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

Illustration of a Nanotyrannus fighting with a young Tyrannosaurus rex Raul Martin 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 adds weight to the controversial idea that another smaller species, called , lived alongside the king of the dinosaurs. Read more “This is the most famous fossil animal in the world,” says at the University of Bath in the UK.

“There are a lot of people looking at it. And we can’t agree. ” Advertisement The for decades.

A 1960s paper argued that a skull unearthed two decades earlier in Montana’s Hell Creek Formation was from a that had died before reaching full maturity. But in the 1980s, others argued that differences between the skull and known specimens showed this fossil was in fact an adult of a distinct species, which they named More recent work based on additional fossils disputed this, arguing that the variation and full-size 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 .

Now, Longrich and at the University of Chicago have brought together six lines of evidence in favour of identifying the smaller fossils as . This included cataloguing more than 150 discrete characteristics that differ between the fossils at issue and known 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 .

Sign up to our Wild Wild Life newsletter A monthly celebration of the biodiversity of our planet’s animals, plants and other organisms. 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 rather than juvenile They estimate these adult would have weighed between 1000 to 2000 kilograms, up to a quarter of an adult 8000 kilograms. “Any way you graph that data you can’t get that animal to turn into a ,” 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 because it differs in critical ways from the hypothesised fossils.

“It’s an animal that’s smaller than the but it’s got the morphology,” says Longrich. Read more Some outside researchers say they remain unconvinced that the smaller fossils are indeed a separate species. “I have no problem with being a real thing if science shows that,” says at Oklahoma State University, who a 2020 study of growth rings that pointed to the juvenile explanation.

“I’m not convinced that their interpretation is more accurate than ours,” she says, adding that a specimen of a fully-grown would be needed to resolve the different interpretations. at Carthage College in Wisconsin, who has long argued that the fossils are juvenile , adds that the front-bone found in Berkeley is too incomplete to sway him. “I don’t take this seriously at all,” he says.

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. Journal reference Topics:.