Open Paleo

Nanotyrannus

Bakker et al., 1988 NAN-oh-tye-RAN-us /ˌnænoʊˈtaɪrænəs/ Disputed

Originally proposed by Bakker, Williams and Currie (1988) as Latin nano ("small") plus tyrannus ("tyrant"); the derivation is now usually corrected to Greek nanos ("dwarf" or "pygmy") and tyrannos ("king"), in reference to the small body size relative to other tyrannosauroids.

72.2–66.0 million years ago Maastrichtian, Late Cretaceous
Late Cretaceous stages

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.

Under dispute

The validity of Nanotyrannus is actively contested. After the 1988 erection of the genus, Carr (1999) and Woodward and colleagues (2020) argued that the holotype CMNH 7541 is an immature Tyrannosaurus rex, citing tooth-grain immaturity and femoral osteohistology. Longrich and Saitta (2024), Zanno and Napoli (2025) — who erected a second species, N. lethaeus — and Griffin and colleagues (2025, ceratobranchial histology) reaffirmed Nanotyrannus as a distinct, skeletally mature taxon. No broad consensus yet.

Diagnosis

  1. Muzzle width greatly constricted, only one fourth the width of the temporal region.
  2. Extraordinarily wide basicranium between basitubera and basipterygoid processes.
  3. Two large pneumatic foramina placed one behind the other near the midline of the basisphenoid.
  4. Dorsal surface of the nasal lacks strong ridges and striae.
  5. Parietal wing of the occiput with a sharp angle between dorsal and lateral edges.
  6. Basicranial boxwork with flat ventral floor.

Species (2)

Nanotyrannus lancensis (Gilmore, 1946) Disputed

Type species

First named as Gorgosaurus lancensis

From Latin lancensis ("of the Lance"), referring to the Lance Formation — Gilmore's (1946) original (and mistaken) attribution of the holotype's geological context; the actual horizon is the Hell Creek Formation.

Holotype
CMNH 7541 , Cleveland Museum of Natural History
Material
A nearly complete skull with tightly articulated lower jaws (~57 cm premaxilla-to-quadrate); the right squamosal, postfrontal, quadratojugal and upper half of the right quadrate are missing, and many teeth are incomplete.
Horizon
Hell Creek Formation, Maastrichtian, Late Cretaceous , 72.2–66.0 million years ago
Locality
Sand Creek, Montana, United States

Synonyms

  • Gorgosaurus lancensis reassigned

    Original combination established by Gilmore (1946) for CMNH 7541, based on apparent affinities with G. sternbergi and G. libratus. Bakker, Williams and Currie (1988) reassigned the species to a new genus Nanotyrannus, yielding Nanotyrannus lancensis (Gilmore, 1946).

  • Larsonvenator elegans junior subjective

    Named by Sancarlo and Paul (2026) as a new genus and species on NCSM 40000, which Zanno and Napoli (2025) referred to N. lancensis. Sancarlo and Paul hold that its low, long-snouted skull and high tooth count fit no previously named taxon.

Nanotyrannus lethaeus Zanno & Napoli, 2025

From Latin Lethaeus, the adjective of Lethe (the mythological river of forgetfulness flowing through the Greco-Roman underworld); the name references both the Hell Creek Formation and the river's mythological role.

Holotype
BMRP 2002.4.1 , Burpee Museum of Natural History
Material
Partial skeleton ("Jane") including skull, axial column, pelvic and hindlimb elements; estimated body mass at death ~819 kg, asymptotic adult mass modeled at ~1,264 kg.
Horizon
Hell Creek Formation, Maastrichtian, Late Cretaceous , 72.2–66.0 million years ago
Locality
Montana, United States

Synonyms

  • Gilmoretyrannus lethaeus reassigned

    Combination proposed by Sancarlo and Paul (2026), who erected the new genus Gilmoretyrannus for the species on the grounds that BMRP 2002.4.1 differs from the Nanotyrannus holotype CMNH 7541 more than species within other tyrannosaur genera differ from one another.

Synonyms of the genus

Name Type Reason
Clevelanotyrannus nomen nudum Unpublished manuscript name for CMNH 7541 that predated the formal description; the published 1988 paper (Bakker, Williams and Currie) used Nanotyrannus instead, so Clevelanotyrannus was never validly published.

Notable specimens

  • “Bloody Mary”

    NCSM 40000 North Carolina Museum of Natural Sciences Found 2006 iconic

    Virtually complete Hell Creek skeleton, the most complete known specimen of the genus and part of the Dueling Dinosaurs. Histology shows 25 cyclical growth marks and skeletal maturity, with death at age 17-22, central evidence that Nanotyrannus is a valid, mature taxon distinct from Tyrannosaurus.

    Zanno & Napoli (2025)

References

  1. Bakker, R. T.; Williams, M.; Currie, P. J. (1988). Nanotyrannus, a new genus of pygmy tyrannosaur, from the latest Cretaceous of Montana. Hunteria 1(5): 1-30.

