For many years, scientists believed that the dinosaur known as Nanotyrannus was simply a young Tyrannosaurus rex. However, new research published in Nature has determined that Nanotyrannus is its own distinct species. This finding challenges previous assumptions about T. rex growth and development and may influence how paleontologists view the evolution of these large predators.
The fossil at the center of this study comes from the “Dueling Dinosaurs” specimen found in Montana, which features a Triceratops and a small tyrannosaur locked together. Researchers have now confirmed that the smaller dinosaur is Nanotyrannus lancensis.
By analyzing growth rings, spinal fusion, and developmental anatomy, the team concluded that this specimen was around 20 years old and fully mature when it died. Its physical traits—such as larger forelimbs, more teeth, fewer tail vertebrae, and unique skull nerve structures—were established early in life and are not consistent with those of T. rex.
“For Nanotyrannus to be a juvenile T. rex, it would need to defy everything we know about vertebrate growth,” said James Napoli, co-author of the study and researcher in the Department of Anatomical Sciences at Stony Brook University’s Renaissance School of Medicine. “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,” said Lindsay Zanno, associate research professor at North Carolina State University and head of paleontology at the North Carolina Museum of Natural Sciences.
Napoli and Zanno reviewed over 200 tyrannosaur fossils during their work. They identified another skeleton previously thought to be a teenage T. rex but slightly different from Nanotyrannus lancensis. This led them to name a new species: Nanotyrannus lethaeus—a reference to Greek mythology’s River Lethe for its long-overlooked status.
This research suggests that earlier studies modeling T. rex behavior using Nanotyrannus fossils were based on two separate animals rather than one species at different life stages. The findings also indicate multiple tyrannosaur species lived side by side shortly before dinosaurs went extinct.
“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,” Napoli said.
Researchers further concluded there are two types within this genus: N. lancensis and N. lethaeus—the latter likely being somewhat larger as an adult than previously thought for members of this group.
The confirmation that Nanotyrannus is valid increases estimates for predator diversity near the end of the Cretaceous period and raises questions about whether other small dinosaur species have been misidentified due to similar confusion over growth stages versus distinct species identities.
Funding for this project came from several sources including state agencies in North Carolina, NC State University, Friends of the North Carolina Museum of Natural Sciences, and supporters of the Dueling Dinosaurs Capital Campaign.
