Smithsonian Magazine has an article about teenage T. rex fossils.  The article discusses a recent paper that appeared in Science Advances.  That paper readdresses the two sleek, and slender tyrannosaurs, nicknamed Jane and Petey, at the Burpee Museum of Natural History in Rockford, Illinois and concludes that the two specimens are juvenile T. rexes, not Nanotyrannus as was previously published.

Maxing out around 40 feet in length and up to 9 tons in weight, adult T. rex were a force to be reckoned with. But the most well-studied T. rex fossils have also been the biggest—adults that had wrapped up most of the growing process in their early twenties, study author Holly Woodward, a paleontologist at Oklahoma State University, says in a statement. As such, exactly how these infamous behemoths—which likely entered the world as pigeon-sized hatchlings—attained these staggering sizes has long been mysterious.

So Woodward and her team decided to re-analyze two dinosaur specimens housed at Illinois’s Burpee Museum of Natural History. The pair, nicknamed Jane and Petey, are sleek, slender, and around 20 feet in length, bearing some resemblance to T. rex, but middling in size. As such, while most researchers believe Jane and Petey are probably T. rex teens, others have previously contended they were adult members of another species entirely—a diminutive dinosaur called Nanotyrannus.

But when the researchers analyzed microstructures in the specimens’ leg bones, they discovered that both Jane and Petey almost certainly juveniles at the time of their death. The fibers in their bones were haphazard, and teemed with ancient traces of blood vessels—signs that the dinosaurian duo were in the midst of rapid growth.