A fossil once thought to be from a juvenile Tyrannosaurus rex has been confirmed as an entirely different species — and a fully grown adult at that. The dinosaur, known as Nanotyrannus, had puzzled paleontologists for decades. Now, new evidence confirms it lived as a separate, smaller and more agile predator alongside the mighty T. rex.

The species sparked scientific debate because of its size — roughly half the length and only one-tenth the body mass of an adult T. rex. While early interpretations assumed the remains belonged to a teenage T. rex, the fossil’s distinct features told a different story.

“Nanotyrannus is a small-bodied predator designed for speed. It’s very agile and has long powerful arms larger than those of the T. rex,” said Lindsay Zanno, a palaeontologist at North Carolina State University. Unlike T. rex, Nanotyrannus had a shorter tail, sharper and less curved teeth, and body proportions built for quick movement rather than sheer power.

The findings, published in Nature on 30 October, are based on the famous ‘Duelling Dinosaurs’ discovery — a Nanotyrannus and Triceratops fossilized mid-battle in Montana’s Hell Creek Formation in 2006. Dating back 67 million years, the fossils offer a rare snapshot of predator-prey interaction during the late Cretaceous period.

To determine the dinosaur’s age, researchers analysed growth rings inside its fossilized bones — similar to counting rings in a tree trunk. The rings revealed growth patterns that had slowed significantly, confirming the animal had reached adulthood. Scientists also compared its growth rate to crocodiles, modern relatives of dinosaurs, to further support the conclusion.

The result settles a long-running scientific debate: Nanotyrannus was not a teenage T. rex — it was a distinct, fast-moving predator that hunted in the shadow of the giant tyrannosaur.

This discovery reshapes our understanding of late-Cretaceous ecosystems, suggesting multiple large predators coexisted in North America shortly before the dinosaurs’ extinction.