 Several species were incorrectly assigned to Gorgosaurus in the twentieth century . A complete skull of a small tyrannosaurid ( <unk> <unk> ) , found in the younger , late Maastrichtian @-@ age Hell Creek Formation of Montana , was named Gorgosaurus lancensis by Charles Whitney Gilmore in 1946 . This specimen was renamed Nanotyrannus by Bob Bakker and colleagues in 1988 . Currently , many paleontologists regard Nanotyrannus as a juvenile Tyrannosaurus rex . Similarly , Evgeny Maleev created the names Gorgosaurus lancinator and Gorgosaurus novojilovi for two small tyrannosaurid specimens ( PIN 553 @-@ 1 and PIN 552 @-@ 2 ) from the Nemegt Formation of Mongolia in 1955 . Kenneth Carpenter renamed the smaller specimen Maleevosaurus novojilovi in 1992 , but both are now considered juveniles of Tarbosaurus bataar . 
