Paleontologists thought fossils found in Montana were juvenile T. rex. It's actually a new species of dinosaur

Fossils paleontologists initially thought belonged to a juvenile Tyrannosaurus rex actually belong to a new species of dinosaur that belongs to a distinct genus altogether.

The 67 million-year-old fossils, found in the Hell Creek Formation in Montana, are the skeleton of small-bodied dinosaur called Nanotyrannus lancensis from the late Cretaceous period, according to a paper published Thursday in Nature.

The Nanotyrannus's small skull has been the source of debate among dinosaur experts for at least four decades, according to the paper. Earlier research suggested it belonged to a young T. rex, but others argued that it possessed features only seen in this particular species.

But a new, more complete skeleton found in the Hell Creek Formation helped to make those distinctions, according to the paper.

The new specimen is "distinguishable" from T. rex and shares features with the N.  lancensis skull found decades earlier -- confirming that it belongs to a separate genus, the researchers said.

Analysis of the skeleton of the Nanotyrannus indicates that it was a nearly full-grown adult, rather than a juvenile T. rex, and weighed about 1,500 pounds -- compared to about 14,000 to 18,000 pounds that an adult T. rex typically weighed.

Modeling also revealed distinct bone-growth trajectories between the two species, further supporting the notion that they are separate species.

N. lancensis was likely also a predator that coexisted with T. rex within 1 million years of the dinosaur extinction at the end of the Cretaceous period, according to the paper.

The findings indicate that dinosaur diversity was "rich" in this region during the latest years of the Cretaceous, prior to the impact from the asteroid that caused the mass extinction of dinosaurs and other species, according to the paper.

The discovery could also have important implications for the evolution of tyrannosauroids, the authors noted.