A new analysis of growth rings in Tyrannosaurus rex bones indicates the dinosaur took 35 to 40 years to reach maturity, slower than earlier estimates. The research, based on 17 specimens, also fuels debate over whether smaller fossils represent a separate species like Nanotyrannus. Experts praise the study's thoroughness and its potential to reshape views on dinosaur development.
Researchers have conducted the largest-ever examination of Tyrannosaurus rex growth, analyzing thigh and shin bones from 17 individuals ranging from juveniles to adults. By studying bone tissue microstructure, including annual growth rings similar to those in tree trunks, the team found that T. rex developed more slowly than previously believed. Earlier studies from the early 2000s, based on at most seven specimens, suggested the predator reached its 8-tonne maximum weight in about two decades and lived to around 30 years.
"The last big T. rex growth studies were done in the early 2000s, based on, at most, seven specimens," explained Holly Ballard at Oklahoma State University, lead author of the study published in PeerJ. The new work highlights variable growth rates influenced by environmental conditions, with the earliest rings often destroyed as the bone marrow cavity expanded. Only two of the 17 specimens reached full adult size, implying that the most successful T. rex lived to about 40 years, though few achieved that age.
Two specimens showed slower growth, prompting speculation they could belong to a different species, such as Nanotyrannus, or represent injured or environmentally stunted T. rex. "We propose, based on their growth differences, they are either a different species, or perhaps they are sick or injured T. rex, or perhaps dwarfed for some environmental reason," Ballard said.
Lindsay Zanno at the North Carolina Museum of Natural Sciences called it "the most thorough examination of Tyrannosaurus growth yet conducted." Her own research last year on a Montana fossil supported Nanotyrannus lancensis as a small, fully grown tyrannosaur at 20 years. "It’s exciting to finally have a growth curve for Tyrannosaurus that we can feel confident in," Zanno added.
Thomas Carr at Carthage College urged caution on species distinctions but foresaw wider impacts. "I expect that the estimates of growth curves of other dinosaurs will now have to be revisited," he said, predicting a shift in understanding dinosaur development overall.
