a mature individual.