A study published in Science and led by Princeton University and the Cleveland Museum of Natural History (CMNH) has resolved a longstanding debate among paleontologists about the identity of a small tyrannosauroid skull discovered in 1942. The research concludes that Nanotyrannus lancensis was its own species, not just a juvenile Tyrannosaurus rex.
Christopher Griffin, assistant professor of geosciences at Princeton and lead scientist on the project, noted that prior consensus considered the specimen to be a young T. rex. Caitlin Colleary, CMNH vertebrate paleontology curator, collaborated with Griffin to apply age-dating techniques on dinosaur bones to clarify the issue.
“As a curator, I often have to find a balance between conservation and discovery,” said Colleary. “If our only objective was conservation, we would keep a lot of amazing fossils locked in cabinets, but we collect them to learn about the past, about ancient creatures.”
The team used microscopic cross-sections of the hyoid bone from the Nanotyrannus skull to determine age. Unlike skulls that remodel over time and obscure age records, these throat bones preserve growth rings similar to those found in long bones.
“This isn’t a new technique, but up until now, age dating has been exclusively used on limb bones and ribs,” Griffin explained. “The novelty of this study is showing that these slender throat bones preserve a record of growth like limbs and ribs do.”
Colleary emphasized the significance of using destructive analysis for scientific gain: “Historically, a lot of curators probably wouldn’t have approved a destructive analysis like this on a holotype. But because I do molecular research, I know the value of hidden information inside a fossil. This is a perfect example of using destructive analyses appropriately, because we gained so much more information than we lost.”
Griffin confirmed that hyoid bones show clear adult growth patterns across modern birds and reptilian relatives before examining the Nanotyrannus specimen. When he observed tightly clustered rings—an external fundamental system indicating full maturity—he realized it represented an adult animal.
“I saw what, for all the world, looked like EFS, the external fundamental system,” said Griffin. “An EFS is unambiguous. You see it, and boom, you know this thing’s full-grown. My first thought was, ‘Oof, this is going to be a lot more work.’”
To strengthen their findings against previous theories about juvenile T. rex specimens, Griffin expanded his research to include various theropods and other animals’ hyoid measurements—a process complicated by difficulties locating uncatalogued hyoids in museum collections.
Despite its name suggesting otherwise, Nanotyrannus was roughly six feet tall at the hip as an adult—smaller than T. rex but still sizable for predatory dinosaurs. Its holotype skull measures nearly two feet long compared with adult T. rex skulls ranging from four to six feet.
“It’s still a big animal; it’s just that Tyrannosaurus rex is so big that by comparison Nanotyrannus seems really small — about a tenth of the size by total body mass,” said Griffin.
The confirmation of Nanotyrannus as an independent species suggests greater ecological diversity among late Cretaceous predators in western North America than previously believed.
“Before we thought there was only one major predator at the end of the Cretaceous in western North America,” said Griffin. “But now we know there was more diversity, more ecological complexity leading up to the mass extinction.”
Griffin compiled his database using digital microscopy funded by Princeton University’s endowment and shared all data via an online repository associated with their publication.
“This fancy microscope takes enormous high-resolution whole-slide images,” he said. “We posted every single piece of data that we have on an online repository that goes with this paper… So whoever downloads these big files is seeing exactly what I see when the slide is on my microscope.”
While conducting their research another group in North Carolina identified a complete skeleton matching Nanotyrannus lancensis characteristics; their findings published in Nature also support recognition of Nanotyrannus as distinct from T. rex.
Colleary commented on both studies: “It’s great to confirm that our Nanotyrannus was fully grown. It’s also really exciting to have also contributed a new tool — hyoid age dating — to the field of vertebrate paleontology… We’ve invested in the care of this specimen for more than 80 years, and that’s what enables studies like this to happen.”
The article “A diminutive tyrannosaur lived alongside Tyrannosaurus rex” by Christopher T. Griffin et al., appears in Science (DOI: 10.1126/science.adx8706). Research support came from Princeton University and Yale University endowments along with National Science Foundation funding.
