New Study Shows How Tyrannosaurus rex Grew Up

Friday, January 3, 2020

A juvenile Tyrannosaurus rex, based on the Jane specimen, walking through a lake. Image credit: PaleoEquii / CC BY-SA 4.0.

In the early 2000s, the fossilized skeletons of two small tyrannosaurs were collected from the famous Hell Creek Formation of Carter County, Montana. Nicknamed Jane and Petey, the individuals would have been slightly taller than a draft horse and twice as long. Settling a debate about whether Jane and Petey represent a separate genus of pygmy tyrannosaurs (Nanotyrannus) or rather just juveniles of Tyrannosaurus rex, an analysis of sliced bones from the two specimens suggests the latter.

“Historically, many museums would collect the biggest, most impressive fossils of a dinosaur species for display and ignore the others,” said Dr. Holly Woodward, a paleontologist in the Oklahoma State University Center for Health Sciences.

“The problem is that those smaller fossils may be from younger animals. So, for a long while we’ve had large gaps in our understanding of how dinosaurs grew up, and T. rex is no exception.”

The smaller size of the Jane and Petey specimens is what make them so incredibly important.

Not only can paleontologists study how the bones and proportions changed as T. rex matured, but they can also utilize paleohistology– the study of fossil bone microstructure– to learn about juvenile growth rates and ages.

In the study, Dr. Woodward and colleagues removed thin slices from the leg bones of Jane and Petey and examined them at high magnification.

The researchers compared the organization of bone fibers and other microstructures in the specimens, finding that they appeared to have been growing, as evidenced by growth rings in the bone in a spaced-out pattern not typically seen in adults.

The bones also lacked the closely spaced series of lines present in adults that signals growth is complete.

The authors determined that small T. rex were growing as fast as modern-day warm-blooded animals.

They also found that Jane and Petey were teenaged T. rex when they died; 13 and 15 years old, respectively.

The results also support that a skull specimen at the Cleveland Museum of Natural History, which was classified as Nanotyrannus in 1988, is actually a young T. rex.

“Because it took T. rex up to twenty years to reach adult size, the tyrant king probably underwent drastic changes as it matured,” Dr. Woodward said.

“Juveniles such as Jane and Petey were fast, fleet footed, and had knife-like teeth for cutting, whereas adults were lumbering bone crushers.”

“Not only that, but we discovered that growing T. rex could do a neat trick: if its food source was scarce during a particular year, it just didn’t grow as much. And if food was plentiful, it grew a lot.”

The findings appear in the January 1, 2020 issue of the journal Science Advances.


Holly N. Woodward et al. 2020. Growing up Tyrannosaurus rex: Osteohistology refutes the pygmy ‘Nanotyrannus’ and supports ontogenetic niche partitioning in juvenile TyrannosaurusScience Advances 6 (1): eaax6250; doi: 10.1126/sciadv.aax6250