A new study suggests that Tyrannosaurus rex took about 40 years to reach its full size, estimated at roughly eight tons. This finding extends previous growth estimates by 15 years; earlier estimates indicated the species reached adult size around age 25.

The study, published in the journal PeerJ, analyzed 17 tyrannosaur fossils ranging from young juveniles to large adults, belonging to what researchers describe as the Tyrannosaurus rex species complex. The team used statistical models to combine information from multiple specimens and examined thin slices of fossil bone under specialized lighting to reveal growth rings. Researchers found that circularly polarized and cross-polarized light can reveal a new type of growth ring in dinosaur bones.

Holly Woodward, a professor of anatomy at Oklahoma State University, led the research effort. "This is the largest data set ever assembled for Tyrannosaurus rex," Woodward said. "Examining the growth rings preserved in the fossilized bones allowed us to reconstruct the animals' year-by-year growth histories."

Nathan Myhrvold, a mathematician and paleobiologist at Intellectual Ventures, led the statistical analysis for the study. "We came up with a new statistical approach that stitches together growth records from different specimens to estimate the growth trajectory of T. rex across all stages of life in greater detail than any previous study," Myhrvold said. "The composite growth curve provides a much more realistic view of how Tyrannosaurus grew and how much they varied in size." He added, "Interpreting multiple closely spaced growth marks is tricky." Myhrvold also stated, "We found strong evidence that the protocols typically used in growth studies may need to be revised."

Two specimens, nicknamed Jane and Petey, stood out from the rest of the fossils in the study. Jack Horner, a coauthor of the study affiliated with Chapman University, said, "A four-decade growth phase may have allowed younger tyrannosaurs to fill a variety of ecological roles within their environments." Horner added, "That could be one factor that allowed them to dominate the end of the Cretaceous Period as apex carnivores."