Tiny T. rex fossils may be distinct species – but not everyone agrees

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Palaeontologists can’t agree on whether fossils from several small dinosaurs represent juvenile Tyrannosaurus rex or smaller adults of a separate species that lived alongside them

By James Dinneen

Illustration of a Nanotyrannus fighting with a young Tyrannosaurus rex

Raul Martin

Tyrannosaurus rex may not have been the only large carnivore reigning over North America during the Late Cretaceous after all. A reinterpretation of several fossils of small dinosaurs sometimes categorised as young T. rex adds weight to the controversial idea that another smaller species, called Nanotyrannus lancensis, lived alongside the king of the dinosaurs.

“This is the most famous fossil animal in the world,” says Nicholas Longrich at the University of Bath in the UK. “There are a lot of people looking at it. And we can’t agree.”

The debate has divided palaeontologists for decades. A 1960s paper argued that a skull unearthed two decades earlier in Montana’s Hell Creek Formation was from a T. rex that had died before reaching full maturity. But in the 1980s, others argued that differences between the skull and known T. rex specimens showed this fossil was in fact an adult of a distinct species, which they named Nanotyrannus lancensis.

More recent work based on additional fossils disputed this, arguing that the variation between the smaller fossils and full-size T. rex fossils were differences of age, not of species. The debate has implications for making sense of the ecology and diversity of dinosaurs in the period just before they went extinct.

Now, Longrich and Evan Saitta at the University of Chicago have brought together six lines of evidence in favour of identifying the smaller fossils as Nanotyrranus. This included cataloguing more than 150 discrete characteristics that differ between the fossils at issue and known T. rex fossils, including things like a narrower snout and smoother teeth. They argue it would be implausible for all of these to change between a juvenile and an adult T. rex.

The researchers also analysed patterns of growth rings in bones, which form as bones pause their growth each year. As an animal’s growth slows, the distance between the rings shrinks. The researchers found a tightening of these rings, which they say suggests the smaller fossils are young adult Nanotyrannus rather than juvenile T. rex. They estimate these adult Nanotyrannus would have weighed between 1000 to 2000 kilograms, up to a quarter of an adult T. rex’s 8000 kilograms. “Any way you graph that data you can’t get that animal to turn into a T. rex,” says Longrich.

The newest piece of evidence is a front-bone fossil – which sits between the eye and skull – that Longrich unearthed from an archive at the University of California Museum of Paleontology, which the researchers interpret as a juvenile T. rex because it differs in critical ways from the hypothesised Nanotyrannus fossils. “It’s an animal that’s smaller than the Nanotyrannus but it’s got the T. rex morphology,” says Longrich.

Some outside researchers say they remain unconvinced that the smaller fossils are indeed a separate species. “I have no problem with Nanotyrannus being a real thing if science shows that,” says Holly Woodward at Oklahoma State University, who authored a 2020 study of growth rings that pointed to the juvenile T. rex explanation. “I’m not convinced that their interpretation is more accurate than ours,” she says, adding that a specimen of a fully-grown Nanotyrannus would be needed to resolve the different interpretations.

Thomas Carr at Carthage College in Wisconsin, who has long argued that the fossils are juvenile T. rex, adds that the front-bone found in Berkeley is too incomplete to sway him. “I don’t take this seriously at all,” he says.

Scott Persons at the College of Charleston in South Carolina is more welcoming of what he says is a new perspective on a long-stagnant divide among palaeontologists. “This new paper will not resolve the debate, but I am optimistic that this paper will shake things up a great deal,” he says.

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