Nanotyrannus

Nanotyrannus (/ˈnænoʊˌtaɪˌrænəs, -tɪ- /, NAN-o-ti-RAN-us) 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.

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.

Nanotyrannus was a mid-sized tyrannosauroid. The adult N. lancensis may have weighed nearly 704 kg (1,552 lb) based on the "Bloody Mary" specimen. N. lethaeus was larger, and mature individuals may have reached a body mass of about 1,200 kg (2,600 lb). The morphology of Nanotyrannus combines characteristics from more basal tyrannosauroids, such as Moros and Xiongguanlong, with more derived tyrannosaurids. Unlike Tyrannosaurus, Nanotyrannus had a slender, low skull, and small crests or horns immediately in front of its eyes. The muscle attachment sites at the back of its skull were large, suggesting that it could strongly flex its head from side to side. The teeth at the front of its upper jaw were not serrated, unlike many other tyrannosaurs. The rest of the teeth were serrated and compressed from side to side, a condition known as ziphodonty, which is not seen in tyrannosaurids. Nanotyrannus was longer-legged than contemporary taxa, suggesting that it was a pursuit predator, and had forelimbs which were both absolutely and proportionally larger than its relatives.

Nanotyrannus is the only definite representative of the clade Nanotyrannidae, though one phylogenetic hypothesis suggests close relations to Appalachiosaurus and Dryptosaurus within this clade, and that Nanotyrannus descended from a lineage of Appalachian tyrannosaurs which migrated to the other North American island continent, Laramidia, with the recession of the Western Interior Seaway.