Return of the short (tyrant) king: A new paper by Dinosaur Institute researcher shows Nanotyrannus was not a juvenile T. Rex
Dinosaur Institute researcher shows throat bones accurately record maturity in dinosaurs, establishing Nanotyrannus was a fully grown, entirely distinct species from T. rex
Natural History Museum of Los Angeles County
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A Late Cretaceous face-off between an adult Nanotyrannus (left) and two juvenile T. rex, with a sub-adult T. rex watching from a distance. The scene evokes a preface to the NHMLAC’s famous T. rex trio on display in the Jane G. Pisano Dinosaur Hall. Artwork by Jorge Gonzalez.
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Credit: Artwork by Jorge Gonzalez.
Los Angeles, CA (December 4, 2025)—For decades, paleontologists argued over the lone skull used to establish the distinct species Nanotyrannus. Was it truly a separate species or simply a juvenile Tyrannosaurus rex? A new paper published in Science has definitively shown that Nanotyrannus is, in fact, nearly fully grown and not an immature T. rex, at the same time revealing new insights into how these giant predators achieved such terrifying sizes so quickly.
A multi-institutional team, including Dinosaur Institute Postdoctoral Fellow, Dr. Zach Morris, examined the much-debated Nanotyrannus holotype—the specimen used to name a new species—particularly its throat bone. The team examined the bone’s microscopic structure, comparing it to those of living birds, crocodilians, and extinct dinosaurs—including the Dino Hall’s singular T. rex growth series—to establish 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 competed with juvenile T. rex for prey in Late Cretaceous North America.
“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.
Just as cutting a tree and counting its growth reveals its age, cross-sectioning a dinosaur’s bones can reveal how mature it is. Researchers examine thin slices of fossil bone under a microscope, 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 these do not survive intact in every fossil specimen. The Nanotyrannus holotype is mostly a skull, and skulls are rife with sinus cavities and other irregularities that make them unsuitable for the technique. However, the hyoid—a throat bone that supports the tongue—of the specimen was potentially able to answer this question once and for all.
“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!”
Since no one had previously demonstrated that hyoid bones preserve a useful record of an animal's growth, the team needed to prove that the throat bone could be a reliable gauge for maturity. To test its viability, Dr. Griffin brought together a team of experts to build a database comparing the hyoids of a range of species, from living lizards, crocodiles, and birds to 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 oversaw the sampling and analysis of the juvenile and sub-adult “Thomas" in NHM’s one-of-a-kind 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.” Having the T. rex growth series available 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."
The discovery also begins to paint a different picture of the world of Late Cretaceous North America, which until recently saw T. rex as the lone apex predator prior to the end-Cretaceous mass extinction. “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, 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.”
About the Dinosaur Institute
The Dinosaur Institute (DI) houses NHMLAC’s collection of Mesozoic tetrapods (four-limbed vertebrates), dating from 250 million to 65.5 million years ago. This collection includes fossils of dinosaurs spanning the Mesozoic Era, as well as fossils of other tetrapods that lived alongside the dinosaurs, such as flying and marine reptiles, crocodiles, turtles, amphibians, and early mammals. The DI maintains an active paleontological training program, supporting undergraduates, PhD students, and Postdoctoral fellows.
About the Natural History Museum of Los Angeles County:
The Natural History Museum (NHM) is one of Los Angeles’ oldest cultural institutions and anchor of the evolving cultural, educational, and entertainment nexus in Exposition Park. NHM’s collection ranges from 4.5-million-year-old meteorites to newly discovered species explored throughout immersive visitor experiences such as Age of Mammals and the award-winning Dinosaur Hall. The outdoor 3.5-acre Nature Gardens and indoor Nature Lab look at people’s relationship with the environment in L.A., while another beloved permanent exhibit, Becoming Los Angeles, examines how L.A. has changed over time. NHM also features industry-leading habitat dioramas, an exquisite gem and mineral hall, a hands-on Discovery Center, and behind-the-scenes experiences such as the Dino Lab, where fossils are prepared in public view. NHM recently opened NHM Commons, a new community-focused wing designed to open new doors to natural history and celebrate the intersections of science, nature, and culture.
