A complete dinosaur skeleton has provided strong evidence that fossils long debated as juvenile Tyrannosaurus rex belong to a separate species called Nanotyrannus. Researchers analyzed a specimen from the 'Duelling Dinosaurs' site, confirming it as a fully grown individual distinct from T. rex. The findings challenge decades of palaeontological arguments.
The debate over whether certain small tyrannosaur fossils from the late Cretaceous period represent young Tyrannosaurus rex or a distinct species has persisted for decades. It began with a skull discovered in Montana's Hell Creek Formation in the 1940s, initially classified as Gorgosaurus and later suggested to be a juvenile T. rex. In 1988, researchers proposed it as an adult of a new species, Nanotyrannus lancensis, though many experts continued to view such fossils as immature T. rex.
A breakthrough came with the analysis of a complete skeleton from the 'Duelling Dinosaurs' specimen, unearthed in 2006 by commercial fossil hunters. This fossil, featuring a Triceratops buried alongside the tyrannosaur about 67 million years ago, was acquired in 2020 by the North Carolina Museum of Natural Sciences. "When we acquired the specimen, we knew it was exceptional," says Lindsay Zanno at the museum. "We had no idea it would turn decades of research on the world's most famous dinosaur on its head."
Zanno and colleague James Napoli at Stony Brook University examined the remains, finding distinct features: different nerve and sinus patterns in the skull, more teeth, larger hands, and a shorter tail—traits that do not change with growth. The individual was fully grown at around 20 years old, weighing 700 kilograms and measuring 5.5 meters long, about one-tenth the mass and half the length of an adult T. rex.
The team reanalyzed over 200 tyrannosaur fossils, reclassifying the near-complete 'Jane' specimen from Hell Creek as a new species, Nanotyrannus lethaeus. "We only have one skeleton of N. lethaeus, but its anatomy suggests it was a larger species," Zanno notes, citing unique sinus patterns and bone shapes.
Experts largely welcome the evidence for Nanotyrannus as distinct. Scott Persons at the South Carolina State Museum calls it "one of the scariest dinosaur predators," comparing it to a cheetah versus T. rex's lion-like build. Thomas Carr at Carthage College deems the Duelling Dinosaurs evidence "pretty conclusive" for a non-T. rex species but questions Jane's classification, noting the scarcity of confirmed juvenile T. rex fossils. Holly Ballard at Oklahoma State University accepts the specimen's near-adult size but argues Jane was still growing and larger than N. lancensis, reigniting old debates.
The study, published in Nature, underscores the need for more Hell Creek tyrannosaur collections to clarify early T. rex growth.