At academic conferences and inside the pages of scientific journals, palaeontologists have long argued about the features of the iconic Tyrannosaurus rex.
But a new study, published today in Nature, has the potential to finally put their argument to rest.
Historically, most dinosaur experts believed there was only one species of tyrannosaur roaming around North America between 69 – 66 million years ago in the late Cretaceous period.
Then in the 1940s, palaeontologists dug up a small skull that appeared come from a juvenile T. rex.
In the years since, there has been debate over whether the small dinosaur was actually a T. rex or another type of dinosaur completely.
Now, a new analysis of a controversial fossil known as "Duelling Dinosaurs" suggests the small tyrannosaur belongs to Nanotyrannus or "tiny terrors".
According to Lindsay Zanno, a palaeontologist at North Carolina State University, the topic was "so controversial" the research team needed multiple lines of evidence to confirm their findings.
"The debate has raged for decades, largely due to the lack of well-preserved skeleton of Nanotyrannus lancensis," Dr Zanno, one of the study's lead authors, said.
"This ... doesn't just settle the debate. It flips decades of T. rex research on its head."
The Duelling Dinosaurs
Scientific debate over the true nature of small, T. rex-like fossils has been raging since 1988, according to Curtin University palaeontologist Stephen Poropat.
A number of these "juvenile" skeletons have been found in the famous Hell Creek Formation in Montana, which is also where many adult T. rex have also been found.
"In many ways that debate has been coloured by the fact that Nanotyrannus comes from the same area and same-aged rocks as Tyrannosaurus rex," Dr Poropat, who was not involved in the study, said.
"Also because it's relatively closely related to Tyrannosaurus rex, and it looks superficially similar."
It was widely believed that the T. rex underwent a rapid period of growth around the age of 15, expanding to its final body weight of up to 8,000 kilograms.
"I had thought that the Tyrannosaurus going through this massive change was quite cool, if unusual … and yet it did seem to make sense," Dr Poropat said.
While other researchers tried in the past to make the case these smaller specimens were another species, it had been difficult as many fossils were incomplete, with some being just skulls or teeth.
That changed with the Duelling Dinosaurs.
The large fossil, which was first discovered in 2006, features two dinosaurs — an almost complete Triceratops horridus and a tyrannosaur known as "Bloody Mary".
The tyrannosaur would have been much smaller than a adult T. rex, at around 5 metres tall and weighing only around 700 kilograms.
While Bloody Mary was originally thought to be a juvenile T. rex, Dr Zanno said it was now clear this was not the case.
"The Duelling Dinosaurs tyrannosaur is the first complete skeleton of Nanotyrannus lancensis," she said.
"It was obvious from the beginning that the specimen didn't match expectations for a juvenile T. rex."
How do we know it's not a T. rex?
When the team began investigating the fossil in 2021, they found multiple reasons it didn't fit the criteria for a juvenile T. rex.
The first issue was its age.
Dr Zanno said growth rings found on the dinosaur's femur bone in its leg suggested it was around 20 years old, and "nearly fully grown" when it died.
"When growth rings are far apart, an animal is growing quickly, and as the animal approaches maturity and growth slows down, the lines become compacted," she said.
"Similarly, parts of the spine of our Nanotyrannus lancensis specimen had already begun to fuse, which would not happen if the animal was on the precipice of the extreme growth spurt."
They also showed that Nanotyrannus had more teeth, and that its skull nerve and skull sinuses were hugely different from larger T. rex — rendering them, from a biological perspective, virtually incompatible.
For Dr Poropat, one of the best examples showing the specimen was not a juvenile T. rex was simply the length of its arms.
"In absolute terms, proportions of Nanotyrannus' forearms are more elongated than the arms of much larger specimens of Tyrannosaurus rex," he said.
"It would be like a human child having longer arms as a child than as an adult."
It's not the first time that some of these points have been put forward.
In 2013, palaeontologist Pete Larson noted at the Society of Vertebrate Paleontology conference that both the tyrannosaur's arm length and teeth count indicated the specimen was Nanotyrannus, not Tyrannosaurus.
However, this was controversial at the time, as the fossil was being privately held, and research couldn't be done to confirm or deny Larson's assertion.
Now that the fossil is in scientific hands, this paper is able to provide a more detailed argument.
"I think they have demonstrated very convincingly that this is a different species to Tyrannosaurus rex," Dr Poropat said.
Is this the final word?
Dr Poropat notes that while "it would be very nice to believe this is the final word on it," other palaeontologic debates suggest there might be further discussion down the track.
Dr Zanno also noted more research was needed to be done to uncover which fossils were Nanotyrannus and which were T. rex, now there was an "anatomical road map" to confirm which species is which.
"There are at least four Nanotyrannus skeletons in museums in the US and several juvenile T. rex as well," she said.
"Now that we know Nanotyrannus is real, we need to learn about its biology. We know virtually nothing about this animal.
"It's an exciting time to study tyrannosaurs!"