Dr. Morris studying the hyoid, or throat bone, of “Thomas” in the Dinosaur Institute collections. Image by Stephanie Abramowicz.
CNHM Dinosaur Institute Postdoctoral Fellow Dr. Zach Morris compares a cast of the Nanotyrannus skull to the juvenile (left) and teenage (right) skeletons featured in the Dinosaur Hall’s iconic T. rex growth series. Image by Stephanie Abramowicz.
The T. rex growth series at the Natural History Museum of Los Angeles County’s Jane G. Pisano Dinosaur Hall is the only display of its kind on Earth. Courtesy of the Natural History Museum of Los Angeles County
Credit
Image by Stephanie Abramowicz
A size comparison of the hyoid in juvenile and subadult T. rex with Nanotyrannus (above). While Nanotyrannus would have been slightly smaller than the NHM’s juvenile T. rex, the signs of maturity are clear under the microscope (below). The greater number and more closely spaced growth rings (8 vs. ~2 in the teenage T. rex!) reveal the holotype of Nanotyrannus was both fully mature and a distinct species.
Credit
Dr. Zach Morris
Journal
Science
Method of Research
Observational study
Subject of Research
Animals
Article Title
A Diminutive Tyrannosaur Lived Alongside Tyrannosaurus rex
Article Publication Date
4-Dec-2025
This fossil bird choked to death on rocks, and no one knows why
The new species, named after electro-funk band Chromeo, helps tell the larger story of why only one small group of dinosaurs survived extinction
Field Museum
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The unlucky fossil bird, preserved with over 800 tiny rocks in its throat (visible as the gray mass next to the left of its neck bones).
view moreCredit: Photo courtesy of Jingmai O’Connor.
A fossil only tells part of the story. When an animal’s body is preserved as a fossil, there are often pieces missing, and even a perfectly-preserved body doesn’t tell the whole story of how that animal behaved, how it lived, and how it ultimately died. But the cause of death for one unlucky bird that lived about 120 million years ago is clearer: the cluster of rocks in its throat tells scientists that it probably choked to death. The reason why this bird was swallowing rocks in the first place is more of a mystery, and one that gets into the bigger picture of dinosaur and bird evolution.
Jingmai O’Connor, the associate curator of fossil reptiles at Chicago’s Field Museum and lead author of a new paper describing the new species in the journal Palaeontologica Electronica, came across the fossil at the Shandong Tianyu Museum of Nature in China.
“There are thousands of bird fossils at the Shandong Tianyu Museum, but on my last trip to visit their collections, this one really jumped out at me,” says O’Connor. “I immediately knew it was a new species.”
The fossil was tiny— about the size of a modern sparrow— but it had features in common with a larger fossil bird called Longipteryx. “It had really big teeth at the end of its beak, just like Longipteryx, but it’s a tiny little guy. So based on that, I knew it was something new.”
When O’Connor examined the fossil under a microscope, she saw something puzzling. “I noticed that it had this really weird mass of stones in its esophagus, right up against the neck bones,” says O’Connor. “This is really weird, because in all of the fossils that I know of, no one has ever found a mass of stones inside the throat of an animal.” The placement and chemical composition of the rocks suggested that they really were swallowed by the animal during its life, instead of just washing up near its body in the lakebed where its fossil formed.
It’s common enough for animals to swallow stones that there’s even a term for rocks swallowed by animals, either intentionally or by accident: gastroliths. Some birds, including chickens, swallow small stones that are stored in a muscular stomach called a gizzard, which they use to help grind up their food. But in the thousands of fossil birds in the same group as this little fossil, none have ever been found with gizzard stones.
