A new analysis of a fossil from Montana's Dueling Dinosaurs site has confirmed that Nanotyrannus lancensis was a mature species, not a juvenile Tyrannosaurus rex. This discovery challenges decades of paleontological assumptions about tyrannosaur growth and diversity. Published in Nature, the research reveals greater predator variety in the late Cretaceous period.
The debate over Nanotyrannus' identity has long divided paleontologists, with some viewing fossils as young T. rex specimens and others as evidence of a separate species. A remarkably preserved skeleton from the Dueling Dinosaurs discovery in Montana, featuring a Triceratops locked in combat with a smaller tyrannosaur, provides definitive proof otherwise.
Scientists, led by Lindsay Zanno, associate research professor at North Carolina State University and head of paleontology at the North Carolina Museum of Natural Sciences, analyzed bone growth rings, spinal fusion, and developmental anatomy. The tyrannosaur was approximately 20 years old at death, indicating full maturity. Key differences include longer arms, more teeth, fewer tail vertebrae, and unique skull nerve structures, which emerged early and contradict T. rex growth patterns.
"This fossil doesn't just settle the debate. It flips decades of T. rex research on its head," Zanno stated. Co-author James Napoli, an anatomist at Stony Brook University, added, "For Nanotyrannus to be a juvenile T. rex, it would need to defy everything we know about vertebrate growth. It's not just unlikely—it's impossible."
The team reviewed over 200 tyrannosaur fossils, identifying one as a new species, Nanotyrannus lethaeus, named after the River Lethe for its long 'forgotten' status. This suggests multiple tyrannosaur species coexisted in the final million years before the asteroid impact ended the dinosaur era, painting a more competitive late Cretaceous ecosystem.
"This discovery paints a richer, more competitive picture of the last days of the dinosaurs," Zanno explained. "With enormous size, a powerful bite force and stereoscopic vision, T. rex was a formidable predator, but it did not reign uncontested. Darting alongside was Nanotyrannus—a leaner, swifter and more agile hunter."
The study, published in Nature in 2025, was supported by the State of North Carolina, NC State University, the Friends of the North Carolina Museum of Natural Sciences, and the Dueling Dinosaurs Capital Campaign.