Tyrannosaurus rex is arguably the world’s most famous dinosaur, yet paleontologists are still learning more about this giant “tyrant lizard king.” T. rex roamed the western United States during the Cretaceous Period about 145 to 66 million years ago, and was likely alive when the meteor that k icked off all non-avian dinosaur’s extinction struck. Based on the annual growth rings (like those on trees) within fossilized leg bones, scientists  estimate that T. rex usually reach adulthood at around 25 years old. However, new research argues that their growth phase lasted  significantly longer. They may have become fully grown—approximately eight tons—after 40 years. The paper was recently published in the journal PeerJ. To produce an updated timeline of tyrannosaur growth, scientists studied 17 tyrannosaur specimens of all 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,” Nathan Myhrvold, a mathematician and paleobiologist at Intellectual Ventures and co-author of the new study said in a statement. “The composite growth curve provides a much more realistic view of how Tyrannosaurus grew and how much they varied in size.” The team then studied bone slices from the specimens with a type of light that unveils previously overlooked growth rings within the bones. The T. rex bone slices—or cross sections—only consist of the animal’s most recent one to two decades.  “Examining the growth rings preserved in the fossilized bones allowed us to reconstruct the animals’ year-by-year growth histories,” said Holly Woodward, a study co-author  and a professor of anatomy at Oklahoma State University.  Together, with co-author and Chapman University paleontologist Jack Horner, Myhrvold and Woodward, assembled a large  T. rex data set. From all of this data, it appears that the iconic beast followed the tortoise’s advice—it grew more slowly and steadily than what researchers thought.  A graph showing how the Tyrannosaurus rex may have grown Image: Dr. Holly Woodward Ballard. What’s more, the dinosaur’s long growth period might have enabled it to carry out diverse ecological roles before becoming fully grown, Horner explained. This also might be one of the reasons why they ruled at the top of the food chain toward the end of the Cretaceous Period.  The study also indicates that some of the 17 tyrannosaur specimens might actually not be T. rexes, joining other research suggesting the misidentification of some T. rex specimens. For example, a 2025 study argues that some small fossils, which some researchers thought were young T. rexes, actually belonged to a smaller relative, Nanotyrannus.  According to the team, these earlier propositions are still controversial and heavily debated. However, their study highlights the possibility that two famous specimens, dubbed Jane and Petey, might not be the same species, as well as other potential reasons why their growth curves are statistically incompatible. Interestingly, these are the same specimens that other recent  research categorized as two different Nanotyrannus species.  The post Tyrannosaurus rex took 40 years to reach full size appeared first on Popular Science. ...read more read less