New study reveals the T. rex didn’t reach full-size until age 40

Wednesday, January 14, 2026

Media Contact: Sara Plummer | Senior Communications Coordinator | 918-561-1282 | sara.plummer@okstate.edu

For decades, scientists have been counting annual growth rings, similar to tree rings, found inside fossilized leg bones of Tyrannosaurus rex to estimate how old the giant carnivores were when they died and how quickly they grew to adulthood. 

“Best estimates from previous studies were that T. rex typically stopped growing around age 25,” said Dr. Holly Woodward, professor of anatomy and paleontology at Oklahoma State University Center for Health Sciences and the study’s lead author.

An extensive new study of 17 tyrannosaur specimens, ranging from early juveniles to massive adults, shows the king of carnivores took about 40 years to reach its full-grown size of around eight tons. 

The new analysis, the most complete life history ever conducted on T. rex, gives a more complete and accurate picture of tyrannosaurs’ growth by using advanced statistical algorithms and examining slices of bone under a special kind of light that reveals hidden growth rings not counted in previous studies. 

In addition to extending the growth phase of Tyrannosaurus by 15 years, the results suggest that some of the specimens may not be T. rex at all, but rather members of other species or different for some other reason. The study was published in January in the journal PeerJ.

“This is the largest data set ever assembled for Tyrannosaurus rex,” said Woodward, whose research specialty is paleohistology, the study of prehistoric bone structures. “Examining the growth rings preserved in the fossilized bones allowed us to reconstruct the animals’ year-by-year growth histories.” 

Unlike the rings visible on a tree stump, a cross section of T. rex bone records only the last 10 to 20 years of the animal’s life.

“We came up with a new statistical approach that stitches together growth records from different specimens to estimate the growth trajectory of T. rex across all stages of life in greater detail than any previous study,” said Nathan Myhrvold, a mathematician and paleobiologist at Intellectual Ventures who led the statistical analysis. “The composite growth curve provides a much more realistic view of how Tyrannosaurus grew and how much they varied in size.”

The research shows that rather than racing to adulthood, Tyrannosaurus grew more slowly and steadily than previously believed. 

“A four-decade growth phase may have allowed younger tyrannosaurs to fill a variety of ecological roles within their environments,” said the study’s coauthor Jack Horner, a lecturer and presidential fellow at Chapman University in California. “That could be one factor that allowed them to dominate the end of the Cretaceous Period as apex carnivores.”

Although Tyrannosaurus rex is the best-known species of this group of dinosaurs, recent studies have proposed that some specimens previously identified as T. rex may in fact be members of other related species. 

Woodward said to help shed light on the question, the new study includes data from 17 specimens that are part of the Tyrannosaurus rex species group but could include other species or subspecies.  

“One important finding of the study is that the growth curves of two of the more famous specimens, known by their nicknames ‘Jane’ and ‘Petey,’ are statistically incompatible with the others. Although growth records alone cannot establish whether they were separate species, the evidence suggests that intriguing possibility, among other possible explanations,” she said. 

While conducting their research, Woodward, Myhrvold and Horner discovered using different types of light revealed a new kind of dinosaur growth ring that helped resolve longstanding problems reconciling the growth of some specimens. 

This discovery, backed up in the paper by robust statistical evidence, could be important in reevaluating the growth of other dinosaurs beyond T. rex, said Myhrvold.

“Even after more than a century of study, Tyrannosaurus rex continues to surprise paleontologists,” Woodward said. “By combining expanded sampling, innovative statistics and careful bone analysis, the new study offers a clearer, more accurate picture of Tyrannosaurus rex as a living animal, growing from juvenile to giant.”