Tyrannosaurids Took Their Time Growing to 17,000 Pounds

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HHow long did it take a Tyrannosaurus rex to reach its maximum size of over 17,000 pounds?

Growth often levels off as adults, so assessing an animal’s annual growth rates can give you an idea of ​​not only how quickly it grew, but also when it reached maturity. Just like you would a tree, you count and measure the growth rings in slices of T. rex leg bones, with larger rings indicating faster growth.

In a study published yesterday in Peer Jresearchers examined the growth rings of 17 tyrannosaurs fossils to trace their growth pattern.

“Examining growth rings preserved in fossilized bones allowed us to reconstruct the growth history of animals from year to year,” study lead author and Oklahoma State paleontologist Holly Woodward Ballard said in a statement.

Read more: “T. Rex was a slacker”

In the most comprehensive analysis of Tyrannosaurus’ internal bone structure to date, researchers examined the femurs and tibias of young specimens from juveniles to older adults. The use of polarized light made it easier to analyze growth rings that would otherwise be missed. A statistical algorithm led by mathematician Nathan Myhrvold combined data from multiple specimens to establish a growth trajectory for the species.

Previous studies have estimated the time required for T. rex to reach maximum size around 25 years old, but new data indicates it grew more slowly, reaching its maximum size between 35 and 40 years old. This is a very long subadult age.

“A four-decade growth phase could have allowed young tyrannosaurs to fulfill various ecological roles within their environment,” hypothesizes co-author and Chapman University paleontologist Jack Horner. “This could be a factor that allowed them to dominate the late Cretaceous as apex carnivores.”

The new data also fuels an ongoing debate over whether the small T. rex specimens are unique species, members of a hypothetical genus dubbed Nanotyrannus.. Two of the 17 specimens, colloquially called “Jane” and “Petey”, were found to be outliers on these new growth charts. Thus, they could be species other than T. rex, as proposed by a recent related study published in Nature.

Including additional tyrannosaur specimens in statistical models would continue to strengthen our understanding of how these animals became the giants of the Cretaceous.

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Main image: Esprit photo / Shutterstock

• Devin Reese

  Published on January 15, 2026

  Devin Reese is the editor-in-chief of Natural history and a science writer based in Alexandria, Virginia.