Most Dinosaurs Grew Slowly. T. Rexes Grew Like Weeds

Paleontologist Tom Cullen cutting into SUE the T. rex’s thigh bone to learn how T. rex grew. (Credit: David Evans)

(CN) — Teenage years are often awkward for adolescents, but none more so than the Tyrannosaurus rex, according to research Tuesday that revealed the giant dinosaur had to contend with a sudden and intense growth spurt as it aged into an adult.

The study, published in the journal Proceedings of the Royal Society B, detailed the researchers’ findings on the growth rate of theropods, a group of bipedal dinosaurs.

Pete Makovicky, from the University of Minnesota, said in a statement the scientists “wanted to understand how some of them got so big.”

“We also wanted to see if we got the same growth record when we sampled a variety of different bones from the same skeleton. All these questions about how theropods grew could impact our understanding of the evolution of the group,” he said.

Theropod dinosaurs are distant cousins of modern-day birds, but researchers have previously been unable to determine their growth patterns. The research team decided to take a look at the fossils of theropods, using a method similar to mark the age of dead trees.

“Inside the bones as an animal grows, there are markings like tree rings that record roughly how old the animal is, how much it’s growing each year, and a number of other factors,” said Tom Cullen, lead author and postdoctoral researcher at the North Carolina Museum of Natural Sciences.

The scientists analyzed these markings to help determine the dinosaur’s age and how much it grew by the amount of space between them.

“You can see all the little areas where the bone cells have grown, and the structure of the blood vessels that passed through the bone,” Cullen said. “These vascular canals tell you roughly how fast the bone was growing. If the canals are more organized, the bone was being laid down more slowly, and if the structure is chaotic, it grew more quickly.”

For the T. rex, researchers estimate it put on approximately 35-45 pounds a week during its teenage years. An adult could grow as big as 42 feet long and weigh about 16,000 pounds.

“The amount of calories T. rex would have needed during its growth spurt would have been ridiculous,” Cullen said.

The T. rex, as well as other dinosaurs of the coelurosaur family, seemed to all have gone through massive growth spurts during their adolescence. 

“We can’t say for sure, but there could be some kind of a selection pressure for the coelurosaurs to grow quickly to keep up with their prey, or pressure for the allosauroids to keep growing in size since their prey were also increasing in size,” Cullen said. “But it’s pretty speculative. It could be that even if the sauropods kept growing their whole lives, they had so many offspring that there was always something small to eat.”

Cullen said that while scientists still have questions about the growth of dinosaurs, he hopes this research will help advance their knowledge of the ancient creatures.

“I’m really proud of this work. It’s the culmination of many, many years of small projects building towards sort of a central goal of trying to understand growth in these animals and understand the many factors that influence these patterns. This doesn’t resolve it, but this is a really big step forward,” he said.

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