    Erects Nanotyrannus lancensis on CMNH 7541 (transferred from Gorgosaurus lancensis Gilmore 1946); diagnosed on muzzle constriction, ziphodont maxillary teeth, and basicranial pneumatic pattern.

    bakker1988b
  2. Gilmore, C. W. (1946). A new carnivorous dinosaur from the Lance Formation of Montana. Smithsonian Miscellaneous Collections 106(13): 1-19.

    Original description of CMNH 7541 as Gorgosaurus lancensis on a nearly complete skull collected by D.H. Dunkle in 1942 from Carter County, Montana; published posthumously.

    gilmore1946a
  3. Carr, T. D. (1999). Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria). Journal of Vertebrate Paleontology 19(3): 497-520. doi:10.1080/02724634.1999.10011161

    Anti-validity: "I thus consider Nanotyrannus lancensis Bakker et al., 1988 a subjective junior synonym of Tyrannosaurus rex." Cites immature bone grain + 13 T. rex synapomorphies in CMNH 7541.

    carr1999a
  4. Woodward, H. N.; Tremaine, K.; Williams, S. A.; Zanno, L. E.; Horner, J. R.; Myhrvold, N. (2020). Growing up Tyrannosaurus rex: Osteohistology refutes the pygmy 'Nanotyrannus' and supports ontogenetic niche partitioning in juvenile Tyrannosaurus. Science Advances 6(1): eaax6250. doi:10.1126/sciadv.aax6250

    Anti-validity histology: femur/tibia osteohistology of BMRP 2002.4.1 and 2006.4.4 shows woven-parallel cortex to periosteum with no EFS, interpreted as skeletally immature juvenile T. rex.

    woodward2020a
  5. Longrich, N. R.; Saitta, E. T. (2024). Taxonomic Status of Nanotyrannus lancensis (Dinosauria: Tyrannosauroidea) — A Distinct Taxon of Small-Bodied Tyrannosaur. Fossil Studies 2(1): 1-65. doi:10.3390/fossils2010001

    Pro-validity: distinct taxon differing from T. rex in >150 characters; histology interpreted as mature subadults; phylogenetic analyses suggest Nanotyrannus may lie outside Tyrannosauridae.

    longrich2024d
  6. Zanno, L. E.; Napoli, J. G. (2025). Nanotyrannus and Tyrannosaurus coexisted at the close of the Cretaceous. Nature 648: 357-367. doi:10.1038/s41586-025-09801-6

    Pro-validity. NCSM 40000 ("Bloody Mary") interpreted as mature N. lancensis (~700 kg); erects N. lethaeus on BMRP 2002.4.1 ("Jane"); erects Nanotyrannidae.

    zanno2025a
  7. Voris, J. T.; Zelenitsky, D. K.; Kobayashi, Y.; Modesto, S. P.; Therrien, F.; Tsutsumi, H.; Chinzorig, T.; Tsogtbaatar, K. (2025). A new Mongolian tyrannosauroid and the evolution of Eutyrannosauria. Nature 642(8069): 973-979. Springer Science and Business Media LLC. doi:10.1038/s41586-025-08964-6

    Excludes N. lancensis from its phylogenetic analysis as a "problematic" taxon.

    voris2025a
  8. Paul, G. S. (2025). A presentation of the current data on the exceptionally diverse non-tyrannosaurid eutyrannosaur and Tyrannosaurini genera and species of western North America during the end Cretaceous North American interchange. Mesozoic 2(2): 85-138. doi:10.11646/mesozoic.2.2.1

    Pro-validity under the Multiple Small Theropod Taxa Hypothesis; treats Hell Creek small graciles as multiple non-tyrannosaurid eutyrannosaurs; provisional diagnoses for Nanotyrannus and Stygivenator.

    paul2025b
  9. Griffin, C. T.; Bugos, J.; Poust, A. W.; Morris, Z. S.; Sombathy, R. S.; D'Emic, M. D.; O'Connor, P. M.; Petermann, H.; Fabbri, M.; Colleary, C. (2025). A diminutive tyrannosaur lived alongside Tyrannosaurus rex. Science. doi:10.1126/science.adx8706

    Pro-validity: ceratobranchial I histology of CMNH 7541 indicates the specimen was nearing or had reached skeletal maturity, supporting taxonomic distinctness from coeval Tyrannosaurus rex.

    griffin2025a
  10. Woodward, H. N.; Myhrvold, N. P.; Horner, J. R. (2026). Prolonged growth and extended subadult development in the Tyrannosaurus rex species complex revealed by expanded histological sampling and statistical modeling. PeerJ. doi:10.7717/peerj.20469

    BMRP 2002.4.1 and 2006.4.4 "do not fit with other Tyrannosaurus specimens"; authors note data "do not directly bear on the Nanotyrannus or multi-taxic proposals" — agnostic but compatible.

    woodward2026a
  11. Sancarlo, F.; Paul, G. S. (2026). The taxonomics of the lithe basal eutyrannosaur genera and species of late Maastrichtian western North America. ZooNova 54: 1-65. Afriherp Communications. doi:10.5281/zenodo.22944014

    Proposes Gilmoretyrannus for N. lethaeus (BMRP 2002.4.1) and Larsonvenator elegans for NCSM 40000; its own analysis recovers both outside Tyrannosauridae with CMNH 7541.

    sancarlo2026a