For years, scientists believed Tyrannosaurus rex reached its adult size at around age 25. But a major new study suggests the iconic predator may have taken much longer to mature. According to the research, T. rex likely continued growing for about 40 years before reaching its maximum size of roughly eight tons.
The findings come from an analysis of 17 tyrannosaur fossils spanning a wide range of ages, from young juveniles to massive adults. Researchers say the work provides the most detailed reconstruction yet of how Tyrannosaurus grew throughout its life.
The study was published in the journal PeerJ.
Reading the Growth Rings Hidden Inside Dinosaur Bones
To estimate the age of dinosaurs, paleontologists often examine growth rings preserved inside fossilized bones. These rings are somewhat similar to the annual rings found in tree trunks. Each growth mark can provide clues about how quickly an animal was growing and how old it was when it died.
For decades, scientists have used these rings to reconstruct the life history of Tyrannosaurus rex. However, the new study employed more advanced techniques than earlier investigations. Researchers examined thin slices of fossil bone under specialized lighting that can reveal growth rings that are difficult to detect using standard methods.
The team also used sophisticated statistical models to combine information from multiple specimens. This allowed them to create a more complete picture of growth across the entire lifespan of T. rex.
The results indicate that Tyrannosaurus remained in a growth phase about 15 years longer than previously thought.
In addition, the findings suggest that some fossils traditionally assigned to T. rex may actually belong to other closely related species, or differ for other biological reasons.
Largest T. Rex Dataset Ever Assembled
“This is the largest data set ever assembled for Tyrannosaurus rex,” says Holly Woodward, a professor of anatomy at Oklahoma State University who led the research effort. “Examining the growth rings preserved in the fossilized bones allowed us to reconstruct the animals’ year-by-year growth histories.”
Unlike a tree stump, which preserves rings from an organism’s entire life, dinosaur bones provide only a partial record. A cross section of a T. rex leg bone typically preserves information from just the final 10 to 20 years of the animal’s life.
To overcome that limitation, the researchers combined growth records from multiple individuals of different ages.
“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,” explains Nathan Myhrvold, a mathematician and paleobiologist at Intellectual Ventures who led the statistical analysis.
“The composite growth curve provides a much more realistic view of how Tyrannosaurus grew and how much they varied in size.”
A Slower Path to Becoming a Giant Predator
The new results paint a different picture of Tyrannosaurus development than earlier studies.
Instead of rapidly reaching adulthood, T. rex appears to have grown at a steadier pace over several decades. According to the researchers, this prolonged growth period may have helped younger tyrannosaurs occupy different ecological niches as they matured.
In ecology, a niche refers to the role an organism plays within its environment, including what it eats, where it lives, and how it interacts with other species.
“A four-decade growth phase may have allowed younger tyrannosaurs to fill a variety of ecological roles within their environments,” says coauthor Jack Horner of Chapman University. “That could be one factor that allowed them to dominate the end of the Cretaceous Period as apex carnivores.”
The Cretaceous Period ended about 66 million years ago, shortly before the extinction of non-avian dinosaurs.
Are Some Famous T. Rex Fossils Actually Different Species?
The study also contributes to an ongoing debate among paleontologists.
Although Tyrannosaurus rex is one of the most famous dinosaurs ever discovered, some researchers have argued that not every fossil labeled as T. rex necessarily belongs to the same species.
One controversial proposal suggests that several smaller fossils represent a separate dinosaur called Nanotyrannus rather than young Tyrannosaurus individuals. Other researchers have proposed that even some large specimens may belong to multiple closely related species.
The issue remains unresolved.
To investigate the question, the researchers included 17 fossils belonging to what they describe as the “Tyrannosaurus rex species complex,” a term acknowledging the possibility that more than one species or subspecies could be represented.
Two particularly famous specimens, nicknamed “Jane” and “Petey,” stood out from the rest. Their growth patterns differed significantly from those of the other fossils in the study.
While growth data alone cannot determine whether these animals belonged to different species, the unusual patterns make that possibility worthy of further investigation.
The researchers note that a separate recent study by Zanno and Napoli reached a similar conclusion using different methods, classifying Jane and Petey as two distinct species of Nanotyrannus.
Hidden Growth Rings Could Change Dinosaur Research
Another important finding involves the discovery of previously overlooked growth markers inside dinosaur bones.
Woodward, Myhrvold, and Horner found that circularly polarized and cross-polarized light can reveal a new type of growth ring. These hidden features may help explain discrepancies that have puzzled researchers studying dinosaur growth.
Because the approach is supported by strong statistical evidence, it could influence how scientists examine fossils in future studies, not only for T. rex but for many other dinosaur species as well.
“Interpreting multiple closely spaced growth marks is tricky,” Myhrvold says. “We found strong evidence that the protocols typically used in growth studies may need to be revised.”
A New Look at the Life of Tyrannosaurus Rex
More than a century after Tyrannosaurus rex was first described by scientists, the giant predator continues to reveal new surprises.
By combining a larger fossil sample, improved imaging techniques, and innovative statistical analysis, the new research provides one of the clearest views yet of how T. rex developed from a young dinosaur into one of the largest and most formidable land predators in Earth’s history.
The findings suggest that the king of dinosaurs may have taken far longer to grow up than anyone previously realized.