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. Dinosaur bones are like trees: each year is represented by a new ring, and paleontologists can count these concentric rings to determine a fossil’s age. But new research suggests that in the case of Tyrannosaurus rex, some growth rings have so far escaped detection. This means the king of the tyrant lizards lived longer than experts realized, and never stopped growing. The new analysis was able to build a more complete and accurate picture of tyrannosaur growth by examining bone sections under a special type of light, which reveals hidden growth rings not counted in previous studies. Rather than growing rapidly to adulthood, Tyrannosaurus rex grew more slowly and steadily than previously believed. “A four-decade growth spurt may have allowed younger tyrannosaurs to fulfill a variety of ecological roles within their environments. This may be one of the factors that allowed them to dominate the late Cretaceous period as top-of-the-food-chain carnivores,” says co-author Jack Horner of Chapman University.