Was Nanotyrannus Real? The Ongoing Debate about a Small Tyrannosaur

Reconstruction of Nanotyrannus lancensis (Image source：Nobu Tamura， CC BY-SA 4.0 )

Nanotyrannus lancensis, named in 1988, is a tyrannosauroid that appears noticeably smaller than Tyrannosaurus. This size difference is the reason for the name “dwarf tyrant.” Its body length is estimated at slightly over five meters, with a body mass roughly between one and two tonnes.

Despite clear differences in size and certain anatomical details between Nanotyrannus and Tyrannosaurus, the debate over whether Nanotyrannus represents a valid taxon has never truly disappeared. The main reason for this controversy lies in the fact that fossils assigned to Nanotyrannus come from exactly the same formations and time period as Tyrannosaurus. Furthermore, every known specimen attributed to Nanotyrannus appears to represent a subadult individual. In contrast, specimens of juvenile or subadult Tyrannosaurus are relatively scarce.

Because of this, many paleontologists have proposed that Nanotyrannus may simply represent immature individuals of Tyrannosaurus. Morphological differences between the two could then be explained by individual variation or by changes occurring during growth. In 2020, new research further strengthened this interpretation, leading many researchers to consider Nanotyrannus an invalid taxon. Under that view, fossils previously assigned to it are interpreted as subadult specimens of Tyrannosaurus.

However, this interpretation introduces another problem: the dramatic difference in body size between the two. Imagine a person who stands only 80 centimeters tall throughout middle and high school but suddenly grows to 170 centimeters upon entering university. Such an abrupt change would appear rather unusual.

Ecological niche separation

To explain this phenomenon, researchers have proposed a hypothesis that combines the above observations with broader sampling patterns of Late Cretaceous carnivorous dinosaurs. The conclusion suggests that medium-sized carnivorous dinosaurs were generally uncommon during the Late Cretaceous. One possible explanation is that subadult individuals of large carnivorous species occupied the ecological niches that would otherwise have been filled by medium-sized predators.

This ecological arrangement could reduce competition between juvenile and adult individuals of the same species while also limiting pressure from other predatory groups.

Subadult Tyrannosaurus provides a classic example. These younger animals differed dramatically in size from full-grown adults. Their bodies were more lightly built, and they were likely much faster runners. Such characteristics would have allowed them to pursue smaller, more agile prey than those typically targeted by massive adult individuals.

A reversal of interpretation?

Since the debate has never truly ended, some researchers continue to argue that Nanotyrannus should not be regarded as a juvenile Tyrannosaurus. In 2022, one study suggested that Nanotyrannus might instead be more closely related to Dryptosaurus, a tyrannosauroid from eastern North America. However, Dryptosaurus itself does not belong to Tyrannosauridae but instead lies within the broader superfamily Tyrannosauroidea.

Because that study offered relatively limited anatomical description and its analysis was considered insufficiently rigorous, it did not attract much attention at the time. Interest returned in early January of this year, when a new research paper on Nanotyrannus was published. This study reignited discussion about the true identity of the animal.

Six lines of evidence

In the new paper, the research team evaluated whether Nanotyrannus truly belongs to Tyrannosaurus by examining six separate lines of evidence.

1. Diversity of carnivorous dinosaurs

Even within Tyrannosauridae, multiple species are often found coexisting within the same ecosystem. For example, in southern Canada during the Late Cretaceous, the smaller Gorgosaurus lived alongside the larger Daspletosaurus. In Mongolia, the slender Alioramus coexisted with the larger Tarbosaurus. Similar patterns also occur among other groups of carnivorous dinosaurs such as abelisaurids.

Another pattern seen among predators is that species diversity often increases as body size decreases. Smaller-bodied predators tend to exist in greater numbers of species than large predators. If this rule also applies to tyrannosaurs, one might expect smaller species to be more numerous than giant ones.

Researchers also pointed out that both ancient and modern ecosystems rarely contain only a single apex predator. If Tyrannosaurus truly were the sole top predator of its ecosystem, that situation would be highly unusual.

