For over 30 years, paleontologists have debated the true identity of a small, predatory dinosaur known as Nanotyrannus. Was it a distinct species that coexisted with the colossal Tyrannosaurus rex in Late Cretaceous North America, or were its fossils simply juvenile T. rex specimens not yet fully grown? A groundbreaking new study published in *Nature* offers compelling evidence that Nanotyrannus was indeed a separate, smaller tyrannosaur species, living alongside its larger relative in what is now Montana. This conclusion stems from a detailed analysis of the remarkable “Dueling Dinosaurs” fossil—a rare and striking specimen discovered in 2006 that captures a triceratops locked in combat with a small tyrannosaur-like dinosaur, both showing significant injuries.
The research team, led by paleontologists Lindsay Zanno of North Carolina State University and James G. Napoli of Stony Brook University, focused on this fossil to unravel the mystery surrounding Nanotyrannus. Zanno emphasizes that the Nanotyrannus specimen from the Dueling Dinosaurs fossil was fully grown, yet only half the length and one-tenth the body mass of an adult T. rex. “There is no scenario in which this animal morphs into a T. rex,” she states firmly, underscoring that Nanotyrannus must have been a smaller, sleeker predator that occupied a different ecological niche alongside T. rex.
One of the most decisive pieces of evidence came from microscopic examination of the dinosaur’s bone structure, particularly a feature called the external fundamental system (EFS). The EFS is a series of tightly packed growth rings in the bone that indicate the animal has ceased growing. Napoli explains that the presence of an EFS in the Nanotyrannus specimen confirms it was a mature individual, rather than a juvenile T. rex. This discovery significantly strengthens the argument that Nanotyrannus was a distinct genus cohabiting the same environment as T. rex, specifically in the Hell Creek Formation of Montana—a fossil-rich site dating to the final million years of the Cretaceous period.
Beyond bone maturity, the researchers employed high-resolution scans to study the Nanotyrannus skull and found it differed in several key anatomical features from T. rex. Notably, Nanotyrannus possessed more tooth sockets than any known T. rex specimen, regardless of age. Additionally, the paths of cranial nerves and sinuses within the skull were distinct, suggesting developmental differences that would have been fixed from early embryonic stages throughout the dinosaur’s life. These anatomical distinctions further support the classification of Nanotyrannus as a unique species.
To better understand its evolutionary context, the team performed computer-based phylogenetic analyses. These placed Nanotyrannus just outside the direct lineage of T. rex, forming a new clade named Nanotyrannidae. This group potentially originated in eastern North America, highlighting a previously unrecognized diversity among tyrannosaurs in this region during the Late Cretaceous. Among the notable discoveries from the Dueling Dinosaurs fossil was the first complete tail ever found for a Nanotyrannus, providing additional anatomical data to refine the species’ characterization.
The study also revisited two earlier important fossils from the Hell Creek Formation: a skull originally unearthed by researchers from the Cleveland Museum of Natural History that first helped define Nanotyrannus in 1988, and a specimen nicknamed “Jane,” discovered by the Burpee Museum of Natural History team. By comparing these fossils with the newly analyzed Dueling Dinosaurs specimen, Zanno and Napoli confirmed that the Dueling Dinosaurs fossil belongs to the species *Nanotyrannus lancensis*, named in 1988. They also identified “Jane” as a separate species within the genus, which they designated *Nanotyrannus lethaeus*.
This research builds on work going back decades, including observations by paleontologist Philip J. Currie, who co-authored the paper naming Nanotyrannus in 1988. Currie highlights the higher tooth count in Nanotyrannus as a clear differentiator from T. rex. The idea that multiple tyrannosaur species could coexist is not unprecedented; for example, in Asia, the large Tarbosaurus lived alongside the smaller, more agile Alioramus, with both likely dividing prey types between them.
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