A new analysis of fossils believed to be juveniles of T. rex now shows they were adults of a small tyrannosaur, with narrower jaws, longer legs, and bigger arms than T. rex. The species, Nanotyrannus lancensis, was first named decades ago but later reinterpreted as a young T. rex.

The first skull of Nanotyrannus was found in Montana in 1942, but for decades, paleontologists have gone back and forth on whether it was a separate species, or simply a juvenile of the much larger T. rex.

Dr. Nick Longrich, from the Milner Centre for Evolution at the University of Bath, and Dr. Evan Saitta, from the University of Chicago, have re-analyzed the fossils, looking at growth rings, the anatomy of Nanotyrannus, and a previously unrecognized fossil of a young T. rex.