Fossils previously thought to belong to a juvenile tyrannosaurus rex have been identified as a fully mature carnivore of a distinct species, resolving a long-standing debate in paleontology.

This controversy originated from a skull unearthed in the Hell Creek Formation in Montana during the 1940s, which was initially identified as gorgosaurus. It was later proposed to be a juvenile tyrannosaurus. In 1988, other researchers posited that the fossil represented an adult of a smaller related species, which they designated Nanotyrannus lansensis.

Since then, a number of additional fossils classified as Nanotyrannus have been discovered, although many paleontologists contend that they are merely tyrannosaurus juveniles.

Now, researchers have examined a complete skeleton for the first time, providing compelling evidence that Nanotyrannus is indeed a separate species.

The skeleton is among a pair of specimens known as “Dueling Dinosaurs,” found by private fossil hunters in 2006, which includes a triceratops and what was initially believed to be a juvenile T. rex buried approximately 67 million years ago.

It was only in 2020, when the fossil came into the possession of the North Carolina Museum of Natural Sciences, that paleontologists could conduct a thorough analysis of it.

“Upon receiving the specimen, we recognized it was extraordinary,” remarks Lindsey Zanno from the North Carolina Museum of Natural Science. “We had no inkling it would radically alter decades of research concerning the world’s most renowned dinosaur.”

Mr. Zanno collaborated with his colleagues, including James Naples, a postdoctoral researcher at Stony Brook University in New York. She had initially supported the juvenile tyrannosaurus theory but was compelled to reevaluate her stance based on the findings.

“Nanotyrannus displays unique nerve and sinus patterns, a greater number of teeth, larger hands, and shorter tails. These traits remain consistent as the species develops from juvenile to adult,” she notes.

Zanno and Napoli’s examination of the dinosaur’s limb bones confirmed that it was a fully mature specimen, approximately 20 years old, weighing around 700 kilograms and measuring about 5.5 meters in length. “This is roughly one-tenth the weight of an adult human and half the length of a tyrannosaurus,” Zanno explains.

Additionally, Zanno and Napoli reanalyzed 200 tyrannosaur fossils and concluded that another nearly complete skeleton, known as Jane from the Hell Creek Formation, is misclassified as a tyrannosaurid fossil. They propose that Jane is actually a new species within this genus, designated Nanotyrannus letaeus.

“Although we possess only one skeleton of N. Retheus, its anatomy suggests it was part of a larger species,” remarks Zanno. “The configuration of the palatal sinuses and the shape of the bone behind the eye are distinctive.”

Scott Parsons, a researcher at the South Carolina State Museum, asserts that this latest study clarifies the debate surrounding Nanotyrannus being its own genus and species.

“In my opinion, Nanotyrannus was among the most formidable dinosaur predators, and being pursued by one would be quite a terrifying experience,” Parsons remarks, noting its long legs and fearsome thumb claws.

“In essence, we can compare Nanotyrannus and tyrannosaurus to modern-day cheetahs and lions. While they shared a similar overall anatomy, they had distinct hunting strategies.”

Thomas Carr from Carthage College in Wisconsin has expressed that the new findings are “quite definitive” that the dueling dinosaur specimen represents an almost adult species, one that differs from tyrannosaurus.

Additionally, Holly Ballard from Oklahoma State University, who led the 2020 research, stated that there is “no contention” regarding the team’s conclusion that the fossil belonged to an individual nearing adult size.

However, neither Ballard nor Carr are fully convinced that the other fossil, Jane, constitutes a new species of Nanotyrannus. “Jane is still maturing and large,” emphasizes Ballard, suggesting it may be a new taxon rather than a juvenile tyrannosaurus. “We’re returning to the old debates,” Ballard laments.

“Moreover, in the Hell Creek Formation, if there are numerous small tyrannosaurs in circulation, where are the juvenile tyrannosaurus? ” Kerr questions, stating that it complicates the fossil record. “We simply haven’t uncovered enough tyrannosaurs to fully elucidate what was occurring during the growth phases of the tyrannosaurids in the Hell Creek Formation.”

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