Nanotyrannus confirmed: Dueling dinosaurs fossil rewrites the story of T. rex

What if everything we know about T. rex growth is wrong? A complete tyrannosaur skeleton has just ended one of paleontology’s longest-running debates – whether Nanotyrannus is a distinct species, or just a teenage version of Tyrannosaurus rex. 

The fossil, part of the legendary “Dueling Dinosaurs” specimen unearthed in Montana, contains two dinosaurs locked in prehistoric combat: a Triceratops and a small-bodied tyrannosaur. That tyrannosaur is now confirmed to be a fully grown Nanotyrannus lancensis – not a teenage T. rex, as many scientists once believed.

“This fossil doesn’t just settle the debate. It flips decades of T. rex research on its head,” says Lindsay Zanno, associate research professor at North Carolina State University, head of paleontology at the North Carolina Museum of Natural Sciences and co-author of the study published in Nature.

Using growth rings, spinal fusion data and developmental anatomy, the researchers demonstrated that the specimen was around 20 years old and physically mature when it died. Its skeletal features – including larger forelimbs, more teeth, fewer tail vertebrae, and distinct skull nerve patterns – are features fixed early in development and biologically incompatible with T. rex.

“For Nanotyrannus to be a juvenile T. rex, it would need to defy everything we know about vertebrate growth,” says James Napoli, anatomist at Stony Brook University and co-author of the study. “It’s not just unlikely – it’s impossible.”

The implications are profound. For years, paleontologists used Nanotyrannus fossils to model T. rex growth and behavior. This new evidence reveals that those studies were based on two entirely different animals – and that multiple tyrannosaur species inhabited the same ecosystems in the final million years before the asteroid impact.

As part of their research, Zanno and Napoli examined over 200 tyrannosaur fossils. They discovered that one skeleton, formerly thought to represent a teenage T. rex, was slightly different than the Dueling Dinosaurs’ Nanotyrannus lancensis. They named this fossil a new species of Nanotyrannus, dubbed N. lethaeus. The name references the River Lethe from Greek mythology – a nod to how this species remained hidden in plain sight and “forgotten” for decades.

Confirmation of the validity of Nanotyrannus means that predator diversity in the last million years of the Cretaceous was much higher than previously thought, and hints that other small-bodied dinosaur species might also be victims of mistaken identity.

“This discovery paints a richer, more competitive picture of the last days of the dinosaurs,” Zanno says. “With enormous size, a powerful bite force and stereoscopic vision, T. rex was a formidable predator, but it did not reign uncontested. Darting alongside was Nanotyrannus – a leaner, swifter and more agile hunter.”

This work, published in Nature, was supported by the State of North Carolina, NC State University, the Friends of the North Carolina Museum of Natural Sciences, and the Dueling Dinosaurs Capital Campaign. 

-30-

Note to editors: An abstract follows.

“Nanotyrannus and T. rex coexisted at the close of the Cretaceous”

DOI: 10.1038/s41586-025-09801-6

Authors: Lindsay Zanno, North Carolina State University and the North Carolina Museum of Natural Sciences; James Napoli, Stony Brook University 
Published: Oct. 30 in Nature

Abstract:
Tyrannosaurus rex ranks among the most comprehensively studied extinct vertebrates and a model system for dinosaur paleobiology. As one of the last surviving non-avian dinosaurs, Tyrannosaurus is a crucial datum for assessing terrestrial biodiversity, ecosystem structure, and biogeographic exchange immediately preceding the end-Cretaceous mass extinction —one of Earth’s greatest biological catastrophes. Paleobiological studies of Tyrannosaurus, including ontogenetic niche partitioning, feeding, locomotor biomechanics, and life history have drawn upon an expanding skeletal sample comprising multiple hypothesized growth stages—and yet the Tyrannosaurus hypodigm remains controversial. A key outstanding question relates to specimens considered to exemplify immature Tyrannosaurus, which have been argued to represent the distinct taxon Nanotyrannus. Here, we describe an exceptionally well-preserved, near somatically mature tyrannosaur skeleton (NCSM 40000) from the Hell Creek Formation that shares autapomorphies with the holotype specimen of N. lancensis. We couple comparative anatomy, longitudinal growth models, observations on ontogenetic character invariance, and a novel phylogenetic dataset to test the validity of Nanotyrannus, demonstrating conclusively that this taxon is distinguishable from Tyrannosaurus, sits outside Tyrannosauridae, and unexpectedly contains two species—N. lancensis and N. lethaeus, sp. nov. Our results prompt a re-evaluation of dozens of existing hypotheses based on currently indefensible ontogenetic trajectories. Finally, we document at least two co-occurring, ecomorphologically distinct genera in the Maastrichtian of North America, demonstrating that tyrannosauroid alpha diversity was thriving within one million years of the end-Cretaceous extinction.