Tyrannosaurus rex lived longer and took more time to reach its maximum size than previously thought, according to a new study.

Scientists have long counted annual growth rings in fossilized Tyrannosaurus rex leg bones to calculate both their age at death and how fast they grew to adult size. These dinosaurs typically stopped growing around 25 years old and lived to around 30, research has shown.

However, a new study published in the journal PeerJ on Wednesday details how a team of scientists used polarized light to reveal previously unseen growth rings from 17 individual specimens. T. rex wouldn’t have reached its maximum size of around 8 tons until it was 35 to 40 years old, the analysis found.

Unlike tree growth rings, dinosaur growth rings only capture the final 10 to 20 years of an individual’s life.

But because the specimens ranged in age from early juveniles to adults, researchers were able to put together a picture of their growth using a new statistical approach that combines records from individuals of different ages.

Drawing on the largest dataset ever put together for Tyrannosaurus rex, researchers were able to reconstruct their growth history year by year and found that the dinosaurs grew much more slowly than previously thought.

“Instead of growing quickly, T. rex spent most of its life in the mid-body size range rather than achieving a total body length of 40 feet quickly,” lead study author Holly Woodward, a professor of anatomy at Oklahoma State University, told CNN on Thursday.

“Additionally, we found that growth ring spacing varied within individuals, with some years showing substantial growth and others very little,” she said. “This variability suggests that growth was flexible and likely influenced by resource availability and possibly environmental conditions.”

The findings help scientists understand more about the king of the dinosaurs and its role in the world tens of millions of years ago, Woodward said.

“I think the study helps reveal why T. rex was so successful as an apex carnivore — that by growing slowly over a longer period of time, T. rex occupied many food niches throughout its life, eventually becoming large enough that it was only really competing with other T. rex for resources,” she said.

Growth rings in T. rex fossils

In addition, differences in the growth curves of some of the specimens involved in the study add to the scientific debate as to whether what paleontologists believed to be a single species named Tyrannosaurus rex is in fact a complex that includes other species or subspecies.

For example, a study published in October uncovered evidence that a specimen thought to have belonged to a teenage T. rex was actually from a different species known as Nanotyrannus.

While the growth rates examined in this study cannot definitively prove the existence of separate species, “the evidence suggests that intriguing possibility, among other possible explanations,” according to the researchers’ statement.

The new study’s findings mesh well with the recent work on Nanotyrannus, said Steve Brusatte, a professor of paleontology and evolution at Scotland’s University of Edinburgh, who was not involved in the study.

“This is good, provocative work that also suggests that there was more variation among T. rex than we used to think, and that some fossils that have long been called T. rex may actually belong to different species,” Brusatte said.

The discovery of a new type of dinosaur growth ring could also have wider implications for paleontologists investigating dinosaur growth rates.

“Interpreting multiple closely spaced growth marks is tricky,” said study coauthor Nathan Myhrvold, a mathematician and paleobiologist at invention and investment business Intellectual Ventures, in the statement.

“We found strong evidence that the protocols typically used in growth studies may need to be revised,” he said.

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