New Analysis of a Fossil Discovery Rewrites the Story of the Tyrannosaurus Rex

Stony Brook paleontologist co-authors Nature paper that provides evidence Nanotyrannus was not a teenage T. rex

STONY BROOK, NY, November 4, 2025 - For decades paleontologists believed that the dinosaur discovered in the 1940s - Nanotyrannus - was a juvenile or "teenaged" Tyrannosaurus Rex. Now new research published in Nature has revealed this dinosaur was its own species, not a young T. rex, a finding that rewrites what scientists have suspected about T. rex's growth and development and could even alter views on T. rex evolution.

T. 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. The newly studied 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, now confirmed as Nanotyrannus lancensis.

Using growth rings, spinal fusion data and developmental anatomy, the research team 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 arrangements - 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," states James Napoli, PhD, a vertebrate paleontologist, co-author of the study, and Researcher in the Department of Anatomical Sciences in the Renaissance School of Medicine (RSOM) at Stony Brook University. "It's not just unlikely - it's impossible."

"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.

As part of their research, Napoli and Zanno examined over 200 tyrannosaur fossils. They discovered that one skeleton, formerly thought to represent a teenage T. rex, was slightly different from 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.

A new name for a smaller, speedier dinosaur

The implications of the new research 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.

Napoli emphasizes that the paper conclusively reveals the former scientific consensus was wrong, and that Nanotyrannus is a real species of medium-sized, long-legged, and speedy tyrannosaur that lived alongside T. rex at the end of the Age of Dinosaurs.

"We illustrated this using a spectacular new specimen that, for the first time, showed us that Nanotyrannus had very long arms with a vestigial third finger (not short arms with two fingers like in T. rex), and which preserved a growth record in bone microstructure indicating that it had reached adulthood, and therefore could not have grown up to be a T. rex," explains Napoli.

The researchers also recognized that Nanotyrannus included two species - Nanotyrannus lancensis, and a new species Nanotyrannus lethaeus, which would have been slightly larger as an adult. And they showed that many of the growth-related changes prior scientists had inferred in T. rex were not actually biologically possible, further confirming the result while casting doubt on many prior studies of T. rex and how it grew and evolved.

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 work 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.