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

A new analysis of fossils thought to be T. rex juveniles now shows that they were adults of a small tyrannosaur, with narrower jaws, longer legs and larger arms than those of the T. rex. rex. The species, Nanotyrannus lancensis, was first named decades ago, but later reinterpreted as a juvenile T. rex.

The first Nanotyrannus skull was discovered in Montana in 1942, but for decades paleontologists debated whether it was a separate species or simply a juvenile of the much larger T. rex.

Dr Nick Longrich, from the Milner Center for Evolution at the University of Bath, and Dr Evan Saitta, from the University of Chicago, reanalyzed the fossils, examining growth rings, the anatomy of Nanotyrannus and an individual hitherto unknown. fossil of a young T. rex.

By measuring the growth rings of Nanotyrannus bones, the researchers showed that they grew closer to the outside of the bone as growth slowed. This suggests that these animals were almost life-size; not fast-growing juveniles.

Growth modeling of the fossils showed that the animals would have reached a maximum of about 900 to 1,500 kilograms and five meters, or about 15 percent of the size of the giant T. rex, which reached 8,000 kilograms and nine meters or more.

The researchers published their results in Fossil studies.

“When I saw these results, I was blown away,” Longrich said. “I didn’t expect it to be this conclusive. If these were young T. rexes, they would have to grow like crazy, gaining hundreds of kilos a year, but we don’t see that. We tried to model the data in a way that

Supporting the existence of distinct species, the researchers found no evidence of fossils combining the characteristics of Nanotyrannus and T. rex, which would exist if one evolved into the other. Each fossil examined could be identified with certainty as belonging to one species or another.

The growth patterns of other tyrannosaurs were also inconsistent with the hypothesis that they were young T. rexes.

Dr Longrich said: “If you look at juveniles of other tyrannosaurs, they have many of the distinctive features of adults. A very young Tarbosaurus, a close relative of T. rex, exhibits distinctive features from adults. In the same way. kittens look like cats and puppies look like dogs, the juveniles of different tyrannosaurs are distinctive. And Nanotyrannus just doesn’t look like a T. rex. It might grow in a way that’s completely unlike any other tyrannosaur, or any other dinosaur, but it’s more likely that it just isn’t a T. rex. »

But this raises a mystery: If Nanotyrannus is not a juvenile T. rex, then why has no one ever found a young T. rex?

“That was always one of the big questions. Well, it turns out we had found one,” Longrich said. “But the fossil was collected years ago, stuck in a box of unidentified bones in a museum drawer, and then forgotten.”

The research led Longrich and co-author Evan Saitta to an earlier fossil discovery, stored in a San Francisco museum, that they identified as a juvenile tyrannosaurus.

This young T. rex is represented by a skull bone – the frontal bone – with distinctive characteristics that associate it with Tyrannosaurus, but which are not visible in Nanotyrannus. It is a small animal with a skull approximately 45 cm long and a body length of approximately 5 meters.

Dr Longrich said: “Yes, it’s only one specimen and one bone, but it only takes one. The skull bones of T. rex are very distinctive; nothing else is like them. Young T. rexes exist, they are just incredibly rare, like juveniles of most dinosaurs.

The researchers say these results provide strong evidence that Nanotyrannus is a distinct species, not closely related to Tyrannosaurus. It was lighter and had longer limbs than its stocky cousin. It also had larger arms, unlike the famous short-armed T. rex.

“The arms are actually longer than those of the T. rex. Even the largest T. rex has shorter arms and smaller claws than these smaller Nanotyrannus. This was an animal whose arms were actually pretty formidable weapons. It’s really just a completely different animal: small, fast, agile. T. rex relied on size and strength, but this animal relied on speed.

The long arms and other features suggest that it was only distantly related to T. rex and may have been outside the family Tyrannosauridae, of which T. rex is a member, in its own family of predatory dinosaurs.

The new study is the latest in a series of publications on the problem, dating back decades.

Longrich said: “Nanotyrannus is very controversial in paleontology. Not too long ago, it seemed like we finally had this problem sorted, and it involved a young T. rex.

“I was very skeptical about Nanotyrannus myself until about six years ago, when I looked closely at the fossils and was surprised to realize that we had been wrong all these years. “

The authors suggest that given the difficulty of distinguishing dinosaurs based on their often incomplete skeletons, we may be underestimating the diversity of dinosaurs and other fossil species.

Longrich said: “It’s amazing to think how much we still don’t know about the most famous of all dinosaurs. It makes you wonder what else we could have done.”