​​​​​​ISTANBUL

A fossil once thought to belong to a teenage Tyrannosaurus rex has been identified as a separate species of tyrannosaur that lived alongside T. rex at the end of the dinosaur age, according to scientists.

In a study published Thursday in the journal Nature, paleontologists Lindsay Zanno and James Napoli said their research shows the animal known as Nanotyrannus lancensis was not a young T. rex, as many experts had believed, but a full-grown predator of its own kind.

The findings are based on an exceptionally well-preserved skeleton from Montana’s Hell Creek Formation, part of a fossil find known as the “dueling dinosaurs.” The bones, which include much of the skeleton, helped researchers compare features such as skull shape and bone growth patterns.

The analysis revealed that Nanotyrannus had distinctive physical traits that set it apart from Tyrannosaurus rex and even placed it outside the tyrannosaur family. The study also identified a second species, Nanotyrannus lethaeus, suggesting that at least two types of tyrannosaurs shared the same Late Cretaceous ecosystem roughly 67 million years ago.

Researchers said the discovery challenges decades of assumptions that smaller tyrannosaur fossils were merely immature T. rex specimens.

Instead, the results indicate that several different large predators were thriving in North America shortly before the mass extinction that wiped out the dinosaurs.