T. rex Took 40 Years to Reach 8 Tons, Extending Growth Timeline by 15 Years

Q: Has new evidence finally settled the 80-year debate over the pygmy tyrant dinosaur, Nanotyrannus?

Not completely, but the balance of evidence has shifted strongly toward *Nanotyrannus* being a real, separate tyrannosaur rather than just a juvenile *T. rex*. The June 2026 *PeerJ* study does not by itself “settle” the debate, but it adds important support. Holly Woodward and colleagues analyzed 17 tyrannosaur fossils using improved growth-ring detection and statistical modeling, and found that specimens such as “Jane” and “Petey” grew differently from the main *T. rex* sample. That suggests they may not belong to *T. rex* at all. This builds on stronger evidence published in late 2025 and early 2026. Studies of the “Dueling Dinosaurs” tyrannosaur and the original Cleveland skull reported bone histology indicating those smaller animals were mature or near maturity when they died. If correct, that is the key point: a mature small tyrannosaur cannot simply be an immature *T. rex*. Researchers also reported anatomical differences, including proportions and skeletal details, that do not fit a straightforward juvenile growth stage of *T. rex*. Taken together, these findings have moved much of the field toward recognizing *Nanotyrannus* as distinct. But “finally settled” is still too strong. Some paleontologists remain unconvinced and argue the fossils can still be explained as unusual young *T. rex* individuals. So the debate is not entirely over, but it is much closer to resolution than at any point in the past 80 years.

3 articles · Updated · The Economic Times · Jun 23

About 40 years—not 25—was the time T. rex likely needed to reach its adult weight of roughly eight tons, according to a new PeerJ study led by Holly Woodward.
Thin slices of fossil leg bones revealed annual growth rings, including previously hidden ones, showing a slow, steady rise in size rather than the rapid-maturation model that dominated since a 2004 study.
Two famous fossils, Jane and Petey, did not match the growth pattern of the other specimens, raising the possibility that they were not T. rex at all but another tyrannosaur lineage.
The imaging methods used to uncover those rings could force paleontologists to revisit growth estimates for other dinosaurs, suggesting decades-old aging protocols may need revision.