A group of researchers has estimated how long it took tyrannosaurus rex to reach its full size — and the time period is significantly longer than scientists previously believed.

The dinosaur is believed to have been Earth’s largest land predator. It may have taken around 40 years — not 30 — for it to reach its full size, according to the new study.

The findings were published in the journal PeerJ. The growth trajectory “is more gradual than expected,” the lead author of the study, Holly Woodward of Oklahoma State University, told Reuters. 

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“Rather than T. rex ratcheting up to adult size quickly, it spent a lot of its life at juvenile-to-subadult sizes,” the paleohistologist said.

The study analyzed bone tissue microstructure in the leg bones of 17 T. rex fossils — discovering that it took around four decades for the dinosaurs to reach a maximum size of roughly eight tons.

Paleontologists have long believed it took around 30 years for these dinosaurs to reach full size.

Researchers also found growth marks in the bones that were only visible using polarized light.

Some of these marks were found on leg bones and represented annual growth, much like tree trunks. Researchers analyzed the remains of both juveniles and fully grown adults.

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“We also found that growth-ring spacing in individual T. rex was variable,” Woodward told Reuters. 

“T. rex had a flexible growth pattern. Some years it didn’t grow much, while other years it grew a lot.”

Study co-author Jack Horner of Chapman University told Reuters the new estimate is based on a fresh statistical approach that took growth records from different specimens into account.

“We don’t know for certain which of these estimates [is] more accurate, since we don’t have living T. rexes to measure, but these new estimates make more sense logically and statistically, considering the size these dinosaurs [attained],” Horner said.

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Growth pacing likely depended on resource availability and environmental conditions, added Woodward.

“In other words, if conditions weren’t great, it didn’t spend energy on growing — but when conditions were good, it could grow larger,” she said.

“This flexibility allowed it to survive harsh times while growing larger than other carnivores, so it could outcompete others for resources. Ultimately, T. rex was only competing against other T. rex for food.”

Fossils have shown scientists that T. rex could measure upwards of 40 feet long — and had exceptional bite strength.

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The T. rex prowled western North America during the Cretaceous Period. The dinosaurs were largely wiped out by the asteroid strike in Mexico’s Yucatán Peninsula 66 million years ago.

The recent study is just one of many interesting dinosaur-related discoveries made in the last few months.

Australian researchers announced in November that they’d found the traces of a dinosaur that may have been limping over 150 million years ago in Colorado.

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Earlier last year, scientists unearthed a new dinosaur, plus its ancient leftovers, in a tourist hot spot in Argentina.

Reuters contributed reporting.