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, 60 centimeters (2.0 ft) 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 5.2 meters (17 ft) 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.

In 2001, a more complete juvenile tyrannosaur (nicknamed "Jane", catalog number BMRP 2002.4.1), belonging to the same species as the original Nanotyrannus specimen, was uncovered.
This discovery prompted a conference on tyrannosaurs focused on the issues of Nanotyrannus validity at the Burpee Museum of Natural History in 2005.
Several paleontologists who had previously published opinions that N. lancensis was a valid species, including Currie and Williams, saw the discovery of "Jane" as a confirmation that Nanotyrannus was, in fact, a juvenile T. rex.
Peter Larson continued to support the hypothesis that N. lancensis was a separate but closely related species, based on skull features such as two more teeth in both jaws than T. rex; as well as proportionately larger hands with phalanges on the third metacarpal and different wishbone anatomy in an undescribed specimen.
He also argued that Stygivenator, generally considered to be a juvenile T. rex, may be a younger Nanotyrannus specimen.
Later research revealed that other tyrannosaurids such as Gorgosaurus also experienced reduction in tooth count during growth, and given the disparity in tooth count between individuals of the same age group in this genus and Tyrannosaurus, this feature may also be due to individual variation.
In 2013, Carr noted that all of the differences claimed to support Nanotyrannus have turned out to be individually or ontogenetically variable features or products of distortion of the bones.

In 2016, analysis of limb proportions by Persons and Currie suggested Nanotyrannus specimens to have differing cursoriality levels, potentially separating it from T. rex.
However, paleontologist Manabu Sakomoto has commented that this conclusion may be impacted by low sample size, and the discrepancy does not necessarily reflect taxonomic distinction.
In 2016, Joshua Schmerge argued for Nanotyrannus' validity based on skull features, including a dentary groove in BMRP 2002.4.1's skull.
According to Schmerge, as that feature is absent in T. rex and found only in Dryptosaurus and albertosaurines, this suggests Nanotyrannus is a distinct taxon within the Albertosaurinae.
The same year, Carr and colleagues noted that this was not sufficient enough to clarify Nanotyrannus' validity or classification, being a common and ontogenetically variable feature among tyrannosauroids.

A 2020 study by Holly Woodward and colleagues showed the specimens referred to Nanotyrannus were all ontogenetically immature and found it probable that these specimens belonged to T. rex.
The same year, Carr published a paper on T. rex's growth history, finding that CMNH 7541 fit within the expected ontogenetic variation of the taxon and displayed juvenile characteristics found in other specimens.
It was classified as a juve
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, 60 centimeters (2.0 ft) 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 5.2 meters (17 ft) 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.
In 2001, a more complete juvenile tyrannosaur (nicknamed "Jane", catalog number BMRP 2002.4.1), belonging to the same species as the original Nanotyrannus specimen, was uncovered. This discovery prompted a conference on tyrannosaurs focused on the issues of Nanotyrannus validity at the Burpee Museum of Natural History in 2005. Several paleontologists who had previously published opinions that N. lancensis was a valid species, including Currie and Williams, saw the discovery of "Jane" as a confirmation that Nanotyrannus was, in fact, a juvenile T. rex. Peter Larson continued to support the hypothesis that N. lancensis was a separate but closely related species, based on skull features such as two more teeth in both jaws than T. rex; as well as proportionately larger hands with phalanges on the third metacarpal and different wishbone anatomy in an undescribed specimen. He also argued that Stygivenator, generally considered to be a juvenile T. rex, may be a younger Nanotyrannus specimen. Later research revealed that other tyrannosaurids such as Gorgosaurus also experienced reduction in tooth count during growth, and given the disparity in tooth count between individuals of the same age group in this genus and Tyrannosaurus, this feature may also be due to individual variation. In 2013, Carr noted that all of the differences claimed to support Nanotyrannus have turned out to be individually or ontogenetically variable features or products of distortion of the bones.
In 2016, analysis of limb proportions by Persons and Currie suggested Nanotyrannus specimens to have differing cursoriality levels, potentially separating it from T. rex. However, paleontologist Manabu Sakomoto has commented that this conclusion may be impacted by low sample size, and the discrepancy does not necessarily reflect taxonomic distinction. In 2016, Joshua Schmerge argued for Nanotyrannus' validity based on skull features, including a dentary groove in BMRP 2002.4.1's skull. According to Schmerge, as that feature is absent in T. rex and found only in Dryptosaurus and albertosaurines, this suggests Nanotyrannus is a distinct taxon within the Albertosaurinae. The same year, Carr and colleagues noted that this was not sufficient enough to clarify Nanotyrannus' validity or classification, being a common and ontogenetically variable feature among tyrannosauroids. 
A 2020 study by Holly Woodward and colleagues showed the specimens referred to Nanotyrannus were all ontogenetically immature and found it probable that these specimens belonged to T. rex. The same year, Carr published a paper on T. rex's growth history, finding that CMNH 7541 fit within the expected ontogenetic variation of the taxon and displayed juvenile characteristics found in other specimens. It was classified as a juve
Tyrannosaurus