New research shows 'juvenile' T. rex fossils are a distinct species of small tyrannosaur

According to a recent research, fossils that were previously thought to be juvenile Tyrannosaurus rex were really adults of a smaller tyrannosaur with larger arms, longer legs, and narrower jaws. After being identified decades ago, the species Nanotyrannus lancensis was subsequently reinterpreted as a juvenile Tyrannosaurus rex.

Paleontologists have debated for decades whether the 1942 discovery of the first Nanotyrannus skull in Montana was a new species or merely a juvenile of the much bigger T. rex.

Reexamining the fossils, Drs. Nick Longrich of the Milner Centre for Evolution at the University of Bath and Evan Saitta of the University of Chicago examined growth rings, Nanotyrannus's morphology, and an as-yet-undiscovered fossil of a juvenile Tyrannosaurus rex.

Researchers measured the growth rings of Nanotyrannus bones and found that as development slowed, the rings were closer together near the exterior of the bone. It implies that these animals were not rapidly developing juveniles but rather were almost at full size.

Based on growth models, the animals' greatest size was estimated to be between 900 and 1,500 kg and five meters, or around 15% of the massive T. rex's size, which reached up to 8,000 kg and nine meters or more.

The results of the study have been reported in Fossil Studies.

Longrich remarked, "I was pretty blown away when I saw these results." "It surprised me because it was so definitive. We're not seeing them develop like rapidly, gaining hundreds of kilos a year, if they were juvenile Tyrannosaurus rex. Despite our best efforts, we were unable to achieve satisfactory growth rates no matter how we modelled the data. The theory that these creatures are juvenile Tyrannosaurus rex appears to be coming to an end."

The lack of fossils that combined traits from Nanotyrannus and T. rex—which would exist if one species evolved into the other—supported the presence of separate species. They could positively identify each fossil they looked at as belonging to a particular species.

The idea that they were juvenile Tyrannosaurus rex was further refuted by the development trends in other tyrannosaurs.

"Jungles of other tyrannosaurs exhibit many of the distinguishing characteristics of the adults," Dr. Longrich said. A very young Tarbosaurus, a close cousin of Tyrannosaurus rex, has characteristics unique to the adults. Just as kittens resemble cats and puppies resemble dogs, so too are the juveniles of certain tyrannosaurs unique. Furthermore, Nanotyrannus doesn't resemble a Tyrannosaurus rex at all. It's possible that it is developing in a manner that sets it apart from all other dinosaurs, including tyrannosaurs, but it's more likely that it is simply not a T. rex."

However, this begs the question, why hasn't a young Tyrannosaurus rex been discovered if Nanotyrannus isn't a juvenile?

That has always been a major concern. As it happens, we had discovered one," Longrich remarked. "But the fossil was collected years ago, stuck in a box of unidentified bones in a museum drawer, and then forgotten."

Through their investigation, Longrich and co-author Evan Saitta were able to identify a juvenile Tyrannosaurus from a prior specimen found and kept in a San Francisco museum.

The frontal bone of the baby Tyrannosaurus rex's skull is a representation of it; it has unique characteristics that identify it as a Tyrannosaurus but are absent from Nanotyrannus. It originates from a small mammal, around five meters in length and with a 45 centimeter long skull.

"Yes, it's just one specimen and one bone, but it only takes one," Dr. Longrich stated. None of the other skull bones resembles the unique appearance of T. rex skull bones. Young Tyrannosaurus rex are real, they're simply extremely rare, much as other dinosaur juveniles."

These results, according to the researchers, provide compelling evidence that Nanotyrannus is a distinct species that is not closely linked to Tyrannosaurus. Compared to its densely packed sibling, it had longer legs and a lighter build. Also, in contrast to the well-known T. rex with short arms, it possessed longer arms.

"In actuality, the arms are longer than T. rex's. The limbs and claws of even the largest Tyrannosaurus rex are shorter than those of these tiny Nanotyrannus. The arms of this animal were really rather strong weapons. It's basically simply a tiny, swift, and nimble mammal all together. Whereas T. rex depended on power and bulk, this animal depended on speed."

It may have existed outside of the Tyrannosauridae family, of which T. rex is a member, in its own family of predatory dinosaurs, as shown by its long arms and other characteristics, which indicate that it was only distantly related to T. rex.

The new research is the most recent in a decades-long sequence of papers on the issue.

"Nanotyrannus is highly controversial in paleontology," Longrich stated. It felt like we'd figured out this problem with the baby T. rex not too long ago.

"I was very skeptical about Nanotyrannus myself until about six years ago when I took a close look at the fossils and was surprised to realize we'd gotten it wrong all these years."

The authors speculate that we may be underestimating the variety of dinosaurs and other ancient animals since it is difficult to distinguish between them based just on their sometimes fragmentary bones.

"It's astounding to consider how much we still don't know about the most well-known of all the dinosaurs," commented Longrich. It begs the question, "What else have we done wrong?"

Provided by University of Bath