Board Thread:General Discussion/@comment-213.107.92.35-20150103092457/@comment-17754282-20150103142706

It may well be a juvenile T rex, it may well be a different species. We can't really be sure until more examples of Tyrannosaurus and Nanotyrannus at different growth stages are found.

Unfortunately Nanotyrannus having fewer teeth in its lower jaw that Tyrannosaurus cannot be used as proof that they are separate animals as juvenile Gorgosaurus are known to have more teeth than adult Gorgosaurus. On the other hand, juvenile Tarbosaurus are known to have the same number of teeth as adult Tarbosaurus, so maybe the opposite is true.

Again though using Tarbosaurus as evidence is controversial as it is currently unknown how closely related Tarbosaurus and Tyrannosaurus actually are - some say they are sister taxon, others say they are the same animal and others argue that they are the end result of two lines of Tyrannosaurs which evolved similarly.