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Sometimes the simplest explanation is the right one. Two controversial dinosaur skeletons have been held up as evidence that Tyrannosaurus rex shared its environment with a second, tiny species of tyrannosaur. Now, a detailed study of the fossilised leg bones suggests the diminutive dinos are really just teenage T. rexes.

The two skeletons, one nearly complete, were discovered in the early 2000s in rocks known as the Hell Creek Formation, which spans Montana, Wyoming and North and South Dakota. The formation also yielded some of the first T. rex bones, early in the 20th century.

The more complete specimen is nicknamed Jane, and the other is nicknamed Petey. Along with one other small skull found in 1942, they have been used to argue for a new type of dinosaur called Nanotyrannus, which was like a T. rex but smaller.

As researchers performed more detailed analysis of the specimens, it seemed more and more likely that the small predators were simply young T. rexes, but there was still some disagreement. The newest analysis by Holly Woodward at Oklahoma State University and her colleagues may be the final nail in the coffin of Nanotyrannus.

Read more: T. rex evolved into a monster predator by dumbing down its brain

All modern vertebrates have a period every year where bone growth briefly pauses. We don’t know exactly why this happens, but it leaves a circle in every bone that shows when the growth stopped, Woodward says. “We can just count the rings like with a tree to find the dinosaur’s age.”

Woodward and her team counted the rings in Jane and Petey’s femurs and tibias. They found that Jane was probably around 13 years old and Petey about 15 when they died. Other fossils such as Sue, one of the largest and most complete T. rex skeletons we’ve found, have shown that T. rexes lived to around 30, so that makes Jane and Petey adolescents. Because they are so young, there is no need to invoke a whole new species to explain their small size.

“We know the T. rex as this giant king of dinosaurs,” says Woodward. “But it didn’t start that way, and we didn’t know much about how it got from baby T. rex-sized… to Sue-sized.”

At a little more than half Sue’s 12.3-metre body length, these young dinos are starting to fill in that gap.