A detailed analysis of growth rings in bones from 17 Tyrannosaurus rex specimens shows the dinosaur took 35-40 years to mature to eight tons, far slower than prior estimates of around 20-25 years. Published in PeerJ, the research also bolsters debate over whether smaller fossils represent separate species like Nanotyrannus. The findings reshape understanding of tyrannosaur life history and ecology.
Paleontologists have refined estimates of Tyrannosaurus rex growth by examining annual growth rings in fossilized leg bones from 17 specimens, ranging from juveniles to adults—the largest dataset assembled for the species. Previous studies from the early 2000s, using at most seven specimens, suggested T. rex reached its maximum 8-tonne size in about 20-25 years and lived to around 30.
Led by Holly Woodward of Oklahoma State University, the team sliced bones for microscopic analysis, employing advanced techniques like circularly polarized and cross-polarized light to detect previously overlooked rings. Nathan Myhrvold developed new statistical modeling to create a composite growth curve across life stages. "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."
Myhrvold added: "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."
The results indicate a prolonged growth phase of roughly 35-40 years, with variable rates influenced by environment. Only a few specimens reached full adult size, suggesting successful T. rex lived up to 40 years. Coauthor Jack Horner of Chapman University noted: "A four-decade growth phase may have allowed younger tyrannosaurs to fill a variety of ecological roles... That could be one factor that allowed them to dominate the end of the Cretaceous Period as apex carnivores."
The study, titled "Prolonged growth and extended subadult development in the Tyrannosaurus rex species complex revealed by expanded histological sampling and statistical modeling," also scrutinizes the 'T. rex species complex.' Specimens like 'Jane' and 'Petey' exhibited distinct, slower growth, supporting hypotheses they represent Nanotyrannus or other relatives rather than immature T. rex—possibly due to injury, illness, or environmental factors.
Experts praised the work. Lindsay Zanno of the North Carolina Museum of Natural Sciences called it "the most thorough examination of Tyrannosaurus growth yet conducted," aligning with her prior research on Nanotyrannus. Thomas Carr of Carthage College urged caution on species distinctions but predicted: "I expect that the estimates of growth curves of other dinosaurs will now have to be revisited."
