New research shows the Hell Creek "mini T. rex" is a separate species, Nanotyrannus lancensis. Bone growth rings indicate the animal was about 20 years old and nearly fully grown. Anatomical differences — proportionally larger forelimbs, more teeth and fewer tail vertebrae — distinguish it from T. rex. The finding forces a review of past studies that mixed Nanotyrannus and T. rex specimens and raises new questions about how two different predators shared the late Cretaceous landscape.
‘Mini T. rex’ Confirmed as Its Own Species — New Study Rewrites a 40-Year Debate
New research shows the Hell Creek "mini T. rex" is a separate species, Nanotyrannus lancensis. Bone growth rings indicate the animal was about 20 years old and nearly fully grown. Anatomical differences — proportionally larger forelimbs, more teeth and fewer tail vertebrae — distinguish it from T. rex. The finding forces a review of past studies that mixed Nanotyrannus and T. rex specimens and raises new questions about how two different predators shared the late Cretaceous landscape.
09:14, 04.11.2025Science
New study identifies the Hell Creek "mini" tyrant as Nanotyrannus lancensis
For more than four decades, paleontologists have debated whether a small skull discovered in Montana’s Hell Creek Formation belonged to a juvenile Tyrannosaurus rex or to a distinct species. A new paper brings fresh evidence: the specimen represents a separate species, Nanotyrannus lancensis, not a teenaged T. rex.
History of the specimen
The skull was first found in the 1940s and originally named Gorgosaurus lancensis because of its diminutive size. In 1988 it was reclassified as Nanotyrannus lancensis, but later researchers argued the bones were simply from a young T. rex. That interpretation influenced decades of research into how T. rex grew and matured.
What the new study found
The research team studied the rare "dueling dinosaurs" specimen at the North Carolina Museum of Natural Sciences — a fossil pair long thought to show a Triceratops locked in combat with a small T. rex. Detailed analysis reveals the combatant is a Nanotyrannus.
Scientists sectioned limb bones and examined growth rings (lines of arrested growth). Those rings indicate the animal was roughly 20 years old at death and nearly fully grown, not a juvenile. Anatomical comparisons further support species-level differences: relative to T. rex, the specimen has proportionally larger forelimbs, more teeth, and fewer caudal (tail) vertebrae. These traits are not consistent with an immature T. rex developing into an adult form.
“For it to change from that to an adult Tyrannosaurus would defy our knowledge of how vertebrates grow,” said Lindsay Zanno, co-author of the study. “It just doesn't hold up as a teen rex.”
Implications
Researchers estimate the Nanotyrannus weighed about 700 kg, far smaller than an adult T. rex (roughly 6,700–8,200 kg). The team proposes Nanotyrannus was a close relative that diverged from other tyrannosaurs around 100 million years ago. Because earlier studies of T. rex biology sometimes included material now attributed to Nanotyrannus, many conclusions about growth, ecology, and behavior may need reassessment.
Steve Brusatte, a leading paleontologist who previously treated similar small skeletons as juvenile T. rex, said the new specimen convincingly supports Nanotyrannus as a distinct, long-armed, smaller tyrannosaur.
Zanno framed the discovery as an opportunity rather than a setback: it opens new questions about how two differently built apex predators — one optimized for brute force, the other for speed — coexisted and competed in the final chapters of the Cretaceous.
Bottom line
The new analysis overturns a long-standing assumption that all small, Nanotyrannus-like fossils from Hell Creek are juvenile T. rex. It strengthens the case for Nanotyrannus lancensis as a distinct species and prompts a careful re-evaluation of studies that mixed data from both animals.