For years, scientists believed that Nanotyrannus, a dinosaur discovered in the 1940s, was simply a young Tyrannosaurus rex. However, new research published in Nature challenges this idea by confirming that Nanotyrannus is its own distinct species.
The study focused on fossils from the “Dueling Dinosaurs” specimen found in Montana, which features a Triceratops and a small tyrannosaur locked together. Researchers determined that the smaller dinosaur was Nanotyrannus lancensis and not a juvenile T. rex as previously thought.
By analyzing growth rings, spinal fusion, and developmental anatomy, the team concluded that the specimen was about 20 years old and had reached physical maturity at death. The fossil’s characteristics—such as larger forelimbs, more teeth, fewer tail vertebrae, and unique skull nerve patterns—are traits established early in development and inconsistent 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, vertebrate paleontologist and co-author of the study from 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 of North Carolina State University and co-author of the study.
Napoli and Zanno examined over 200 tyrannosaur fossils during their research. They identified another skeleton similar to but distinct from Nanotyrannus lancensis among these specimens. This led them to name a new species: Nanotyrannus lethaeus—a reference to Greek mythology’s River Lethe for how this species remained overlooked for so long.
This discovery affects how scientists understand both T. rex growth patterns and predator diversity at the end of the Cretaceous period. Previously, many studies used Nanotyrannus fossils as models for young T. rexes; now researchers know these were two different animals sharing ecosystems 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.
The findings also suggest higher predator diversity than once believed during dinosaurs’ final era and raise questions about whether other small-bodied dinosaur fossils might have been misidentified.
The project received support from several organizations including NC State University and Stony Brook University.