For decades, scientists have been counting annual growth rings—similar to tree rings—inside fossilized leg bones of Tyrannosaurus rex to estimate how old the giant carnivores were when they died and how quickly they grew to adulthood. The best estimates from previous studies were that T. rex typically stopped growing at around age 25.
An extensive new study of 17 tyrannosaur specimens, ranging from early juveniles to massive adults, now concludes that the king of carnivores took 40 years to reach its full-grown size of around eight tons. The new analysis—the most complete life history ever conducted on T. rex—was able to assemble a more complete and accurate picture of tyrannosaurs' growth by using advanced statistical algorithms and examining slices of bone under a special kind of light, which reveals hidden growth rings not counted in previous studies. In addition to extending the growth phase of Tyrannosaurus by 15 years, the results suggest that some of the specimens may not be T. rex at all, but rather members of other species or different for some other reason. The study is published in the journal PeerJ .
"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 the rings visible on a tree stump, a cross section of T. rex bone records only the last 10 to 20 years of the animal's life.
"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."
Rather than racing to adulthood, Tyrannosaurus grew more slowly and steadily than previously believed. "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."
Although Tyrannosaurus rex is the best-known species of this group of dinosaurs, recent studies have proposed that some specimens previously identified as T. rex may in fact be members of other related species. Some scientists, for example, have argued that certain smaller specimens represent a small-bodied species, dubbed Nanotyrannus, rather than juvenile specimens of Tyrannosaurus rex.
Others have suggested that even the large specimens might belong to two or three different species.
These proposals remain controversial, however.
To help shed light on the question, the new study includes data from 17 specimens as part of the
"Tyrannosaurus rex species complex," noting that the complex could include other species or subspecies. One important finding of the study is that the growth curves of two of the more famous specimens, known by their nicknames "Jane" and "Petey," are statistically incompatible with the others. Although growth records alone cannot establish whether they were separate species, the evidence suggests that intriguing possibility, among other possible explanations. An independent analysis in a recent paper by Zanno and Napoli used different methods to classify Jane and Petey as each belonging to a different species of Nanotyrannus.
The discovery by Woodward, Myhrvold, and Horner that circularly polarized and cross-polarized light reveal a new kind of dinosaur growth ring helps to resolve longstanding problems reconciling the growth of some specimens. This discovery, backed up in the paper by robust statistical evidence, could be important in reevaluating the growth of other dinosaurs beyond T. rex. "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."
Even after more than a century of study, Tyrannosaurus rex continues to surprise paleontologists. By combining expanded sampling, innovative statistics, and careful bone analysis, the new study offers a clearer, more accurate picture of Tyrannosaurus rex as a living animal, growing from juvenile to giant.
Read the full open access article at https://peerj.com/articles/20469/
