For decades, paleontologists have argued over one mystery: was Nanotyrannus a real species, or just a teenage Tyrannosaurus rex?
Now, a new fossil has finally ended the debate—and rewritten the story of the world’s most famous dinosaur.
The breakthrough came from the famous “Dueling Dinosaurs” fossil found in Montana.
The fossil captures two creatures locked in a prehistoric battle: a Triceratops and a smaller predator that was long thought to be a young T. rex.
But new research published in Nature has confirmed that the smaller dinosaur is actually a fully grown Nanotyrannus lancensis—a separate species that lived alongside T. rex.
“This fossil doesn’t just end the argument. It changes everything we thought we knew about T. rex,” says Lindsay Zanno, a paleontologist at North Carolina State University and co-author of the study.
The team analyzed the bones in remarkable detail, studying growth rings (similar to those in trees), spinal fusion, and bone structure.
They found that the Nanotyrannus was around 20 years old when it died—an adult, not a teenager. Its skeleton had unique traits that never appear in T. rex, including longer arms, more teeth, fewer tail bones, and different skull nerve patterns.
“For Nanotyrannus to be a juvenile T. rex, it would have to break every rule of vertebrate growth,” says James Napoli of Stony Brook University. “That’s simply impossible.”
This discovery has huge implications. For years, scientists used Nanotyrannus fossils as examples of young T. rex individuals to study how the species grew.
But this new evidence shows those studies were based on two completely different animals.
It also means that late Cretaceous North America hosted more than one top predator—something paleontologists never expected.
While comparing over 200 tyrannosaur fossils, the researchers also identified another distinct species, which they named Nanotyrannus lethaeus. Its name comes from the River Lethe in Greek mythology, symbolizing how the species had been “forgotten” for decades, hidden in plain sight.
Together, these findings reveal a much more diverse and competitive world at the end of the dinosaur era. T. rex may have been the heavyweight champion, but Nanotyrannus was the swift and agile hunter—fast, sharp-eyed, and built for pursuit.
“This discovery gives us a richer picture of life before the asteroid hit,” says Zanno. “T. rex was powerful, but it wasn’t alone at the top. It shared its world with another predator—one that history had almost erased.”
