Return of the Short (Tyrant) King: A New Paper by Dinosaur Institute Researcher Shows Nanotyrannus Was Not a Juvenile T. rex
With the help of our T. rex growth series, researchers shows throat bones accurately record maturity in dinosaurs, establishing Nanotyrannus as a distinct species
Published December 4, 2025
A new paper in Science shows that Nanotyrannus was indeed its own species and not just a young T. rex.
The decades-long debate ended when co-author and Postdoctoral Fellow at the Dinosaur Institute, Dr. Zach Morris, along with a team of researchers from across the country, went for the throat bone of the single skull used to establish Nanotyrannus as a species.
The team examined the throat bone’s microscopic structure, comparing it to distant living relatives and more closely related extinct dinosaurs—including the Dino Hall’s singular T. rex growth series—to show that Nanotyrannus, while smaller, was a fully grown and distinct predator in an ancient ecosystem more diverse than previously imagined. Slightly less than half the size of their massive adult cousins, Nanotyrannus likely competed with juvenile T. rex for prey in Late Cretaceous North America.
When a researcher describes a new species, they need to reference an actual specimen, and this is the holotype. For Nanotyrannus, this specimen is mostly a skull, as well as a throat bone that supports the tongue, or hyoid.
“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. The holotype “What we did not expect was to see it was nearing maturity with clear evidence of the cessation of growth.”
Dinosaur Growth Rings
Counting a tree’s growth rings reveals its age, and similarly, cross-sectioning a dinosaur’s bones can show how mature it is. Using powerful microscopes, Dr. Morris and his colleagues examined thin slices of fossil bone, measuring the organization of the tissues to gauge the age of the animal in life and understand how quickly it grew. Researchers typically use long bones, such as femora or ribs, but the Nanotyrannus holotype is mostly a skull, and skulls don’t preserve growth markers. However, the hyoid of the specimen was potentially able to answer this question once and for all.
Nobody knew if hyoid bones could preserve a useful record of an animal's growth, so the team needed to find out if the throat bone could be a reliable gauge for maturity. “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. Christopher Griffin, lead author and Assistant Professor of Geosciences at Princeton University. He put together a team of experts to build a database comparing hyoid bones across a range of species, including crocodiles, living lizards, birds, and extinct dinosaurs—like T. rex.
Having the juvenile and sub-adult “Thomas" in NHM’s one-of-a-kind T. rex growth series handy helped researchers distinguish Nanotyrannus. “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.” The T. rex growth series provided benchmarks to better understand and assess differences in growth 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.
The study highlights the importance of understanding the maturity of holotype specimens; otherwise, scientists might confuse evolutionary and developmental 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."
“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.
A New Post Doc at the Dino Institute
Dr. Morris is the inaugural Dinosaur Institute Postdoctoral Fellow, where his research focuses on investigating the developmental origins of evolutionary patterns and using museum collections to understand how the skull changes in 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.”
