For decades, paleontologists have debated whether Nanotyrannus lancensis, a smaller relative of the fearsome Tyrannosaurus rex, was a separate species or simply a juvenile T. rex. A groundbreaking new study, bolstered by an exceptionally complete fossil, definitively establishes Nanotyrannus as a distinct species, forever changing our understanding of tyrannosaur evolution and raising new questions about T. rex biology.
A Long-Standing Debate
The controversy stems from a collection of smaller tyrannosaur fossils discovered over the years in western North America. Many paleontologists initially believed these were young T. rex specimens. However, the discovery of a small skull in Montana in 1942, given the name Nanotyrannus lancensis in 1988, fueled the debate, as scientists had limited information about the rest of the animal’s body. This initial finding, based on a single skull, only deepened the mystery.
The Dueling Dinosaurs Discovery
The pivotal evidence comes from the remarkable “Dueling Dinosaurs” fossil—a 67-million-year-old Nanotyrannus skeleton and a Triceratops locked in what appears to be a death struggle. This find, unearthed in the Hell Creek Formation, offered scientists the best opportunity yet to study an adult Nanotyrannus specimen. Lindsay Zanno, a paleontologist at North Carolina State University and North Carolina Museum of Natural Sciences, emphasized the significance of this discovery: “This has been one of the most controversial topics in all of dinosaur paleontology.”
Distinct Features and Growth Stage
Zanno and co-author James Napoli’s analysis revealed that the “Dueling Dinosaurs” Nanotyrannus was approximately 20 years old and nearing full growth—contrasting with the idea that it was a juvenile T. rex. The study found that this Nanotyrannus weighed only about 1,500 pounds (700 kilograms), dramatically smaller than an adult T. rex, which could reach 14,700 to 18,000 pounds (6,700 to 8,200 kg). Crucially, the researchers noted significant differences beyond size, including larger forelimbs, a higher tooth count, fewer tail vertebrae, and distinct patterns in the skull nerves.
A Different Ecological Niche
The findings suggest that Nanotyrannus occupied a different ecological niche than T. rex. While T. rex was a bulky predator with a powerful bite and banana-shaped teeth, Nanotyrannus was smaller, sleeker, and more agile, possessing enlarged hands and claws likely used for prey capture.
The Wider Scientific Response
The findings have largely convinced the scientific community. Dave Hone, a paleontologist at Queen Mary University of London, called the evidence “pretty solid,” while Steve Brusatte, a paleontologist at the University of Edinburgh, declared that the case for Nanotyrannus is “proven beyond a reasonable doubt.” Even Thomas Carr, who previously argued that all the fossils were juvenile T. rex, has changed his stance.
The Jane Skeleton and a Possible New Species
Further complicating the picture is the examination of the “Jane” skeleton—another tyrannosaur fossil from the Hell Creek Formation. Zanno and Napoli argue that Jane differs significantly from both T. rex and the “Dueling Dinosaurs” Nanotyrannus, possibly representing a new species, Nanotyrannus lethaeus. Jane was slightly larger and had unique sinus patterns and a differently shaped bone behind the eye. However, many researchers remain unconvinced about Jane being a distinct species, suggesting it could be a juvenile T. rex.
Rethinking Tyrannosaur Life History
The discovery of a distinct Nanotyrannus species prompts a fundamental re-evaluation of T. rex life history and growth patterns. The lack of juvenile T. rex specimens necessitates a shift in research models, potentially relying on the developmental patterns of Tarbosaurus bataar, a closely related tyrannosaur from Mongolia, where a greater range of developmental stages is documented.
“The overarching mic drop of this paper is that Nanotyrannus is real, its own distinct tyrannosaur species, and that necessitates a fundamental reassessment of tyrannosaur classification and evolution,” Brusatte said.
The new study illuminates that multiple tyrannosaur species coexisted in western North America during the final million years before the asteroid impact. Ultimately, the discovery of Nanotyrannus underscores the complexity of tyrannosaur evolution and challenges long-held assumptions about the life cycle and biology of the “king of the dinosaurs.”
