The largest analysis of Tyrannosaurus rex fossils reveals that this colossal Cretaceous predator took approximately 35 to 40 years to reach maturity, significantly longer than previously believed.

This research suggests that there may have been multiple species of Tyrannosaurus rather than just one, raising questions about whether small specimens attributed to juveniles might actually belong to a distinct species known as Nanotyrannus.

Before this study, it was assumed that Tyrannosaurus rex reached a maximum weight of 8 tons by around 20 years of age and could have a lifespan of only 30 years.

“Previous growth studies on Tyrannosaurus rex were conducted in the early 2000s and were based on a limited sample size of seven specimens,” explains Holly Ballard from Oklahoma State University. “This was not the researchers’ fault; it was the available data at the time.”

Through their recent study, Ballard and her colleagues analyzed thigh and tibia bones from 17 specimens ranging in age from juvenile to adult, making it the largest dataset for Tyrannosaurus growth ever collected.

The research team examined the microstructure of bone tissue, including growth rings formed annually, similar to tree rings. However, the earliest growth rings were often destroyed as the marrow cavity expanded, necessitating a greater number of specimens across overlapping growth stages.

“This is why our sample size and age distribution are so critical and set us apart from past studies,” Ballard emphasizes.

The analysis revealed that Tyrannosaurus rex grows more slowly than initially thought, with growth rates varying based on environmental factors.

However, determining the maximum age of Tyrannosaurus rex remains problematic, as animals cease to grow annual rings upon reaching maturity. “The most successful Tyrannosaurus likely lived to be around 40 years old, with very few individuals achieving that age,” Ballard notes. “In our sample, only two individuals reached full adult size.”

Interestingly, two other specimens exhibited slower growth, indicating they might belong to a different species, such as Nanotyrannus, or that they were possibly Tyrannosaurus rex that were sick or stunted due to environmental factors.

Lindsey Zanno from the North Carolina Museum of Natural Sciences describes this study as one of the most comprehensive investigations of Tyrannosaurus rex growth to date, building on her recent research suggesting that a small specimen from Montana might be a fully grown Tyrannosaurus rex around 20 years old, potentially named Nanotyrannus lansensis.

“I’m thrilled to finally have a growth curve for Tyrannosaurus rex that we can assert with confidence,” Zanno comments, especially regarding Nanotyrannus.

Thomas Carr from Carthage College in Wisconsin believes that this study will likely lead to the separation of Tyrannosaurus rex into different species and expects it to influence future dinosaur research significantly.

“We anticipate that growth estimates for other dinosaurs will undergo reassessment,” Carr states. “Overall, this could transform our understanding of dinosaur growth and development across the spectrum.”

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