Nanotyrannus: Beast of the Week

 This week we will be learning about a dinosaur who's identity has been the subject of a lot of debate or decades.  Check out Nanotyrannus!

Nanotyrannus was a meat-eating dinosaur that lived in what is now Montana in the United States during the latest Cretaceous period, between 67 and 66 million years ago.  From snout to tail it measured between 17 and 20 feet (6.2 meters) long as an adult. The genus name translates to "Small/Dwarf Tyrant" because it was originally believed to be a close relative of Tyrannosaurus rex, but much smaller.  

Watercolor life reconstruction of Nanotyrannus lancensus attacking a baby Tyrannosaurus rex by Christopher DiPiazza.

The first known Nanotyrannus bones were fond in the 1940s and it was initially thought to be a kind of Gorgosaurus. Then in the 1980s it was re-examined and thought by many paleontologists to be worthy of its own genus and was renamed Nanotyrannus.  Then in the 1990s it was suggested by some to be a juvenile Tyrannosaurus and the discovery of a new skeleton, nicknamed "Jane", further strengthened that stance, since that skeleton was indeed of a juvene when it died.  It also exhibited the typical traits paleontologists already knew for sure juvenile tyrannosauroids had, thanks to confirmed juvenile specimens of other taxa, like Albertosaurus.  These traits include proportionally longer arms and legs, which experts suggested allowed younger individuals to fill a different ecological niche, specializing in running down smaller faster prey, before bulking into mature adults.  For decades there was a division among experts on this possibility.  Then finally much more recently in 2025, a formal paper on an extremely complete skeleton, nicknamed "Bloody Mary", deemed Nanotyrannus a valid taxon again.  "Bloody Mary" was not a juvenile, in fact when a cross section of one of their bones was examined, it was estimated they were in their twenties when they died, which is similar to the age of some of the largest adult Tyrannosaurus rex specimens when they died.  Based on this, plus a few other key differences in its skull and limb anatomy, it appeared that Nanotyrannus was indeed it's own kind of dinosaur, and not simply a young T. rex. (although Tyrannosaurus itself still likely looked similar to Nanotyrannus it when it was a juvenile)

Nanotyrannus lethaeus on display at the Burpee Museum in Illinois, USA.  This specimen was thought to be a juvenile Tyrannosaurus rex by many experts.

This latest paper also was able to identify that Nanotyrannus included two species.  Nanotyrannus lethaeus, which includes the skeleton, named "Jane", and was the slightly larger of the two.  Nanotyrannus lancensus was smaller, but had proportionally longer arms, which includes the newer "Bloody Mary" individual.  Another interesting point that the newest paper suggested, was that Nanotyrannus was much less closely related to the group that contains Tyrannosaurus and Gorgosaurus, called the tyrannosaurids, than previously believed.  Even those who thought it was a distinct taxon in the past thought it to be within the tyrannosaurid family.  Thanks to the "Boody Mary" skeleton, which preserved a lot of anatomy not known prior, the new paper suggests Nanotyrannus was outside tyrannosaurids, but still a tyrannosauroid, more similar to dinosaurs like Dryptosaurus, which also had proportionally longer arms, large hand claws, and lived during the late Cretaceous.

Nanotyrannus lancensus skull. (photo: James St. John)

As stated, Nanotyrannus had proportionally long and slender legs, implying it was a fast runner when alive, likely specializing in hunting smaller faster prey, while its much larger cousin, T. rex could take larger, more heavily armored prey.  That being said, juvenile Tyrannosaurus were still almost certainly similar in build to Nanotyrannus, so they would have likely competed with each other on that level at the very least.  

Close up of the skin preserved on Nanotyrannus lancensus' leg.

Nanotyrannus had a narrow, slender snout, filled with proportionally long, bladelike teeth, ideal for slashing meat, not for crushing like adult T. rex had.  Nanotyrannus also possessed little bony crests in front of its eyes, which may have had keratin growing over them in life, forming small display structures, which is a feature common in many tyrannosaurs.  As stated earlier it's arms were proportionally long for a tyrannosaur, and were tipped with two fingers on each hand, each with a hook-shaped claw.  One specimen preserves what appears to be scaly skin on it's leg, which anyone would have assumed it had there, but is still really exciting to have proof of.  It probably also had feathers of some kind on other parts of its body, however. 

That is all for this week!  Comment below!

References

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

Carr, T.D. (1999). "Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria)". Journal of Vertebrate Paleontology. 19 (3): 497–520.

Eberth, David A.; Currie, Philip J. (2010). "Stratigraphy, sedimentology, and taphonomy of the Albertosaurus bonebed (upper Horseshoe Canyon Formation; Maastrichtian), southern Alberta, Canada". Canadian Journal of Earth Sciences. 47 (9): 1119–1143.

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

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.

Larson, P. (2013). The validity of Nanotyrannus lancensis (Theropoda, Lancian - Upper Maastrichtian of North America (PDF). Society of Vertebrate Paleontology 73rd Annual Meeting. p. 159.

Zanno, Lindsay E.; Napoli, James G. (2025-10-30). "Nanotyrannus and Tyrannosaurus coexisted at the close of the Cretaceous". Nature. doi:10.1038/s41586-025-09801-6.