For these reasons, the first line of evidence supports the idea that Nanotyrannus represents a distinct taxon.

2. Morphological differences between Nanotyrannus and Tyrannosaurus

Previous studies combined with new observations produced a detailed list of anatomical differences separating the two animals. In total, 158 diagnostic characters were identified. These distinguishing traits occur across nearly every region of the skull.

If Nanotyrannus truly represented juvenile Tyrannosaurus, one would expect fossils showing intermediate forms between the two. Yet no such transitional specimens have been discovered. Instead, the traits appear discrete rather than continuous.

This second line of evidence therefore also supports the independence of Nanotyrannus.

3. Growth patterns of closely related species

Another possibility is that intermediate forms simply have not yet been discovered. To evaluate this, researchers compared the growth stages of related tyrannosaur species.

Juvenile fossils of Tarbosaurus were compared with adult Tarbosaurus, Nanotyrannus, and adult Tyrannosaurus. The skull morphology of young Tarbosaurus differed significantly from Nanotyrannus but resembled adult Tyrannosaurus and adult Tarbosaurus more closely.

Similarly, juvenile and adult Gorgosaurus show comparable skull morphology despite differences in size.

This third line of evidence again supports the interpretation that Nanotyrannus is distinct.

4. Bone tissue structure

If Nanotyrannus were merely juvenile Tyrannosaurus, its fossils should consistently display immature skeletal characteristics.

One indicator of maturity is the degree of bone fusion. In general, greater fusion suggests a more mature individual. In known Nanotyrannus specimens, most skeletal elements are already fused.

Another indicator is the presence of an external fundamental system (EFS), a thin outer bone layer that forms when growth has essentially stopped. Three Nanotyrannus specimens lack an EFS. This shows they were not extremely old individuals, but it does not rule out the possibility that they were young adults. In fact, many dinosaur fossils never show an EFS; for example, all known Gorgosaurus specimens lack this structure as well.

Growth rings within bones can also be used to reconstruct growth curves. These rings reveal that Nanotyrannus specimens had already entered a phase of slowing growth. Even so, their body size remained more than five times smaller than that of a typical adult Tyrannosaurus.

Thus, apart from the absence of an EFS, the other skeletal evidence supports the idea that Nanotyrannus represents a distinct animal.

5. True juvenile Tyrannosaurus

If Nanotyrannus represented subadult Tyrannosaurus, fossils of genuine juvenile Tyrannosaurus should not exist separately. However, fossils that clearly resemble juvenile Tyrannosaurus have been discovered, some with body sizes comparable to those of Nanotyrannus.

This indicates that Nanotyrannus cannot simply be a young Tyrannosaurus.

Therefore, the fifth line of evidence also supports its independence.

6. Phylogenetic analysis

Finally, researchers conducted a phylogenetic analysis using fossil data attributed to Nanotyrannus. The results suggested that Nanotyrannus lies outside Tyrannosauridae, though still within the broader group Tyrannosauroidea.

Naturally, the sixth line of evidence again supports Nanotyrannus as a separate taxon.

Conclusion

All six lines of investigation indicate that Nanotyrannus does not belong within Tyrannosaurus. Each line of evidence individually may contain some uncertainty. Yet when all six independently point toward the same conclusion, the overall case becomes considerably stronger.

Based on current evidence, approximately eleven known specimens appear to belong either to Nanotyrannus or to close relatives. Exactly how many species or evolutionary lineages these fossils represent remains unclear and will require further research.

The authors of the study conclude with a striking observation: if even an animal as famous and intensively studied as Tyrannosaurus can still hold unresolved mysteries, then there must remain countless discoveries waiting to be made.

Author: Bai Leng

Reference:

Longrich, N. R., Saitta, E. T. (2024). Taxonomic Status of Nanotyrannus lancensis (Dinosauria: Tyrannosauroidea)—A Distinct Taxon of Small-Bodied Tyrannosaur. Fossil Studies.

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