T. rex took 40 years to become fully grown

The largest analysis ever carried out on Tyrannosaurus rex Fossils suggest that the giant, ferocious Cretaceous predator was a late bloomer, taking 35 to 40 years to reach maturity.

The results also deepen the debate over whether there were multiple T. rex species instead of just one and whether the smaller specimens, once thought to be juveniles, are actually a smaller species called Nanotyrannus.

Based on studies carried out twenty years ago, it was believed that T. rex reached its maximum weight of 8 tonnes in around twenty years and the animals probably only lived to the age of 30.

“The last great T. rex Growth studies were done in the early 2000s, based on as many as seven specimens,” says Holly Ballard of Oklahoma State University. “It wasn’t the researchers’ fault, it was just what was available at the time.”

This time, Ballard and his colleagues were able to sample the thigh and tibia bones of 17 individuals, ranging in age from juvenile to full-grown adult, making it the largest collection of growth data ever assembled on T. rex.

Scientists studied the microstructure of bone tissue, including the growth rings that form each year, like those on a tree trunk. However, the first growth rings are destroyed as the medullary cavity enlarges. The team therefore needed to access as many specimens as possible with overlapping growth stages.

“This is why our sample size and age distribution is so large and what sets it apart from previous studies,” says Ballard.

The analysis reveals that T. rex grew more slowly than previously thought and that its growth rate was variable depending on environmental conditions.

However, it is not possible to determine the maximum age T. rex because once animals reached maturity, they stopped laying growth rings. “We can say that the most successful T. rex lived to be around 40 years old, but very few reached that age – only two specimens in our sample reached adult size,” says Ballard.

Two other specimens grew more slowly than the others, opening the possibility that they were other species, such as Nanotyrannusor part of a “Tyrannosaurus complex,” says Ballard. “We propose, based on their growth differences, that they are either a different species or that they are sick or injured. T. rexor perhaps eclipsed for some environmental reason.

Lindsay Zanno of the North Carolina Museum of Natural Sciences says this is the most thorough examination of Tyrannosaurus continued growth. His study of a dinosaur fossil from Montana, published last year, concluded that the specimen was a small tyrannosaur that reached adult size around age 20, tentatively named Nanotyrannus lancensis.

“It’s exciting to finally have a growth chart for Tyrannosaurus that we can trust,” says Zanno. “It’s also exciting to see another study validate our work on Nanotyrannus in just the last few months.

Thomas Carr of Carthage College in Wisconsin is cautious about whether T. rex is expected to be divided into different species, but he predicts the study will have broad ramifications for dinosaur researchers.

“I think estimates of growth curves for other dinosaurs will now need to be revised,” says Carr. “Overall, we will see a shift in our understanding of dinosaur development across the board.”

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