A new dinosaur discovery may finally resolve a decades-old debate: Was a mysterious dinosaur excavated in the 1940s a juvenile Tyrannosaurus rex, or an entirely different species?Initially, scientists had only a tyrannosaur skull, making it difficult to determine if it belonged to a child or an adult. Another skeleton, nicknamed Jane, added to the discussion but did not settle the controversy.

Now, researchers say a complete skeleton uncovered in Montana in 2006 provides key evidence identifying the dinosaur as a distinct species, rather than a young T. rex.

“This discovery rewrites decades of research on Earth’s most famous predator,” said study co-author Lindsay Zanno of the North Carolina Museum of Natural Sciences and North Carolina State University.

Analysis of growth rings in the Montana bones revealed that the dinosaur was an adult, roughly half the size of a fully-grown T. rex. Comparisons with growth patterns in other reptiles, such as crocodiles, indicated that major differences in skull shape, bone structure, nerve patterns, and sinuses were unlikely to develop simply through maturation. The findings suggest the dinosaur is a distant T. rex cousin, Nanotyrannus lancensis, according to a study published Thursday (6 November) in Nature.

Holly Woodward, a fossil expert at Oklahoma State University who was not involved in the study, said the evidence supporting the existence of this T. rex relative is stronger than ever, though she remains unconvinced that other mystery skeletons like Jane represent a new species.

Other paleontologists caution that the debate is not fully resolved. Thomas Carr, a vertebrate paleontologist at Carthage College, said while the new skeleton is indeed an adult, it may be a sister species rather than a distant relative. “I don’t think this study settles everything,” he said, citing similarities in skull shape between T. rex and the mystery specimens.

Understanding this dinosaur is crucial to studying how T. rex grew, said co-author James Napoli of Stony Brook University. Researchers also hope it will clarify whether T. rex was the dominant predator 67 million years ago or if smaller but formidable predators also roamed the Earth.

The skeleton, nicknamed “Dueling Dinosaurs” because it was discovered intertwined with a Triceratops, is currently on display at the North Carolina Museum of Natural Sciences.