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Juvenile 'T. rex' fossils belong to a distinct tyrannosaur species 

A groundbreaking study reevaluates previously identified juvenile T. rex fossils, uncovering they were actually adult specimens of a distinct, smaller tyrannosaur species, Nanotyrannus lancensis. This species, initially named decades prior, was thought to be young T. rex due to similarities in appearance.

Decades of debate

The controversy around Nanotyrannus began with the unearthing of its first skull in Montana in 1942. The scientific community has been divided on its classification, oscillating between a unique tyrannosaur species and a T. rex juvenile. 

This debate led Dr. Nick Longrich from the University of Bath and Dr. Evan Saitta from the University of Chicago to reexamine these fossils, analyzing growth rings, Nanotyrannus anatomy, and a previously unnoticed young T. rex fossil.

Measuring the growth rings

The research, published in the journal Fossil Studies, revealed a crucial detail in the Nanotyrannus bones’ growth rings. 

Measuring the growth rings in Nanotyrannus bones, the experts showed that they became more closely packed towards the outside of the bone, meaning that its growth was slowing, and suggesting that these animals were nearly full size rather than  fast-growing juveniles. 

Conclusive results

In terms of size, Nanotyrannus was significantly smaller than T. rex, with projections indicating a maximum weight of 900-1500 kilograms and length of five meters. In contrast, T. rex could reach up to 8,000 kilograms and over nine meters.

“When I saw these results I was pretty blown away. I didn’t expect it to be quite so conclusive,” Longrich said. “If they were young T. rex they should be growing like crazy, putting on hundreds of kilograms a year, but we’re not seeing that. We tried modeling the data in a lot of different ways and we kept getting low growth rates. This is looking like the end for the hypothesis that these animals are young T. rex.”

Further supporting Nanotyrannus as a separate species, no fossils exhibited a mix of traits from both Nanotyrannus and T. rex. Thus, every fossil they examined could be confidently identified as one species or the other.

Distinctive Tyrannosaur features

The mystery of rare young T. rex fossils was also addressed. A fossil, previously unclassified, was identified in a museum in San Francisco as a juvenile Tyrannosaurus. 

“That young T. rex is represented by a skull bone – the frontal bone – with distinctive features that ally it with Tyrannosaurus, but which aren’t seen in Nanotyrannus. It comes from a small animal, one with a skull about 45 cm long and a body length of around 5 meters,” Longrich explained.

Study implications 

In conclusion, this study presents strong evidence of Nanotyrannus as a distinct species, not closely related to Tyrannosaurus. The differences in physical characteristics, such as larger arms and a lighter build, suggest a separate evolutionary path. 

“It’s amazing to think how much we still don’t know about the most famous of all the dinosaurs. It makes you wonder what else we’ve gotten wrong,” Longrich concluded.

Image Credit: Andrey Atuchin

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