Researchers have long known that the meat-eating Tyrannosaurus rex had a teenage growth spurt, gaining around 35 to 45 pounds per week, to reach its colossal size.
But up until now, it hasn’t been clear whether all dinosaurs shared this growth pattern. Was a T. rex-style period of extreme growth the only way dinos became full-fledged adults?
A study of fossilized bone samples from 11 different dinosaurs published Tuesday has revealed that while some transformed during an adolescent growth spurt, others grew slow and steady.
Like tree trunks, dinosaur bones have rings that show how the creatures grew and aged.
“Most animals have a period every year when they stop growing, traditionally suggested to be in times like winter when food is more scarce. It shows up in the bones as a line, like a tree ring,” said Tom Cullen, a scientific affiliate of Chicago’s Field Museum and the lead author of a new study in Proceedings of the Royal Society B.
“By analyzing these growth lines and examining the bones for new regions of growth, scientists can get a rough estimate of an animal’s age and how much it grew every year. There are also clues in the bone structure,” he said in a news statement.
The dinosaurs’ growth patterns depended on their family, the researchers found. T. rex and its relatives, the coelurosaurs, showed a period of fast growth during adolescence, and then the rate of growth would peter out once they reached adulthood.
However, more distant cousins of the T. rex, the research suggested, could reach a similar size but grew more slowly.
The team sampled bones from a Carcharodontosaurus, a newly identified species from Argentina. It didn’t reach its adult size until its 30s and 40s and lived to be up to 50 years old or more. Despite its advanced age, it had only stopped growing two or three years before becoming part of the fossil record.
For the study, Cullen also took a sample from the Field Museum’s most famous resident dinosaur, Sue the T. rex. She lived to be about 33 years old and is the world’s most complete and best-preserved T. rex.
Cullen used a diamond-tipped coring drill to cut a cylinder of bone around the size of a battery from Sue’s thigh bone — a process he described as “nerve-wracking.”
Paleontologist Tom Cullen, a research associate of Chicago’s Field Museum, cuts into Sue the T. rex’s thigh bone to learn how the massive meat eater grew.
He then sliced samples of bone so thin that light could pass through them. This enabled him to examine the samples under a microscope and analyze the lines showing where new bone had grown year after year. The missing part of Sue’s leg was filled in with brown putty.
Mammals, including humans, tend to go through a growth spurt when young and then stay the same size once reaching maturity. In other animal groups, that’s not always the case.
“Growth rate really varies, there’s no one size fits all,” said Cullen, who is now a postdoctoral researcher at the North Carolina Museum of Natural Sciences in Raleigh. (The museum announced last week that it’s building the new SECU DinoLab to house and study the fossil pair of a Triceratops horridus and a Tyrannosaurus rex, which will go on display in 2022. The “dueling dinosaurs” are some of the most complete dino skeletons ever found, which will contribute to furthering research on dinosaur biology.)
“Birds have super growth spurts and reach adult size really fast, while reptiles like alligators and various lizards and snakes have extended growth. With them, a really, really big individual is probably really old,” he added.
Expanding in size quickly can be an advantage, the study said — it makes it easier for you to hunt other animals, and harder for predators to hunt you.
Conversely, a growth spurt takes a lot of energy and resources, and it’s easier to just get a little bigger every year throughout your life span.
“The amount of calories T. rex would have needed during its growth spurt would have been ridiculous,” Cullen said.
