T. rex's slow growth rewrites the story of the king of dinosaurs

A new study reveals that Tyrannosaurus rex grew far more slowly than previously thought. The giant predators took between 35 to 40 years to reach full size, spending most of their lives at a mid-range weight. Only in old age did they become the towering beasts known from fossils and films.

Researchers found that T. rex juveniles grew at a peak rate of 25 to 100 micrometres per day. This rapid growth slowed dramatically in adulthood, dropping to less than 10 micrometres daily. The extended growth period may have helped them dominate different ecological niches over their lifetimes.

Two smaller specimens, named 'Jane' and 'Petey', grew even more slowly than others. Their unusual features—gracile skulls, elongated snouts, and taller, blade-like teeth—led scientists to question whether they belonged to a separate species. In an October 2025 Nature study, Lindsay Zanno's team proposed that these fossils could represent Nanotyrannus, a distinct but closely related tyrannosaur.

The differences in growth rates and anatomy suggest harsh living conditions, illness, or even the existence of a smaller tyrannosaur species. If confirmed, this would reshape our understanding of T. rex's evolutionary history.

The findings highlight how T. rex's prolonged growth may have contributed to its success as an apex predator. By occupying different roles at various life stages, the species could have reduced competition and thrived for millions of years. The debate over 'Jane' and 'Petey' also keeps open the possibility of discovering new tyrannosaur relatives.