Fossil analysis changes what paleontologists know about how long it took T. rex to grow to full size

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

Scientists have long calculated annual growth rings on fossilized Tyrannosaurus rex leg bones based on their age at death and how fast they grew to adult size. These dinosaurs typically stopped growing at age 25 and lived to about 30, research shows.

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

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

But because the samples ranged in age from early teens to adulthood, the researchers were able to piece together a picture of their growth using a new statistical approach that combines records from people of different ages.

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

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

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

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

“I think the study helps reveal why T. rex was so successful as a top carnivore — that by growing slowly over a long period of time, T. rex occupied many food niches throughout its life, eventually becoming so large that it was just 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 that what paleontologists believed to be a single species called Tyrannosaurus rex was actually a complex that included other species or subspecies.

For example, a study published in October found that juvenile T. Evidence has been found that a specimen thought to be a rex actually belongs to a different species known as a Nanotyrannus.

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

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

“This is good, provocative work that also suggests that T. rex had more variation than we thought, and that some fossils that have long been called T. rex may actually be different species,” Brusatte said.

The discovery of a new type of dinosaur growth ring could have far-reaching implications for paleontologists investigating dinosaur growth rates.

“It’s difficult to interpret growth markers that are very closely spaced,” study co-author Nathan Myhrvold, a mathematician and paleobiologist at the innovation and investment business Intelligent Ventures, said in a statement.

“We found strong evidence that protocols commonly used in developmental studies may need to be modified,” he said.

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