What paleontologists have long identified as Tyrannosaurus rex turns out to be two distinct species, as researchers say the smaller Nanotyrannus lancensis is indeed a separate dinosaur species, giving rise to a late Cretaceous controversy.
The debate has raged for years: were smaller fossils that had traditionally been designated juvenile specimens of T. rex something else entirely? Now, working with a monumental fossil specimen recovered in Montana, paleontologists have finally resolved the two-species conundrum, leaving an enormous number of new questions to address.
Dueling Dinosaurs
Recovered from Montana’s Hell Creek Formation in 2006, the “Dueling Dinosaurs” specimen captures a dramatic prehistoric moment: a Triceratops locked in its final struggle with what had originally been identified as an adolescent T. rex. Nearly twenty years later, detailed analysis now shows the second animal belongs to a distinct species, Nanotyrannus lancensis, according to a new study published in Nature.
“This fossil doesn’t just settle the debate. It flips decades of T. rex research on its head,” said co-author Lindsay Zanno, associate research professor at North Carolina State University, head of paleontology at the North Carolina Museum of Natural Sciences.
Investigating Nanotyrannus lancensis
To reach their conclusion, the team analyzed the fossil from multiple perspectives, including developmental anatomy, growth rings, and spinal fusion patterns. Their findings revealed the animal was a fully grown adult, approximately 20 years old.
They also identified several key anatomical differences from T. rex. Despite its smaller size, Nanotyrannus possessed more teeth, longer forelimbs, and fewer tail vertebrae than its larger cousin. Combined, these traits provided strong evidence that the specimen represented its own species.
“For Nanotyrannus to be a juvenile T. rex, it would need to defy everything we know about vertebrate growth,” said coauthor James Napoli, an anatomist at Stony Brook University. “It’s not just unlikely – it’s impossible.”
A Shift in Paleontology
The findings represent a major shake-up in dinosaur paleontology. For years, models of T. rex growth and behavior assumed that Nanotyrannus fossils represented adolescent tyrannosaurs. With the two species now separated, those models will need to be revisited — and the researchers say more surprises likely await.
The team examined more than 200 fossils in reaching their conclusion. Among them was another specimen long considered a teenage T. rex, which showed features distinct from both T. rex and N. lancensis. The team has named this additional species N. lethacus, after the mythical Greek river of forgetfulness, reflecting how it remained unidentified for decades.
Together, the findings suggest a richer and more competitive landscape in the final chapter of the dinosaur age — with multiple predator species living alongside one another, not just the iconic T. rex.
“This discovery paints a richer, more competitive picture of the last days of the dinosaurs,” Zanno said. “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,” Zanno says, adding that it was “a leaner, swifter and more agile hunter.”
The paper, “Nanotyrannus and T. rex Coexisted at the Close of the Cretaceous,” appeared in Nature on October 15, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.