A new study has settled a long-standing debate by confirming that Nanotyrannus was a fully grown dinosaur species separate from Tyrannosaurus rex. Researchers analyzed the hyoid bone of the Nanotyrannus holotype, revealing maturity signals that distinguish it from juvenile T. rex specimens. This finding suggests a more diverse and competitive tyrannosaur ecosystem in Late Cretaceous North America.
Paleontologists have debated for years whether Nanotyrannus, known from a single skull, represented a true species or merely a young Tyrannosaurus rex. A study published in Science on December 8, 2025, provides definitive evidence that Nanotyrannus was a mature predator, standing at under half the size of an adult T. rex but fully developed.
The research team, led by Dr. Christopher Griffin of Princeton University, examined the hyoid bone—the throat bone supporting the tongue—from the Nanotyrannus holotype. This bone, rarely used for age assessment due to its irregular structure in skulls, proved reliable after comparisons with hyoids from modern reptiles, birds, and other dinosaurs. Microscopic analysis revealed growth patterns indicating maturity, similar to those in T. rex but showing cessation of rapid growth in Nanotyrannus.
"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. Griffin.
Co-author Dr. Zach Morris of the Natural History Museum of Los Angeles County compared the hyoid to T. rex growth series specimens, including the juvenile "Thomas." "What we did not expect was to see it was nearing maturity with clear evidence of the cessation of growth!" Morris noted. These benchmarks confirmed Nanotyrannus competed with young T. rex for prey in a crowded Late Cretaceous landscape.
The study highlights the value of museum collections and non-destructive techniques like 3D scanning, preserving specimens for future research. Senior author Dr. Caitlin Colleary emphasized balancing conservation with discovery. This evidence aligns with recent findings of multiple coexisting tyrannosaur species, reshaping views of prehistoric ecosystems before the end-Cretaceous extinction.