In 2006, paleontologists discovered an exceptionally rare and well-preserved fossil specimen preserved with two dinosaurs together.
The position suggested they were locked in combat at their death and burial.
Unearthed in Montana, the “Dueling Dinosaur” fossil included Triceratops and a small-bodied tyrannosaur.
The discovery led to the long-running debate about the validity of the dinosaur species Nanotyrannus.
The status of Nanotyrannus was long disputed, with paleontologists arguing over whether it was a distinct species or just a juvenile T. rex.
Extensive analysis of the tyrannosaur skeleton preserved in this fossil has settled this long debate.
North Carolina State University confirmed that the tyrannosaur fossil is a fully grown Nanotyrannus lancensis, disproving that it was a teenage T. rex.
“This fossil doesn’t just settle the debate. It flips decades of T. rex research on its head,” said Lindsay Zanno, associate research professor at North Carolina State University, head of paleontology at the North Carolina Museum of Natural Sciences.
Extensive fossil examination
The specimen’s age was around 20 years based on evidence like growth rings, spinal fusion, and developmental anatomy.
The study found that the skeletal features of the Nanotyrannus specimen are biologically incompatible with those expected for a juvenile T. rex.
These distinctive traits — such as larger forelimbs, a greater number of teeth, fewer tail vertebrae, and unique skull nerve patterns — are characteristics that become fixed early in an animal’s development
“For Nanotyrannus to be a juvenile T. rex, it would need to defy everything we know about vertebrate growth,” said James Napoli, anatomist at Stony Brook University and co-author of the study. “It’s not just unlikely – it’s impossible.”
Dinosaur diversity before extinction
The new evidence confirming Nanotyrannus is a distinct species has importance in paleontology.
Decades of studies used Nanotyrannus fossils to model the growth and behavior of T. rex, but this research is now known to be inaccurate because it was based on two entirely different animals.
In addition to confirming N. lancensis, researchers Zanno and Napoli examined over 200 tyrannosaur fossils and made a second discovery.
They identified a separate skeleton, previously mistaken for a teenage T. rex, slightly different from the Nanotyrannus in the Dueling Dinosaurs specimen.
This distinct fossil was named a new species of Nanotyrannus, dubbed N. lethaeus.
The name references the mythical River Lethe to reflect how this species remained “forgotten” or unrecognized within museum collections for decades.
The confirmation of Nanotyrannus as a valid species has changed the understanding of the late Cretaceous ecosystem.
The discovery proves that multiple tyrannosaur species (T. rex and Nanotyrannus) co-existed within the same ecosystems during the final million years leading up to the impact of the asteroid.
This finding paints a “richer, more competitive picture” of the dinosaurs’ last days.
“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,” said Zanno.
The discovery also suggests that other small-bodied dinosaur species currently classified as juveniles of larger species might also be “victims of mistaken identity” and warrant re-examination.
The study was published in the journal Nature.