A new species of small carnivorous dinosaur that lived alongside Tyrannosaurus rex in the late Cretaceous of the Mesozoic has been identified. It had long been thought to be a juvenile Tyrannosaurus, but in fact it was a fully grown different dinosaur. The finding ends a debate that has continued in paleontology for nearly 40 years and completely overturns conventional wisdom about the growth of Tyrannosaurus rex as we knew it.
A North Carolina State University research team said on the 30th (local time) that dinosaur fossils once considered adolescent Tyrannosaurus rex were actually a fully grown, separate dinosaur species. The results were published the same day in the journal Nature.
The team analyzed the so-called "dueling dinosaurs" fossils excavated in Montana in 2006. At first, they thought it had become fossilized as it was, showing a horned Triceratops and a young Tyrannosaurus rex fighting 67 million years ago. Paleontologists, on the premise that it was a juvenile Tyrannosaurus rex, had studied this carnivore's growth and hunting behavior.
A fresh, detailed analysis of the bones showed that this dinosaur was an adult of a different species called "Nanotyrannus lancensis." Through growth rings in the bone that reveal age like tree rings, the degree of vertebral fusion, and skeletal anatomy, they confirmed the individual was about 20 years old at death and fully mature.
Nanotyrannus lancensis is a name attached to a small carnivorous dinosaur skull fossil unearthed in Montana in 1942. It was first classified as Gorgosaurus lancensis, a member of the genus Tyrannosaurus, and later presumed to be a juvenile Tyrannosaurus rex. Then in 1988, other scientists argued it was a new, smaller carnivorous dinosaur and gave it the name Nanotyrannus lancensis. Since then, scientists have debated whether Nanotyrannus is a separate species or a juvenile Tyrannosaurus.
Based on comparative fossils and growth patterns of the two species, the North Carolina State University team estimated that Nanotyrannus had a body length of 5–6 m, 35 caudal vertebrae, and weighed about 700 kg, making it much smaller than Tyrannosaurus. Tyrannosaurus is known to have a body length of 12–13 m, 40–45 caudal vertebrae, and weigh 6,700–8,200 kg.
The analysis found that fully grown Nanotyrannus lancensis had longer arms than Tyrannosaurus rex, more teeth, fewer caudal vertebrae, and different cranial nerve structures. These are characteristics that do not change during growth, leading to the conclusion that this dinosaur could not have been an immature Tyrannosaurus rex.
Lindsay Zanno of North Carolina State University said, "This discovery not only ends a debate, it upends the foundations of Tyrannosaurus rex research." Co-author James Napoli of Stony Brook University said, "If this dinosaur were an immature Tyrannosaurus rex, it would violate the very rules of vertebrate growth," adding, "That is impossible."
The team also found another species of Nanotyrannus, "Nanotyrannus lethaeus," while reexamining about 200 Tyrannosaurus-line fossils. The name comes from Lethe, the river of forgetfulness in Greek mythology, meaning a presence that had long been before our eyes but went unnoticed.
If the findings are correct, several carnivorous dinosaurs likely shared and competed within the same ecosystem in the late Cretaceous. Zanno said, "Even the massive, powerful Tyrannosaurus rex was not the absolute ruler at the time," adding, "The agile, fast Nanotyrannus shows it lived alongside and competed with it."
References
Nature (2025), DOI: https://doi.org/10.1038/s41586-025-09801-6