New analysis of three Montana fossils (~67 million years old) shows Nanotyrannus was a mature, separate genus rather than a juvenile T. rex. Key differences include more teeth, a crest above the eyes, an air sinus in the rear skull and a vestigial third finger. At roughly one-tenth the mass and half the linear size of T. rex, Nanotyrannus appears adapted for speed and pursuit. The team identifies two species and says past studies that treated these fossils as teenage T. rex should be re-evaluated.
Nanotyrannus: A Distinct, Agile Tyrannosaur — Not a Teenage T. rex
New analysis of three Montana fossils (~67 million years old) shows Nanotyrannus was a mature, separate genus rather than a juvenile T. rex. Key differences include more teeth, a crest above the eyes, an air sinus in the rear skull and a vestigial third finger. At roughly one-tenth the mass and half the linear size of T. rex, Nanotyrannus appears adapted for speed and pursuit. The team identifies two species and says past studies that treated these fossils as teenage T. rex should be re-evaluated.
22:22, 03.11.2025Science
Nanotyrannus was a distinct, agile tyrannosaur
New research published in Nature concludes that Nanotyrannus, an active predator that lived in western North America near the end of the Cretaceous, was not merely a juvenile Tyrannosaurus rex but a separate genus with its own species.
The team analyzed three fossil specimens recovered in Montana in 1942, 2001 and 2006, dated to roughly 67 million years ago. Microstructural features of the bones, including annual growth rings, indicate these individuals were mature adults rather than juveniles of T. rex.
Although both animals belong to the tyrannosaur lineage of meat-eating dinosaurs, researchers identified several consistent anatomical differences. Nanotyrannus had more teeth, a small crest above the front of the eyes, an air sinus in a skull bone near the rear of the head, and a vestigial third finger — traits absent in the two-fingered T. rex.
Size and body plan also diverged sharply: Nanotyrannus weighed roughly one-tenth the mass of T. rex — about 1,500 lb (700 kg) versus roughly 15,000 lb (7,000 kg) — and measured about half the linear dimensions (approximately 18 ft / 5.5 m long vs. ~40 ft / 12.2 m). Where T. rex was built for brute force, with a massive skull and thick, banana-shaped teeth, Nanotyrannus appears adapted for speed and pursuit: long legs, a narrow snout with blade-like teeth, and relatively stronger arms for manipulating prey.
“T. rex was a massive predator adapted to wielding incredible bite forces. Nanotyrannus was a slender, agile pursuit predator that could have run circles around the tyrant king,” said Lindsay Zanno, lead author and paleontologist at North Carolina State University.
The study also recognizes two species within the genus: the previously named Nanotyrannus lancensis and a newly designated species, Nanotyrannus lethaeus. One of the fossils analyzed is part of the well-known "Dueling Dinosaurs" specimen, which preserves a predator and prey locked in combat at the time of death.
These findings have important implications. For decades some researchers treated Nanotyrannus specimens as juvenile T. rex, using them to model growth and life history in the tyrant king. Because the Montana specimens are now shown to be adults of a separate genus, prior studies that relied on that assumption should be revisited.
Finally, the coexistence of Nanotyrannus, T. rex and many herbivorous species on the same late-Cretaceous landscapes supports growing evidence that dinosaur diversity remained high up to the asteroid impact about 66 million years ago, rather than declining long beforehand.