Nanotyrannus thumb|Cast of CMNH 7541, the holotype of Nanotyrannus lancensis, sometimes interpreted as a juvenile Tyrannosaurus. Other tyrannosaurid fossils found in the same formations as T. rex were originally classified as separate taxa, including Aublysodon and Albertosaurus megagracilis, the latter being named Dinotyrannus megagracilis in 1995. These fossils are now universally considered to belong to juvenile T. rex. A small but nearly complete skull from Montana, long, might be an exception. This skull, CMNH 7541, was originally classified as a species of Gorgosaurus (G. lancensis) by Charles W. Gilmore in 1946. In 1988, the specimen was re-described by Robert T. Bakker, Phil Currie, and Michael Williams, then the curator of paleontology at the Cleveland Museum of Natural History, where the original specimen was housed and is now on display. Their initial research indicated that the skull bones were fused, and that it therefore represented an adult specimen. In light of this, Bakker and colleagues assigned the skull to a new genus named Nanotyrannus (meaning "dwarf tyrant", for its apparently small adult size). The specimen is estimated to have been around long when it died. However, In 1999, a detailed analysis by Thomas Carr revealed the specimen to be a juvenile, leading Carr and many other paleontologists to consider it a juvenile T. rex individual.
Nanotyrannus thumb|Cast of CMNH 7541 the holotype of Nanotyrannus lancensis sometimes interpreted as a juvenile Tyrannosaurus Other tyrannosaurid fossils found in the same formations as T rex were originally classified as separate taxa including Aublysodon and Albertosaurus megagracilis the latter being named Dinotyrannus megagracilis in 1995 These fossils are now universally considered to belong to juvenile T rex A small but nearly complete skull from Montana long might be an exception This skull CMNH 7541 was originally classified as a species of Gorgosaurus (G lancensis) by Charles W Gilmore in 1946 In 1988 the specimen was re-described by Robert T Bakker Phil Currie and Michael Williams then the curator of paleontology at the Cleveland Museum of Natural History where the original specimen was housed and is now on display Their initial research indicated that the skull bones were fused and that it therefore represented an adult specimen In light of this Bakker and colleagues assigned the skull to a new genus named Nanotyrannus (meaning "dwarf tyrant" for its apparently small adult size) The specimen is estimated to have been around long when it died However In 1999 a detailed analysis by Thomas Carr revealed the specimen to be a juvenile leading Carr and many other paleontologists to consider it a juvenile T rex individual