In the two tyrannosaurs, the fibers “look like pickup sticks,” said Dr. Woodward. “Jane and Petey were growing pretty quickly up until they died.”

The researchers then looked at the bones’ cyclical growth marks. These are the animal equivalent of tree rings — they form during periods of slow growth, and in this way record the passage of years. (For example, the cyclical growth marks in the bones of Svalbard reindeer correspond with polar winters, when the food supply is lowest.)

By counting these growth marks, the researchers found that Jane died at around 13 years old, while Petey was around 15. Experts believed T. rexes reached maturity around 20 years old, and could have lived to about 30.

The marks also indicate how much each dinosaur grew during a particular year. For Jane and Petey, some of the rings are very close together, marking a year of minimal growth. Others are much farther apart, indicating rapid change.

While experts have used morphology to argue both for and against the existence of Nanotyrannus, this internal evidence shows more conclusively that Jane and Petey are not adult specimens of a smaller Tyrannosaur species. Instead, they were “sub-adults with high growth rates,” which means they were most likely juvenile T. rexes, said Thomas Carr, an associate professor of biology at Carthage College in Kenosha, Wis., who had studied Jane before but was not involved with this research.

They also give us a better idea of how this one carnivore was able to dominate the landscape. The uneven spacing of the growth rings suggests that young T. rexes responded to the amount of resources available, growing quickly when food was plentiful, and stopping growth altogether when times were lean.