Small Tyrannosaurus rexes do not represent a distinct dinosaur genus, according to fresh analysis of thinly sliced bones.

Authors of a new study, published this week in the journal Science Advances, claim the shrunken specimens are juvenile T. rexes.

Tyrannosaurus rexes are famous for their size and, for decades, fossil hunters and museums have prioritized the largest specimens. For this reason, and the simple fact that smaller fossils are more fragile and harder to find, scientists know relatively little about the growth and maturation rates of one of the most famous and ferocious dino species.

"Historically, many museums would collect the biggest, most impressive fossils of a dinosaur species for display and ignore the others," lead study author Holly Woodward, a postdoctoral researcher at Oklahoma State University Center for Health Sciences, said in a news release. "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."

For the new study, Woodward and her colleagues examined thin slices of bone sampled from the remains of a pair of small T. rexes named Jane and Petey. The two dinosaur fossils were found nearly 20 years ago in Montana. Today, they're housed in the Burpee Museum of Natural History in Illinois.

Small patterns hidden in the bones' internal structures helped scientists characterize the age and identities of the specimens.

"To me, it's always amazing to find that if you have something like a huge fossilized dinosaur bone, it's fossilized on the microscopic level as well," Woodward said. "And by comparing these fossilized microstructures to similar features found in modern bone, we know they provide clues to metabolism, growth rate, and age."

Specifically, researchers found the growth rings inside the bones were spaced-out, a pattern not usually observed in adults. There were also no tightly bunched parallel lines inside the bones, a sign that bone growth is complete. Both discoveries showed the two dinos were juvenile T. rexes, not members of a unique genus.

The bones showed no evidence that the juveniles had experienced the rapid growth spurt experienced late in the T. rex maturity process. By counting the bones' growth rings, scientists were able to estimate the age of Jane and Petey at 13 and 15 years old, respectively. The new research suggests juvenile T. rexes grew at about the same rates as modern mammals and birds.

But just as the growing environment varies for modern species, life was a mixed bag for young T. rexes. Some years produced more food and friendlier conditions than others.

In good times and bad, researchers suggest even juvenile T. rexes were well equipped to dominate their ecosystems. While adult T. rexes were massive, bone-crushing bruisers, as juveniles they were fast, nimble and armed with knife-like teeth.

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