What if everything we know about T. rex growth is wrong? A complete tyrannosaur skeleton has just ended one of paleontology’s longest-running debates – whether Nanotyrannus is a distinct species, or just a teenage version of Tyrannosaurus rex.

The fossil, part of the legendary “Dueling Dinosaurs” specimen unearthed in Montana, contains two dinosaurs locked in prehistoric combat: a Triceratops and a small-bodied tyrannosaur. That tyrannosaur is now confirmed to be a fully grown Nanotyrannus lancensis – not a teenage T. rex, as many scientists once believed.

“This fossil doesn’t just settle the debate. It flips decades of T. rex research on its head,” says Lindsay Zanno, associate research professor at North Carolina State University, head of paleontology at the North Carolina Museum of Natural Sciences and co-author of the study published in Nature.

Using growth rings, spinal fusion data and developmental anatomy, the researchers demonstrated that the specimen was around 20 years old and physically mature when it died. Its skeletal features – including larger forelimbs, more teeth, fewer tail vertebrae, and distinct skull nerve patterns – are features fixed early in development and biologically incompatible with T. rex.

“For Nanotyrannus to be a juvenile T. rex, it would need to defy everything we know about vertebrate growth,” says James Napoli, anatomist at Stony Brook University and co-author of the study. “It’s not just unlikely – it’s impossible.”

The implications are profound. For years, paleontologists used Nanotyrannus fossils to model T. rex growth and behavior. This new evidence reveals that those studies were based on two entirely different animals – and that multiple tyrannosaur species inhabited the same ecosystems in the final million years before the asteroid impact.

As part of their research, Zanno and Napoli examined over 200 tyrannosaur fossils. They discovered that one skeleton, formerly thought to represent a teenage T. rex, was slightly different than the Dueling Dinosaurs’ Nanotyrannus lancensis. They named this fossil a new species of Nanotyrannus, dubbed N. lethaeus. The name references the River Lethe from Greek mythology – a nod to how this species remained hidden in plain sight and “forgotten” for decades.

Confirmation of the validity of Nanotyrannus means that predator diversity in the last million years of the Cretaceous was much higher than previously thought, and hints that other small-bodied dinosaur species might also be victims of mistaken identity.

“This discovery paints a richer, more competitive picture of the last days of the dinosaurs,” Zanno says. “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.”

IMAGE CREDIT: Anthony Hutchings.