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 mainly 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.
History and validity
Initial discovery
Nanotyrannus is based on CMNH 7541, a skull collected in 1942 by David Hosbrook Dunkle. It was described in 1946 by Charles W. Gilmore, who classified it as a new species in the tyrannosaurid genus Gorgosaurus, G. lancensis. The specific name, lancensis, references the Lance Formation, since Gilmore initially considered the Hell Creek Formation—where the holotype was found—to be a member of the Lance Formation, hence noted as "Hell Creek member", but the type locality has since been corrected as the Hell Creek Formation. In 1988, the specimen was re-described by Robert T. Bakker, Michael Williams, then the curator of paleontology at the Cleveland Museum of Natural History, and Phil Currie, where the original specimen was housed and is now on display. They named the new genus Nanotyrannus for the specimen, stating that the name derives from the Latin words nano, meaning 'small', and tyrannus, meaning 'tyrant'. The etymology has subsequently been clarified to be derived from the Greek words νάνος (nanos), meaning 'dwarf' or 'pygmy', and τύραννος (tyrannos), meaning 'king'. The initial research by Bakker and colleagues indicated that the skull bones were fused, therefore implying it represents an adult specimen. The possibility that the holotype represents a juvenile Tyrannosaurus rex was first suggested by Anatoly Konstantinovich Rozhdestvensky in 1965. Subsequent analysis of the specimen by Thomas Carr in 1999 indicated that it was immature, leading Carr and many other paleontologists to consider it a juvenile specimen of T. rex.
Later finds
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