T. Rex Took 4 Decades to Reach Full Giant Size

Tyrannosaurus rex, it turns out, may have taken its time becoming king. A new analysis of 17 tyrannosaur fossils suggests the giant predator lived longer than thought and never stopped growing, needing about 40 years to reach full size—some 15 years longer than earlier estimates. The study, published in PeerJ, used a bigger fossil sample and a revamped approach to tree-ring-like bands in dinosaur bones that scientists have long relied on to track growth. Early growth records are destroyed as bones expand, but researchers led by Oklahoma State University anatomist Holly Woodward and statistician-paleobiologist Nathan Myhrvold "stitched together" partial records from multiple animals using advanced statistical tools, per Phys.org.

They also examined bone slices under circularly polarized and cross-polarized light, which revealed previously overlooked growth rings. The resulting growth curve paints T. rex not as a fast-maturing sprinter, but as a slow-and-steady heavyweight that ultimately hit about 8 tons. Earlier estimates put T. Rex's lifespan at around 30 years, with growth ending before they were 25.

Study co-author Jack Horner says a four-decade growth window may have allowed younger tyrannosaurs to fill different ecological niches before aging into apex predators, potentially helping the group dominate at the end of the Cretaceous. "It took the prince a lot longer to grow into the king," Steve Brusatte, a paleontologist at the University of Edinburgh who wasn't involved in the study, tells Scientific American.

The work also feeds into a simmering debate over whether all the bones long lumped under T. rex truly belong there. The team treated its sample as a "T. rex species complex" and found that two well-known fossils, "Jane" and "Petey," show growth patterns that don't match the others. That doesn't prove they're separate species, but it lines up with recent research arguing they belong to a smaller cousin genus, Nanotyrannus. Beyond tyrannosaurs, the authors say the newly identified type of growth ring—and the evidence that common aging protocols may be off—could force a reassessment of how scientists measure dinosaur growth across the board.