Nanotyrannus Is Not a Juvenile T. rex: The “Small King of Dinosaurs” Restored from Montana’s Dueling Dinosaurs!

In a landmark study published October 30, 2025, in Nature, paleontologists Lindsay Zanno and James Napoli have definitively proven that Nanotyrannus lancensis is a valid, fully grown species distinct from Tyrannosaurus rex—ending a 40-year debate ignited by the legendary “Dueling Dinosaurs” fossil from Montana’s Hell Creek Formation.

Unearthed in 2006, the spectacular specimen preserves a Triceratops and a small-bodied tyrannosaur locked in combat, with teeth still embedded in bone. Long ᴀssumed to represent a teenage T. rex, the tyrannosaur is now confirmed as an adult Nanotyrannus lancensis approximately 6 meters long—far smaller and more agile than its giant cousin.

Key evidence includes growth rings in the leg and arm bones showing skeletal maturity, plus dozens of anatomical differences in the skull, vertebrae, and limbs that cannot be explained by ontogeny. The team also identified a second species, Nanotyrannus lethaeus, from another Montana specimen.

“This fossil categorically ends the debate,” Zanno states. “Nanotyrannus is not a juvenile T. rex. It belongs to a separate genus entirely.” A December 2025 Science paper independently confirmed the conclusion using throat-bone (ceratobranchial) microstructure.

The discovery dramatically increases late-Cretaceous predator diversity, revealing that T. rex shared its ecosystem with at least one smaller, faster tyrannosaur. All prior growth-rate studies ᴀssuming Nanotyrannus specimens were young T. rex must now be reevaluated.

After decades of controversy, the “short king” has claimed its throne—proving that even the most iconic dinosaurs still hold surprises.