A new study published in Nature challenges the long-held belief that Nanotyrannus was simply a juvenile Tyrannosaurus rex. Researchers, including James Napoli, PhD, from Stony Brook University’s Department of Anatomical Sciences, have provided evidence that Nanotyrannus is its own distinct species.
The fossil at the center of this research comes from the “Dueling Dinosaurs” specimen found in Montana, which features a Triceratops and a small-bodied tyrannosaur now identified as Nanotyrannus lancensis. The team used growth rings, spinal fusion data, and developmental anatomy to determine that the specimen was about 20 years old and physically mature at death. Key skeletal differences—such as larger forelimbs, more teeth, fewer tail vertebrae, and unique skull nerve patterns—were found to be incompatible with T. rex development.
“For Nanotyrannus to be a juvenile T. rex, it would need to defy everything we know about vertebrate growth,” said James Napoli. “It’s not just unlikely – it’s impossible.”
Lindsay Zanno of North Carolina State University added: “This fossil doesn’t just settle the debate. It flips decades of T. rex research on its head.”
Napoli and Zanno examined over 200 tyrannosaur fossils during their research. They identified another skeleton previously thought to be a teenage T. rex as a new species of Nanotyrannus named N. lethaeus—a reference to the River Lethe in Greek mythology due to its hidden status for many years.
The findings suggest that multiple tyrannosaur species lived alongside each other during the final million years before the asteroid impact that ended the Cretaceous period. This means previous studies modeling T. rex growth using Nanotyrannus fossils were based on two different animals.
“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,” explained Napoli.
Researchers also concluded there are two species within Nanotyrannus: N. lancensis and N. lethaeus—the latter being slightly larger as an adult.
These results indicate predator diversity at the end of the Cretaceous was higher than previously believed and raise questions about whether other small dinosaur species may also have been misidentified.
The study received support from several organizations including NC State University and Friends of the North Carolina Museum of Natural Sciences.