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 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, measuring around in length. The adult N. lancensis may have weighed nearly based on the "Bloody Mary" specimen. N. lethaeus was larger, and mature individuals may have reached a body mass of about . 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 , 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 family 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 , and tyrannus, meaning . The etymology has subsequently been clarified to be derived from the Greek words νάνος (nanos), meaning or , and τύραννος (tyrannos), meaning . 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
In 2001, a more complete juvenile tyrannosaur (nicknamed "Jane", accession number BMRP 2002.4.1), suggested to belong to the same species as the original Nanotyrannus specimen, was uncovered. This discovery prompted a conference on tyrannosaurs focused on the issues of the validity of Nanotyrannus, held at the Burpee Museum of Natural History in 2005. Several paleontologists who had previously published opinions that N. lancensis was a valid species, including Currie and Williams, saw the discovery of "Jane" as a confirmation that Nanotyrannus was, in fact, a juvenile T. rex.Currie, Henderson, Horner and Williams (2005). "On tyrannosaur teeth, tooth positions and the taxonomic status of Nanotyrannus lancensis." In "The origin, systematics, and paleobiology of Tyrannosauridae", a symposium hosted jointly by Burpee Museum of Natural History and Northern Illinois University.Henderson (2005). "Nano No More: The death of the pygmy tyrant." In "The origin, systematics, and paleobiology of Tyrannosauridae", a symposium hosted jointly by Burpee Museum of Natural History and Northern Illinois University. Peter Larson continued to support the hypothesis that Nanotyrannus lancensis was a separate but closely related species, and also argued that Stygivenator (LACM 28471), which is generally considered to be a juvenile of Tyrannosaurus rex, could be a younger specimen of Nanotyrannus.
In late 2011, news reports about a 2006 discovery of a new, virtually complete tyrannosaur specimen found alongside a ceratopsid were made. The specimens were studied by Robert Bakker and Peter Larson on-site, who identified the tyrannosaur as Nanotyrannus and the ceratopsian as a taxon possibly distinct from Triceratops. It was impossible to determine whether the tyrannosaur specimen, then nicknamed "Bloody Mary", was distinct from T. rex, as the specimen remained in private hands until 2020, when the ownership of the specimen was decided by the Montana Supreme Court to be given to the land-owners Mary Anne and Lige Murray, who agreed to sell the paired fossils to a U.S.-based museum. The fossil was acquired by the North Carolina Museum of Natural Sciences in 2020, after which the museum built an additional research lab space in which to prepare, study, and display the specimens. In 2025, paleontologists Lindsay Zanno and James Napoli published an initial description of the "Bloody Mary" specimen as part of an extensive revision of the genus Nanotyrannus. They identified this specimen as a skeletally mature individual of Nanotyrannus and provided several lines of evidence supporting the validity of this genus. They also described a second species, N. lethaeus, based on the Jane specimen due to notable differences between it and N. lancensis. The North Carolina Museum of Natural Sciences accessioned and nicknamed the Nanotyrannus and Triceratops specimens respectively as NCSM 40000 ("Manteo") and NCSM 40001 ("Murphy"), and Roberts et al. (2026) identified their locality of origin as the lower portion of the Hell Creek Formation, dated to approximately .
The specific name for the second species, lethaeus, comes from the Latin, and is the adjective of Lethe, referring to the river Lethe from Greco-Roman mythology, which flows through the underworld (often equated to hell). The name references both the Hell Creek Formation, where the specimen was found, and the mythological function of drinking from the Lethe: to forget a prior life and be reincarnated.
Validity
The primary difference that some scientists have used to argue for the validity of Nanotyrannus lancensis concerns the number of teeth. Specimens referred to N. lancensis have 15-16 teeth on each side of the maxilla (upper jaw) and 16-18 teeth on each side of the dentary (lower jaw), whereas adult T. rex specimens had 11–12 tooth positions in the upper jaw and 12–14 in the lower. The exact implications of this difference in tooth count have not been universally accepted, however. In his 1999 study of tyrannosaurid growth patterns, Carr showed that, in Gorgosaurus libratus, the number of teeth decreased as the animal grew, and he used this data to support the hypothesis that N. lancensis is simply a juvenile T. rex. Takanobu Tsujihi and colleagues, who studied growth in the related Tarbosaurus bataar, found little to no decrease in tooth count during growth, even though they had juvenile specimens much younger than the Nanotyrannus specimens. These researchers also noted, however, that both Tyrannosaurus and Gorgosaurus show significant differences in tooth count between individuals of the same age group, and that tooth count may vary on an individual basis unrelated to growth.
Larson has also contended that, along with skull features, Nanotyrannus can also be distinguished from Tyrannosaurus by proportionally larger hands with phalanges on the third metacarpal and in the furcula morphology. Another difference cited in support of the validity of N. lancensis is the presence of a small foramen on the quadratojugal. The holotypes of both Nanotyrannus species have this feature, which is also seen in the adult specimen of Daspletosaurus horneri and an isolated quadratojugal from Alberta, indicating that this feature is seen in other distantly-related tyrannosaurs.
A limb proportion analysis published in 2016 suggested that Nanotyrannus specimens have differing levels of cursoriality, cited as a potential difference between N. lancensis and T. rex. However, paleontologist Manabu Sakomoto commented that this conclusion may be impacted by low sample size, and the discrepancy does not necessarily reflect taxonomic distinction. A 2020 study by Holly N. Woodward and colleagues indicated that the specimens referred to Nanotyrannus are ontogenetically immature and found it probable that these specimens belonged to Tyrannosaurus rex. Yun (2015) claimed that all of the differences used to distinguish Nanotyrannus were individually or ontogenetically variable features, or the products of taphonomic distortion of the bones.
In 2024, Nicholas Longrich and Evan T. Saitta re-examined the holotype and referred specimens of Nanotyrannus. Based on several factors, including differences in morphology, ontogeny, and phylogeny, Longrich and Saitta suggested that Nanotyrannus is a distinct taxon that may fall outside of Tyrannosauridae, based on some of their phylogenetic analyses. Voris et al. (June 2025) suggested that the scoring of Nanotyrannus in a more basal position by Longrich & Saitta (2024) may have been caused by "scoring characteristics related to immaturity" and problematic data sets, supporting the previous interpretation that Nanotyrannus represents a juvenile T. rex. Supplementary Information Later in June 2025, Gregory S. Paul also supported a basal eutyrannosaurian (non-tyrannosaurid) position for Nanotyrannus, emphasizing the validity and distinction of this genus as part of his preferred "multiple small taxa hypothesis" (MSTH) for theropod diversity in the Hell Creek Formation. Based on the MSTH, Paul also argued for the validity of Stygivenator, a Hell Creek theropod known from a fragmentary skull, and claimed that N. lancensis has been treated as a wastebasket taxon that likely comprises fossils of multiple taxa.