Nanotyrannus: Dinosaur skeleton settles long debate over ‘tiny T. rex’ fossils

A dinosaur fossil thought to be a juvenile Tyrannosaurus rex is in fact a fully grown carnivore of a different species, according to researchers who believe they have finally settled a long-running and fierce debate in palaeontology.

The dispute stems from a skull found in Montana’s Hell Creek Formation in the 1940s, originally classified as a Gorgosaurus, then suggested to be a juvenile T. rex. In 1988, other researchers argued that the fossil was in fact an adult of a smaller, related species, which they named Nanotyrannus lancensis.

A number of additional fossils have since been identified as Nanotyrannus, but many palaeontologists believe they are really juvenile T. rex specimens.

Now, for the first time, researchers have analysed a complete skeleton that appears to show beyond doubt that Nanotyrannus is a separate species.

The skeleton is one of a pair from a fossil specimen nicknamed the “Duelling Dinosaurs”, which was discovered by commercial fossil hunters in 2006. The fossil features a Triceratops buried alongside what was originally thought to be a juvenile T. rex around 67 million years ago.

It wasn’t until 2020, when the fossil was purchased by the North Carolina Museum of Natural Sciences, that palaeontologists could comprehensively study the remains.

“When we acquired the specimen, we knew it was exceptional,” says Lindsay Zanno at the North Carolina Museum of Natural Sciences. “We had no idea it would turn decades of research on the world’s most famous dinosaur on its head.”

Zanno, who did the analysis with her colleague James Napoli at Stony Brook University in New York, says she had originally been an adherent of the juvenile T. rex theory, but the evidence has forced her to reconsider.

“Nanotyrannus has different nerve and sinus patterns in the skull, more teeth, large hands and a shorter tail,” she says. “We know that these traits do not change as animals grow from baby to adult.”

According to Zanno and Napoli, detailed analysis of limb bones of the dinosaur confirm it was a fully grown individual about 20 years old, weighing around 700 kilograms and measuring about 5.5 metres in length. “That’s about one-tenth the body mass and one-half the length of a fully grown Tyrannosaurus,” says Zanno.

Zanno and Napoli also reanalysed 200 tyrannosaur fossils and concluded that another near-complete skeleton from the Hell Creek Formation, known as Jane, which was thought to be a T. rex teenager, has also been incorrectly classified. They say Jane is actually a new species in the genus Nanotyrannus, which they call Nanotyrannus lethaeus.

“We only have one skeleton of N. lethaeus, but its anatomy suggests it was a larger species,” says Zanno. “The sinus patterns in the palate and the shape of the bone behind the eye are unique.”

Scott Persons at South Carolina State Museum says the new study resolves the debate about Nanotyrannus being a distinct genus and species.

“For my money, Nanotyrannus was one of the scariest dinosaur predators,” says Persons. “It’s the one I would least like to be chased by. It was extremely long-legged and armed with a wicked thumb claw.

“We can think of Nanotyrannus and Tyrannosaurus as analogues to modern cheetahs and lions. Yes, they have a generally similar body plan, but they were specialised for different ways of hunting.”

Thomas Carr at Carthage College in Wisconsin, who has long sat in the juvenile T.rex camp, says the new evidence is “pretty conclusive” that the Duelling Dinosaur specimen is a “near-adult of a species that is not T. rex”.

And Holly Ballard at Oklahoma State University, who led a 2020 study refuting the Nanotyrannus claims, says she is “fine” with the team’s conclusion that the fossil is of an individual approaching adult size.

But neither Ballard nor Carr is convinced that the other fossil, Jane, represents a new Nanotyrannus species. “Jane is still growing and is already bigger than N. lancensis, so to argue it’s a new taxon instead of a juvenile T. rex,” says Ballard. “We’re back to the same old debates.”

“In addition to that, if every small tyrannosaur from the Hell Creek Formation is Nanotyrannus, then where are the juvenile T. rex?” says Carr. “That part of the picture doesn’t add up. In terms of fossils, we simply haven’t collected enough Hell Creek Formation tyrannosaurs to truly understand what was going on with the early growth stages of T. rex.”

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