FOSSILGUY.COM

---

New research confirms that Nanotyrannus was not a juvenile Tyrannosaurus rex

A remarkably preserved tyrannosaur fossil from Montana confirms that Nanotyrannus was a distinct, fully grown species, and not a juvenile T. rex, rewriting decades of paleontological research.

---

New Study Confirms Nanotyrannus Was a Distinct Species, Not a Juvenile T. rex

This news article is based on a News Release from NCSU and the Journal Article (Zanno, L.E., Napoli, J.G., 2025) from Nature

---

For decades, one of paleontology's most heated debates has centered on Nanotyrannus—a small-bodied predator some researchers claimed was simply a juvenile Tyrannosaurus rex. But new research, published in Nature, has upended that long-standing assumption. The results confirm that Nanotyrannus was not a teenage T. rex, but a distinct, fully grown species that lived alongside it in the final days of the Cretaceous.

The breakthrough comes from the now-famous "Dueling Dinosaurs" fossil, discovered in Montana's Hell Creek Formation. Locked together in what appears to be a deadly encounter, the specimen preserves a Triceratops and a smaller tyrannosaur with astonishing detail—down to growth rings, spinal fusions, and bone textures. After more than a decade of study, researchers led by Dr. Lindsay Zanno (North Carolina Museum of Natural Sciences) and Dr. James Napoli (Stony Brook University) found that the tyrannosaur was about 20 years old and skeletally mature when it died.

"This fossil doesn't just settle the debate—it rewrites decades of T. rex research," said Zanno. The specimen's anatomy diverges sharply from T. rex: it had longer arms, more teeth, fewer tail vertebrae, and a skull pattern distinct from any known juvenile tyrannosaur. These traits are fixed early in development, ruling out the idea that the fossil could represent a young T. rex.

To be a juvenile T. rex, Zanno's team notes, Nanotyrannus would have to violate everything known about vertebrate growth patterns. Instead, the evidence paints a picture of two coexisting apex predators sharing the same Late Cretaceous landscapes of western North America. T. rex—massive, bone-crushing, and dominant—may have occupied one ecological niche, while Nanotyrannus, leaner and faster, hunted different prey or filled a different predatory role.

Adding another twist, the researchers identified subtle differences between known Nanotyrannus fossils. One specimen, previously labeled as a juvenile T. rex, was distinct enough to warrant a new species: Nanotyrannus lethaeus. The name references the River Lethe of Greek mythology—a poetic nod to how the species lay "forgotten" in plain sight for decades.

The implications are far-reaching. Many earlier studies on T. rex growth, metabolism, and behavior were based partly on Nanotyrannus fossils. Those conclusions will now need to be re-examined. If multiple tyrannosaur species coexisted in the Hell Creek ecosystem, predator diversity in the last million years before the asteroid impact was far greater than once believed.

"This discovery paints a richer, more competitive picture of the last days of the dinosaurs," said Zanno. "T. rex may have been the king—but it wasn't the only one ruling the realm."

Context: The Hell Creek Formation Connection
The Hell Creek Formation, spanning Montana and the Dakotas, has yielded some of the most iconic fossils in history—from Tyrannosaurus rex and Triceratops to the plant and fish fossils that mark the Cretaceous–Paleogene boundary. Now, the same formation has revealed that Nanotyrannus survived until the very end of the Cretaceous, coexisting with T. rex in one of Earth's most dynamic ecosystems.

This discovery confirms that the region supported multiple large predators, suggesting complex food webs and evolutionary competition just before the asteroid impact 66 million years ago.

---

---

Journal Article:
Zanno, L.E., Napoli, J.G. Nanotyrannus and Tyrannosaurus coexisted at the close of the Cretaceous. Nature (2025). https://doi.org/10.1038/s41586-025-09801-6.

---

Recommended Dinosaur Books and Educational Items:

---

High quality Dinosaur teeth by Fossilera

---