To help determine if the new little bird was simply the first of its kind to be found with gizzard stones, O’Connor drew upon previous research in which she and her colleagues had CT scanned fossils of birds that definitely did have gizzards. “We had quantified the average volume of the stones, the number of stones that these other fossil birds had in their gizzards, the size of the gizzard stone mass compared to the total size of the bird,” says O’Connor. “We CT scanned this new fossil so we could compare it to these other birds with gizzards.”
The CT scan data indicated that whatever the mass of rocks in the little bird’s throat were, they were not gizzard stones. “We found over 800 tiny stones in this bird’s throat— way more than we would have expected in other birds with gizzards. And based on their density, some of these stones weren’t even really stones, they seemed to be more like tiny clay balls,” says O’Connor. “With these data, we can very clearly say that these stones weren’t swallowed to help the bird crush its food.”
Since the bird’s gastroliths were not gizzard stones, it must have swallowed them for another reason. The researchers can’t be sure, but they have an idea of what that might be.
“When birds are sick, they start doing weird things,” says O’Connor. “So we put forth a tentative hypothesis that this was a sick bird that was eating stones because it was sick. It swallowed too many, and it tried to regurgitate them in one big mass. But the mass of stones was too big, and it got lodged in the esophagus.”
The finding is unusual, not just because it’s the first time, to O’Connor’s knowledge, that a fossil animal has been discovered with a throat full of stones. “It’s pretty rare to be able to know what caused the death of a specific individual in the fossil record,” says O’Connor. “But even though we don’t know why this bird ate all those stones, I’m fairly certain that regurgitation of that mass caused it to choke, and that’s what killed that little bird.”
In addition to its unusual gastroliths, the fossil represents a new species of bird, and thus, a new species of dinosaur. O’Connor named the fossil Chromeornis funkyi (kroh-me-or-nis fun-key) after the techno-funk duo Chromeo, one of her favorite bands, since birds are also known for making beautiful music. (Scientists can’t tell exactly what sounds Chromeornis made— it probably didn’t sing like a modern bird, but O’Connor says that it probably made some sort of vocalizations.)
Chromeo, comprised of David “Dave 1” Macklovitch and Patrick “P-Thugg” Gemayel, were happy to learn that a fossil had been named after them, saying, “We’ve been doing this for 20 years but this is the first time someone’s called us a dinosaur! Jokes aside, this is an incredible honor to add to a career full of surprises. We’re glad to bring a little fossil funk to the great science of paleontology.”
Beyond the fate of this one unlucky bird that choked on rocks, Chromeornis is a piece of a bigger picture of extinction and survival. It’s part of a group of birds called the enantiornithines. During the Cretaceous, this was the most prevalent group of birds, but when the asteroid hit 66 million years ago, enantiornithine birds went extinct along with nearly every other kind of dinosaur— the only group that remained is the one containing the modern birds still alive today.
“During that environmental disaster, the enantiornithines went from being the most successful group of birds to being wiped out,” says O’Connor. “Understanding why they were successful but also why they were vulnerable can help us predict the course of the mass extinction we’re in now. Learning about Chromeornis and other birds that went extinct could ultimately help guide conservation efforts today.”
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Paleontologist Jingmai O'Connor examining Chromeornis under a microscope.
Credit
Courtesy of Jingmai O'Connor
An illustration showing Chromeornis in life.
Credit
Sunny Dror
Close-up of the mass of rocks in the throat of Chromeornis (the rocks are the gray mass just to the left of the neck bones).
Credit
Courtesy of Jingmai O'Connor
Left to right: Chromeo member P-Thugg, paleontologist Jingmai O’Connor, and Chromeo member Dave 1, posing with a picture of Chromeornis funkyi at a Chromeo concert in Chicago on November 20, 2025.
Credit
Photo by Jason Peterson.
Article Title
A new small-bodied longipterygid (Aves: Enantiornithes) from the Aptian Jiufotang Formation preserving unusual gastroliths
Article Publication Date
5-Dec-2025
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