Holly Woodward had 17 sets of bones to work with. That number, modest by some scientific standards, turned out to be enough to rewrite four decades of consensus about the most studied predator in paleontological history.
The professor of anatomy at Oklahoma State University led a team that examined fossilized leg bones from 17 tyrannosaur specimens — juveniles through enormous adults — using microscopic analysis and a specialized lighting technique to detect growth rings that earlier studies had missed. The conclusion, published in the journal PeerJ, is that Tyrannosaurus rex likely took around 40 years to reach its full size, not the 25 years that had been the prevailing estimate.
At maximum, the animal weighed roughly eight tons.
“This is the largest data set ever assembled for Tyrannosaurus rex,” Woodward says in the announcement. “Examining the growth rings preserved in the fossilized bones allowed us to reconstruct the animals’ year-by-year growth histories.”
The Gap in the Record
Those growth rings work like tree rings — annual markers laid down in bone tissue that tell researchers how old an animal was and how fast it was adding mass. The problem is that a cross section of T. rex bone typically captures only the final 10 to 20 years of the animal’s life, not the full sequence. The early years vanish from the record as bone remodels over time.
To close that gap, the team enlisted Nathan Myhrvold, a mathematician and paleobiologist at Intellectual Ventures, who developed a statistical method to bridge specimens of different ages into a single composite growth curve. “We came up with a new statistical approach that stitches together growth records from different specimens to estimate the growth trajectory of T. rex across all stages of life in greater detail than any previous study,” Myhrvold explains. “The composite growth curve provides a much more realistic view of how Tyrannosaurus grew and how much they varied in size.”
The picture that emerged is one of sustained, gradual development — not the burst of rapid growth toward a quick adulthood that earlier models implied.
Ecology of a Slow Grower
Coauthor Jack Horner of Chapman University argues that the extended growth window had ecological consequences. “A four-decade growth phase may have allowed younger tyrannosaurs to fill a variety of ecological roles within their environments,” he says, suggesting it may be one factor that let the species dominate the end of the Cretaceous Period as apex carnivores.
The study also reopens a classification debate that has circled the field for years. Some fossils currently assigned to T. rex may not belong to the species at all — the research hints they could represent distinct species or other meaningful biological differences. Smaller specimens in particular have previously been proposed as candidates for a separate species called Nanotyrannus, a question the field has not resolved. The new growth data adds another layer to that unfinished argument.
According to the researchers, the study represents the most detailed reconstruction of T. rex life history attempted to date.
Photo by Steff Hanson on Unsplash
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