Debate continues over whether the fossilized remains of a small tyrannosaur (Nanotyrannus) were a distinct species or just a juvenile tyrannosaur. A new study claims to have found evidence that a diminutive Tyrannosaurus rex actually walked side by side in evolutionary history with its famous giant cousin, but other scientists aren't convinced.
Tyrannosaurus rex is one of the best-studied dinosaurs, and its massive size is certainly well-documented. However, since the 1940s, several smaller specimens have been discovered, and paleontologists have debated whether these constituted their own dwarf species (named Nanotyrannuslancensis), or were simply young T. rexes.
In 2020, scientists at Oklahoma State University conducted a study and found that the bone microstructure and proportions of two specimens indicated that they had not yet reached full size, thus concluding that they support the juvenile T. rex hypothesis. Like judging the age of a tree, the growth rings on the leg bones indicate they were 13 to 15 years old when they died - in the T. rex's adolescence.
But now, scientists from the University of Bath and the University of Chicago have re-examined several of these small specimens and come to the opposite conclusion - that the bones belong to a separate species. This is thanks to detailed analysis of the animals' growth rings, anatomy and newly discovered young T. rex fossils.
First, the researchers measured the growth rings of the baby T. rex's leg bones and found that the growth rings became tighter on the outside of the bone. This suggests that the dinosaurs' growth rate was slowing down, meaning they were approaching full size.
"If they were young Tyrannosaurus rex, they should have grown like crazy, gaining hundreds of kilograms a year, but we didn't see that," said study co-author Dr. Nick Longrich. "We tried a lot of different ways to model the data, but the growth rate we got was always very low."
Based on their modeling, the researchers estimate that Nanotyrannus could have grown up to 900 to 1,500 kilograms (1,984 to 3,307 pounds) and up to 5 meters (16.4 feet) long. This may not sound too "nano", but it was only 15% the size of an adult T. rex.
Growth rings weren't the only evidence the team used. They studied a series of Tyrannosaurus rex and Nanotiersaurus fossils and found more than 150 characteristics that appeared to differ between the species. In addition to the size of the animal itself, Nanotyrannus tended to have narrower snouts, smoother teeth, longer legs, and larger arms.
"Their arms were actually longer than T. rex," Longrich said. "Even the largest T. rex had shorter arms and claws than these little Nanotilansaurus. The arms of this animal are actually very formidable weapons. It's a completely different animal -- small, fast, agile. T. rex relied on size and strength, whereas this animal relied on speed."
Further evidence, the team says, comes from the lack of fossils that share characteristics of both Tyrannosaurus rex and Tyrannosaurus rex. If one grew into the other, the features should be on a sliding scale - but upon closer inspection, the team found no fossils with mixed features. They tend to fall into one of these categories, supporting the independent species hypothesis.
Finally, scientists claim to have identified a fossil of a young Tyrannosaurus rex - and it doesn't look like the fossils thought to be Nanotyrannus. The skull fragment, which has been gathering dust in a museum drawer for years, is estimated to be from an animal with a skull about 45 centimeters (17.7 inches) long and a total body length of about 5 meters. Upon closer inspection, the researchers found features consistent with Tyrannosaurus rex, but not identical to Nanotyrannus.
While the team says this looks like the end of the mini-T. rex hypothesis, other scientists don't necessarily agree.
Holly Woodward, author of the 2020 study on the young T. rex camp, told New Scientist: "I don't believe their explanation is more accurate than ours. Other experts say the team's newly discovered skull fragment belonging to the young T. rex is too incomplete to be certain."
This new chapter in the Nanotyrannus saga is fascinating, but it's unlikely to be the last. The research was published in the journal Fossil Studies.