Aside from its massive jaws and sharp teeth, Tyrannosaurus rex stands out as one of the largest land predators ever to roam the Earth, but how it grew to such sizes has long puzzled scientists. Growth estimates suggested it bulked up quickly, reaching adult proportions in about two decades before leveling off. Now, a detailed look at bones from across its life stages reveals a different story: T. rex grew more slowly and kept adding size over a much longer lifespan than once thought.
Tyrannosaurus rex lived during the late Cretaceous period in what is now North America, and its fossils have fueled debates about everything from its senses to its speed. Earlier models painted a picture of rapid growth, with young ones ballooning to over 8,000 kilograms in roughly 20 years, living perhaps another decade after that. But those ideas relied on limited samples, often mixing different bones or using partial data. Questions lingered about early growth phases and whether all specimens truly belonged to the same species, given variations in size and shape.
Researchers turned to thin slices from femurs and tibias of 17 individuals, spanning tiny juveniles to massive adults, all from a narrow region in eastern Montana and nearby. They examined layers inside the bones, like tree rings, that mark yearly growth pauses. These include standard lines visible under normal light and subtler bands that only show up under special polarized light. Four models tested different ways to count these marks, but the one including all features fit the data best.
That approach showed T. rex took longer to mature, with growth stretching into the 30s or even 40s. “Our results suggest that the Tyrannosaurus rex species complex grew more gradually and over a longer lifespan than indicated by prior models, with a protracted period of subadult development,” the study states. Bone layers stayed consistent through much of life, hinting at steady but variable growth rates, around 25 to 100 microns daily until late stages. Peak growth hit about 360 kilograms per year, lower than past figures, and animals kept enlarging slowly rather than hitting a quick plateau.
The findings challenge old timelines. One earlier study pegged maximum size by the late teens, but here, maturity arrives later, with some living well into adulthood while still gaining bulk. “We show that during the annual period of active growth, Tyrannosaurus bone apposition rates were between 25 and 100 microns per day from early juvenile through late subadult stages, with slower rates (<10 microns per day) occurring only in the outer cortex of the largest individuals,” the researchers explain.
Citations
H. N. Woodward et al. Prolonged growth and extended subadult development in the Tyrannosaurus rex species complex revealed by expanded histological sampling and statistical modeling. PeerJ. Published online January 14, 2026. DOI: 10.7717/peerj.20469
