Tyrannosaurus rex might not have been the one massive carnivore reigning over North America in the course of the Late Cretaceous in any case. A reinterpretation of a number of fossils of small dinosaurs typically categorised as younger T. rex provides weight to the controversial concept that one other smaller species, referred to as Nanotyrannus lancensis, lived alongside the king of the dinosaurs.

“That is essentially the most well-known fossil animal on the earth,” says Nicholas Longrich on the College of Bathtub within the UK. “There are lots of people taking a look at it. And we will’t agree.”

The talk has divided palaeontologists for many years. A Nineteen Sixties paper argued {that a} cranium unearthed twenty years earlier in Montana’s Hell Creek Formation was from a T. rex that had died earlier than reaching full maturity. However within the Nineteen Eighties, others argued that variations between the cranium and identified T. rex specimens confirmed this fossil was in truth an grownup of a definite species, which they named Nanotyrannus lancensis.

Newer work based mostly on extra fossils disputed this, arguing that the variation between the smaller fossils and full-size T. rex fossils had been variations of age, not of species. The talk has implications for making sense of the ecology and variety of dinosaurs within the interval simply earlier than they went extinct.

Now, Longrich and Evan Saitta on the College of Chicago have introduced collectively six strains of proof in favour of figuring out the smaller fossils as Nanotyrranus. This included cataloguing greater than 150 discrete traits that differ between the fossils at concern and identified T. rex fossils, together with issues like a narrower snout and smoother tooth. They argue it might be implausible for all of those to vary between a juvenile and an grownup T. rex.

The researchers additionally analysed patterns of progress rings in bones, which type as bones pause their progress annually. As an animal’s progress slows, the space between the rings shrinks. The researchers discovered a tightening of those rings, which they are saying suggests the smaller fossils are younger grownup Nanotyrannus fairly than juvenile T. rex. They estimate these grownup Nanotyrannus would have weighed between 1000 to 2000 kilograms, as much as 1 / 4 of an grownup T. rex’s 8000 kilograms. “Any method you graph that information you may’t get that animal to show right into a T. rex,” says Longrich.

The latest piece of proof is a front-bone fossil – which sits between the attention and cranium – that Longrich unearthed from an archive on the College of California Museum of Paleontology, which the researchers interpret as a juvenile T. rex as a result of it differs in important methods from the hypothesised Nanotyrannus fossils. “It’s an animal that’s smaller than the Nanotyrannus nevertheless it’s acquired the T. rex morphology,” says Longrich.

Some outdoors researchers say they continue to be unconvinced that the smaller fossils are certainly a separate species. “I’ve no drawback with Nanotyrannus being an actual factor if science reveals that,” says Holly Woodward at Oklahoma State College, who authored a 2020 research of progress rings that pointed to the juvenile T. rex rationalization. “I’m not satisfied that their interpretation is extra correct than ours,” she says, including {that a} specimen of a fully-grown Nanotyrannus can be wanted to resolve the totally different interpretations.

Thomas Carr at Carthage Faculty in Wisconsin, who has lengthy argued that the fossils are juvenile T. rex, provides that the front-bone present in Berkeley is just too incomplete to sway him. “I don’t take this severely in any respect,” he says.

Scott Persons on the Faculty of Charleston in South Carolina is extra welcoming of what he says is a brand new perspective on a long-stagnant divide amongst palaeontologists. “This new paper won’t resolve the talk, however I’m optimistic that this paper will shake issues up an excellent deal,” he says.