New research has overturned decades of uncertainty by showing that Nanotyrannus was a fully grown predator, not a juvenile T. rex.
For many years, paleontologists have debated whether the single skull used to define the species Nanotyrannus belonged to a separate dinosaur or represented a young Tyrannosaurus rex. That question has now been resolved. A new study published in Science shows that Nanotyrannus was nearly fully mature and not a juvenile T. rex. The research also sheds light on how large tyrannosaurs grew so rapidly into massive apex predators.
A research team that included Dinosaur Institute Postdoctoral Fellow Dr. Zach Morris closely studied the Nanotyrannus holotype—the specimen used to establish the species—with particular attention to its throat bone. By analyzing microscopic details in this bone and comparing them with those of modern birds, crocodilians, and extinct dinosaurs—including the Dino Hall’s T. rex growth series—the team concluded that Nanotyrannus was a fully grown predator that occupied its own ecological niche. Although much smaller than an adult T. rex, it lived in a more diverse Late Cretaceous ecosystem than previously understood and likely competed with juvenile T. rex for prey.
“The identity of the holotype specimen was the key piece in this debate. Discovering that this small skull was actually fully grown shows definitively that it is different from Tyrannosaurus rex,” said Dr. Christopher Griffin, lead author and Assistant Professor of Geosciences at Princeton University.
Using Bone Microstructure to Reveal Age
In paleontology, studying thin slices of bone under a microscope can reveal an animal’s growth rate and stage of maturity, much like counting tree rings. Researchers usually rely on long bones such as ribs or femora for this work, but these are not always preserved. Because the Nanotyrannus holotype is mainly a skull with many air-filled cavities, its bones are typically unsuitable for such analysis. However, the hyoid—a throat bone that supports the tongue—offered a rare opportunity to evaluate the specimen’s maturity.
“When we started this project, it was unclear whether the hyoid preserved a record of a dinosaur’s growth. To be honest, we mostly accepted the hypothesis that Nanotyrannus was a juvenile T. rex, so we expected the microscopic bone structure or histology of the holotype would show this animal was still growing quickly,” said co-author Dr. Morris. “What we did not expect was to see it was nearing maturity with clear evidence of the cessation of growth!”
Validating the Hyoid as a Growth Indicator
Because the usefulness of hyoid bones for assessing growth had never been confirmed, the scientists first needed to test whether this bone type reliably preserves age-related information. Dr. Griffin organized a team to assemble a comparative dataset of hyoids from living lizards, crocodiles, birds, and extinct dinosaurs. “To show that hyoid microstructure would work to test maturity status in Nanotyrannus, we first had to compile strong support for this method across many groups of living reptiles and extinct dinosaurs,” said Dr. Griffin.
Dr. Morris led the sampling and study of juvenile and sub-adult specimens known as “Thomas” from NHM’s unique T. rex growth series. “The growth series in our Dino Hall was critical to demonstrating that the hyoid in Tyrannosaurus showed the same kind of growth record as long bones,” said Morris. “Having a growth series that had already been histologically analyzed meant that we could compare the growth record in the hyoid and the growth record in the long bones and see that they show consistent signals even in these uniquely giant predators.” These comparisons created solid benchmarks for distinguishing developmental differences between T. rex and Nanotyrannus.
“Our teenage Tyrannosaurus looks immature in both its limbs and its hyoid, while Thomas looks like a more mature, but still not quite adult animal. Amusingly enough, Thomas is not nearly as mature as the Nanotyrannus holotype, despite being much larger,” added Morris.
Balancing Conservation and Scientific Insight
The authors emphasize that determining whether a holotype represents a young or fully grown individual is essential, since misinterpretation can lead to confusion between developmental changes and evolutionary differences.
“So many techniques in modern paleontology require some degree of destructive analysis, and as a Curator, I’m always trying to strike a balance between conservation and discovery. We preserved the anatomical data by 3D scanning and molding and casting the hyoid, and there is still more of it for future analyses,” said senior author Dr. Caitlin Colleary of the Cleveland Museum of Natural History (and incidentally, a former undergraduate volunteer in the NHM Dinosaur Institute). “In this instance, it was totally worth it because we gained so much more than we lost.”
A New View of Tyrannosaur Diversity
The findings contribute to a growing body of evidence that Late Cretaceous North America supported several tyrannosaur species, rather than only T. rex before the end-Cretaceous extinction event. “It is remarkable that our study matches findings from other independent lines of evidence, including an analysis published last month, demonstrating that multiple species of tyrannosaurs lived alongside one another. It shows that we need to re-evaluate what we think these ecosystems looked like,” said Dr. Morris.
Dr. Morris is the inaugural Dinosaur Institute Postdoctoral Fellow, with research centered on how developmental changes influence evolutionary patterns and how skull anatomy transforms through the fossil record. “I am fascinated by the ways in which changes during development give rise to the skeletal features which distinguish dinosaurs, birds, crocodylians, and other vertebrates,” said Morris. “This project was an exciting collaboration to study developmental patterns in the fossil record directly.”
“Zach’s expertise in dinosaur growth and development, coupled with his histological skills, was a huge asset to this project. It’s another example of our NHMLAC Post-Docs conducting novel, ground-breaking research,” said Dr. Nate Smith, Gretchen Augustyn Director & Curator of the Dinosaur Institute. “This study also highlights the incredible potential of unique museum collections like our T. rex growth series, which not only inform the public but also provide rich ground for new scientific discoveries.”
Reference: “A diminutive tyrannosaur lived alongside Tyrannosaurus rex” by Christopher T. Griffin, Jeb Bugos, Ashley W. Poust, Zachary S. Morris, Riley S. Sombathy, Michael D. D’Emic, Patrick M. O’Connor, Holger Petermann, Matteo Fabbri and Caitlin Colleary, 4 December 2025, Science.
DOI: 10.1126/science.adx8706
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