The strongest current evidence supports treating Nanotyrannus as distinct from juvenile Tyrannosaurus rex, but it does not settle the identity of every small tyrannosaur fossil. A 2025 study examined the Nanotyrannus holotype’s throat bone and reported evidence of skeletal maturity; a separate 2025 study described a near-mature skeleton that its authors identified as Nanotyrannus.
What is Nanotyrannus?
Nanotyrannus lancensis is the name given to a small-bodied tyrannosaur from Montana’s Hell Creek Formation. The name-bearing specimen, CMNH 7541, is a skull with associated hyoid material. The Cleveland Museum of Natural History reports that its field team discovered the fossil in 1942. Charles Gilmore assigned it to Gorgosaurus in 1945; researchers named it Nanotyrannus lancensis in 1988. See the museum’s history of the specimen.
The central question is whether the holotype represents a small-bodied tyrannosaur or a growing T. rex. That question is distinct from whether other small tyrannosaur fossils, such as Jane, belong to the same species.
Why scientists considered it a juvenile T. rex
Young tyrannosaurs could differ in proportions from adults, with more lightly built skulls and longer legs. This makes it difficult to tell whether a feature distinguishes one species from another or reflects growth within a species. The Cleveland Museum notes that many experts argued the Cleveland skull was a juvenile T. rex.
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A 2020 bone-histology study of Jane and the Cleveland specimen found that the sampled animals were immature and argued that Nanotyrannus was a juvenile T. rex. But immaturity by itself does not establish an animal’s species. The microscopic structure of bone can help indicate growth stage; classification also depends on anatomy and other evidence.
What the 2025 studies reported
Two 2025 studies used different specimens and methods. One examined the Cleveland holotype’s hyoid, or throat bone, to assess its maturity. The other described NCSM 40000, a near-mature tyrannosaur skeleton from the Hell Creek Formation, and assessed its anatomy and classification.
| Study | Specimen and evidence | What the study reported |
|---|---|---|
| 2025 Science study | The Cleveland holotype, CMNH 7541; microscopic examination of its hyoid bone. | The study reported an external fundamental system in the hyoid, interpreted as evidence that the animal was skeletally mature. |
| 2025 Nature study | NCSM 40000, a near-mature Hell Creek skeleton; comparative anatomy, growth models and phylogenetic analysis. | Its authors concluded that Nanotyrannus is distinguishable from Tyrannosaurus and proposed two species, N. lancensis and N. lethaeus. |
The holotype’s hyoid
The hyoid is part of the bone structure that supported the tongue and throat. Because the Cleveland holotype does not provide the kind of complete long-bone sample commonly used in growth studies, the research team investigated whether hyoid microstructure could help assess maturity. The Cleveland Museum says the researchers compared hyoids from living birds and crocodilians and extinct dinosaurs, including T. rex, Coelophysis and Allosaurus.
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The study reported an external fundamental system in the holotype’s hyoid. The Cleveland Museum describes this as evidence that the individual was skeletally mature at death. This finding directly concerns the specimen that anchors the name Nanotyrannus lancensis; it does not by itself classify every small tyrannosaur fossil. Read the museum’s study summary.
The near-mature NCSM 40000 skeleton
The 2025 Nature paper examined NCSM 40000, a near-somatically mature skeleton from the Hell Creek Formation. Its authors used comparative anatomy, growth models and phylogenetic analysis to argue that the specimen shares features with the Nanotyrannus holotype and is distinguishable from Tyrannosaurus. They proposed N. lancensis and N. lethaeus, assigning Jane to the latter species.
Those are the authors’ conclusions, not uncontested findings about every specimen. PubMed records a correction to the paper published in 2026, but the correction’s substantive effect has not been established in the research reviewed for this article. Accordingly, the paper’s claims should be attributed to its authors rather than presented as unqualified current consensus. The original article is available at Nature.
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What this evidence does—and does not—show
The hyoid study addresses the maturity of the holotype. The Nature study addresses the anatomy and classification of a different, near-mature skeleton. These lines of evidence support the distinct-Nanotyrannus interpretation in different ways, but they do not automatically resolve the identity of all small tyrannosaur fossils.
- Holotype, CMNH 7541: The 2025 Science study reported hyoid evidence of skeletal maturity.
- NCSM 40000: The 2025 Nature paper’s authors identified it as a near-mature Nanotyrannus and argued that the genus is distinguishable from Tyrannosaurus.
- Other small fossils: Their maturity and species identity require specimen-specific evidence.
What about Jane?
Jane, specimen BMRP 2002.4.1, is a separate small tyrannosaur skeleton. The hyoid finding concerns the Cleveland holotype, not Jane. The authors of the 2025 Nature paper classified Jane as Nanotyrannus lethaeus; whether Jane belongs to that proposed species or represents a juvenile T. rex remains a separate classification question.
How big was Nanotyrannus?
The Natural History Museum of Los Angeles County estimated that Nanotyrannus was roughly 17 feet long and around 1,500 pounds at maturity. These are the museum’s estimates, not direct measurements of a complete skeleton assigned beyond dispute to the species. The museum also reported eight growth rings in the holotype compared with about two in the teenage T. rex used for comparison. These are study-specific observations, not general statistics for tyrannosaurs.
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What remains unresolved?
The evidence discussed here supports distinguishing the holotype from a very young T. rex, while taxonomic assignments beyond the holotype remain more specific questions. The 2025 Nature authors proposed two species, but the paper’s 2026 correction must be reviewed before its claims are restated without qualification. The genus name and the classification of individual small fossils should therefore be treated carefully.
FAQ
Was Nanotyrannus a juvenile T. rex?
The 2025 Science study reported that the Cleveland holotype’s hyoid showed an external fundamental system, interpreted as evidence of skeletal maturity. That supports the view that the holotype was not simply a very young T. rex. It does not establish the identity of every small tyrannosaur fossil.
What is the difference between the holotype and Jane?
The holotype, CMNH 7541, anchors the name Nanotyrannus lancensis and was the specimen examined in the hyoid study. Jane, BMRP 2002.4.1, is a separate skeleton. The 2025 Nature authors proposed classifying Jane as N. lethaeus.
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Does skeletal maturity mean the animal had reached its absolute maximum size?
Not necessarily. The study’s conclusion concerns skeletal maturity as indicated by bone microstructure; it should not be expanded into a claim that every aspect of body size or development had stopped changing.
Did the 2025 Nature study settle the debate?
Its authors argued that Nanotyrannus is distinguishable from Tyrannosaurus and proposed two species. A correction was published in 2026, but its substantive effect has not been established in the reviewed research. The study’s conclusions should be attributed to its authors, and other specimens should be assessed separately.
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