For 70 years, scientists have bickered and brawled whether Tyrannosaurus rex had a smaller and skinnier cousin, which they named Nanotyrannus. Now, a T. rex called Jane might have just settled the dispute.

Jane is the most complete juvenile T. rex ever found. In 2001, a paleontology team from the Burpee Museum unearthed Jane at the Hell Creek Formation in Montana – a well-known treasure chest of fossils. While a T. rex can grow up to 12 meters (40 feet) in length, Jane stood at just 6 meters (20 feet).

As a near-ideal example of a juvenile T. rex, the 11-year-old Jane provides scientists with crucial information about the different life stages of this dinosaur. The researchers presented their new analysis of Jane at the 75th annual Society of Vertebrate Paleontology conference in Dallas last week. 

The gaps in knowledge about how these dinosaurs changed and developed as they matured had previously confused paleontologists.

In 1988, Robert Bakker analyzed a small tyrannosaur skull that was on display at the Cleveland Museum of Natural History. He concluded that it instead belonged to a separate species that was smaller and lighter, which he dubbed Nanotyrannus lancensis. 

Eleven years later, Thomas Carr, an associate professor of biology at Carthage College in Wisconsin, argued that the skull was not a new species but was instead a juvenile T. rex.

"The extreme changes from the sleek skull of juveniles to the robust skull of adults were too much for some people to believe," Carr said to Live Science. "Regardless, the search was on for a transitional specimen that could test the hypothesis."

That search was successful, according to Carr and his team. Jane demonstrates that T. rex dinosaurs underwent a transition from slinky juveniles to bulky adults. Carr came to this conclusion by looking at the microscopic “growth rings” in Jane’s calf bone. Typically, the bones grow a new ring each year. For Jane, they found nine rings in the bone and space for two more, thus giving her an age of 11 years. They also found evidence of remodeling typical of fast-growing bone, leading them to believe she was a juvenile. 

The size and shape of Jane's skeleton is also what you'd expect as an intermediate between the Cleveland skull and a fully grown T. Rex. Speaking to Live Science, Carr added: "Jane shows us that the gap is, in fact, bridgeable because many features seen in her are more similar to adult T. rex than to the Cleveland skull. The features are exactly what we'd predict are necessary to make the change to a full adult." 

Carr then compared the skull of Jane to Bakker's Nanotyrannus. Both fossils share features once thought to be unique to Nanotyrannus, such as a hole in the jaw bone and a long, low snout, but Carr concluded that these features actually characterize a juvenile T. Rex and do not signify a separate species.

However, it's worth noting that Bakker is still sticking to his guns. He told the journal Science that Carr "has not seen the best specimen” of a Nanotyrranus fossil. Details of that finding have not yet been published. 

Once paleontologists have learned all they can from Jane, the skull will go on display at a museum. Thanks to the remains being found on public land, the museum will receive the specimen for free.