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Even as teenagers, Tyrannosaurus rex dominated the top of the food chain. Researchers can say that with more certainty after confirming that two small T. rex fossils found in the early 2000s are not a pygmy species of tyrannosaurs, but juveniles of the rex species.

In order to settle the decades-long debate, the researchers had to get up close and personal with Jane and Petey, the two fossils named by the staff at the Burpee Museum of Natural History in Rockford, Illinois.

At its peak, T. rex was 40 feet long. But the two small fossils are just a bit taller than a Clydesdale horse and twice its length. When they were found, the fossils were thought to belong to a separate genus of “pygymy” tyrannosaurs called Nanotyrannus.

Researchers sliced into leg bones from the two fossils to understand the age at the time of death for the dinosaurs. Like tree rings, the leg bones of the dinosaurs showed growth rings that put Jane and Petey at 13 and 15 years old, respectively.

The fossils also showed a vascular record, or evidence of blood vessel activity, supporting a rapidly growing young dinosaur. Their study published Wednesday in the journal Science Advances.

The growth rings revealed how the young T. rexes endured years of scarcity.

“The spacing between annual growth rings record how much an individual grows from one year to the next,” said Holly Woodward, lead study author at the Oklahoma State University Center for Health Sciences. “The spacing between the rings within Jane, Petey, and even older individuals is inconsistent — some years the spacing is close together, and other years it’s spread apart.”

This suggests that if food was scarce one year, Jane and Petey didn’t grow as much, but they made up for it by growing even more during years when food was abundant.

Small, juvenile fossils of larger dinosaurs are equally important to researchers as adult specimens because paleohistology, or the study of fossil bone microstructure, can be used to show how the dinosaurs grew as they aged.

“Historically, many museums would collect the biggest, most impressive fossils of a dinosaur species for display and ignore the others,” Woodward said. “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.”

In the case of T. rex, the dinosaur could take 20 years to reach its massive size, meaning it likely went through a number of dramatic changes over time.

Given how much T. rex changed from juvenile to adult, the researchers believe that the different age groups of these dinosaurs fed on varied prey.

“That’s even cooler [than their being a separate genus],” said Scott Williams, study co-author and paleontology lab and field specialist at the Museum of the Rockies. “That tells us they go through a drastic change when they grow up from these sleek, slender, fleet-footed T. rexes with these wonderful knife-like teeth to these big, monster, plodding crushing tyrannosaurs that we are familiar with. It also tells us these animals probably dominated their ecosystems at all ages.”