Pioneering techniques used in this research may soon be applied to other dinosaur fossils and alter the course of paleontology.
A new peer-reviewed study has fundamentally altered scientists' understanding of the Tyrannosaurus rex, revealing that the iconic dinosaur lived much longer and reached its full size much later than previously believed.
Why It Matters This research challenges long-standing assumptions about T. rex's growth rate, lifespan, and even the number of species encompassed by the name “T. rex.“ The findings could significantly impact paleontological methodology, ecological interpretations of dinosaur life, and ongoing debates about the T. rex species complex, affecting academic research and popular perceptions of prehistoric life. What To Know The study, published in PeerJ on Wednesday, analyzed bone samples from 17 Tyrannosaurus rex specimens using advanced microscopic and statistical techniques. Lead author Holly Woodward, a professor of anatomy at Oklahoma State University, and her colleagues used polarized light to reveal previously undetectable growth rings in the dinosaurs' leg bones. These rings, analogous to those found in trees, record periods of growth and slowdown throughout an animal's life. However, unlike tree rings, these dinosaur growth rings only capture the last 10-120 years of the prehistoric creature's life. Previous research had estimated that the T. rex matured rapidly, reaching its full adult size—around eight tons—by age 25, and lived up to 30 years. “I think the study helps reveal why T. rex was so successful as an apex carnivore—that by growing slowly over a longer period of time, T. rex occupied many food niches throughout its life, eventually becoming large enough that it was only really competing with other T. rex for resources,” Woodward told CNN in a recent interview. However, by counting visible and hidden growth rings, Woodward's team found that T. rex experienced a much longer growth period, not reaching full size until between 35 and 40 years old. According to the study, the carnivore underwent its fastest weight gain between ages 14 and 29, sometimes accumulating 800 to 1,200 pounds per year. But after this “teenage“ growth surge, it entered a prolonged subadult phase, gradually increasing in size for another decade or more before reaching adulthood. These findings upend prior assumptions that the T. rex's maturation was comparable to that of modern mammals or birds. The research used a statistical algorithm to piece together year-by-year growth histories by merging data from specimens of various ages. The finer counting of crowded or hidden rings also reduced statistical uncertainty, improving the reliability of the growth curve. The study's dataset is considered the most comprehensive ever assembled for T. rex growth, providing a new reference for future dinosaur histology research. A Tyrannosaurus rex robot dinosaur is seen at the “Jurassic World: The Experience“ exhibition at Espacio Ibercaja Delicias on May 29, 2025, in Madrid. What People Are Saying Holly Woodward, study lead author, told CNN: “Instead of growing quickly, T. rex spent most of its life in the mid-body size range rather than achieving a total body length of 40 feet quickly. Additionally, we found that growth ring spacing varied within individuals, with some years showing substantial growth and others very little. This variability suggests that growth was flexible and likely influenced by resource availability and possibly environmental conditions.” Co-author Nathan Myhrvold, mathematician and paleobiologist at Intellectual Ventures, who led the statistical analysis, told Phys.org: “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 composite growth curve provides a much more realistic view of how Tyrannosaurus grew and how much they varied in size.” Lindsay Zanno, a paleontologist at North Carolina State University, said, per Science.org: “This study is as good as it gets. Those of us working on the life history of predatory dinosaurs have been anxiously awaiting these results for some time.” Steve Brusatte, a professor of paleontology and evolution at Scotland’s University of Edinburgh who was not involved in the study, told CNN: “This is good, provocative work that also suggests that there was more variation among T. rex than we used to think, and that some fossils that have long been called T. rex may actually belong to different species,” What Happens Next These new findings prompt a reevaluation of T. rex's ecological role, suggesting it spent more of its life as a mid-sized predator before ultimately dominating as an apex carnivore. The research also intensifies the debate over whether the T. rex fossil record actually includes multiple species. Two smaller specimens known by their nicknames, “Jane“ and “Petey,“ exhibited growth patterns distinct from other T. rex bones, raising the possibility they belong to a separate species such as Nanotyrannus. According to the study, more work is needed to confirm whether the T. rex label should apply to all current specimens or if a “species complex“ exists.
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