Nanotyrannus ("dwarf tyrant") is a potentially dubious genus of tyrannosaurid dinosaur. It is known only from two specimens (possibly three), which are potentially juvenile specimens of the contemporary genus Tyrannosaurus rex.
Differences from Tyrannosaurus rex
The primary differences that some scientists have used to distinguish Nanotyrannus lancensis from Tyrannosaurus rex primarily concern the number of teeth. Nanotyrannus had more teeth in its upper and lower jaws than an adult Tyrannosaurus. N. lancensis had 14-15 teeth in each side of the upper jaw (maxilla) and 16 teeth in each side of the lower jaw (dentary). T. rex, on the other hand, had 11-12 tooth positions in the upper jaw and 11-14 in the lower. The exact implications of this difference in tooth count has been controversial. In his 1999 study of tyrannosaurid growth patterns, Carr showed that, in Gorgosaurus libratus, the number of teeth decreased as the animal grew, and he used this data to support the hypothesis that N. lancensis is simply a juvenile T. rex. The team of scientists who studied growth in the related Tarbosaurus bataar found little to no decrease in tooth count as that species grew, even though they had juvenile specimens much younger than the Nanotyrannus specimens. These researchers also noted, however, that in both Tyrannosaurus and Gorgosaurus, there were significant differences in tooth count between individuals of the same age group, and that tooth count may vary on an individual basis not related to growth. A juvenile Tarbosaurus skull discovered in 2006 exhibits the same tooth count as do adult Tarbosaurus skulls, supporting the retention of Nanotyrannus as a distinct genus. Larson has also contended that, along with skull features, Nanotyrannus can also be distinguished from Tyrannosaurus by proportionally larger hands with phalanges on the third metacarpal and in the furcula morphology.
Another difference cited by those who support the validity of N. lancensis is the presence of a small foramen, or pit, in the quadratojugal, a bone in the back corner of the skull. Both the holotype and the "Jane" specimen have this feature, suggesting it is not a deformity, and it is not known in any adult tyrannosaurid specimens. It is possible that this is again an individual variant, or that it was a feature lost as the animals grew, though studies of other juvenile tyrannosaurids do not show an equivalent feature. However, most of the differences claimed to support Nanotyrannus turned out to be individually or ontogenetically variable features. The foramen at the quadratojugal is also problematic for the validity of Nanotyrannus, as skull sutures are ambiguous evidence for maturity in archosaurs.
3D models of the brain cavities of the holotype skull and T. rex's skull show that blood vessel positions and those of the optic nerve attachments do not match, possibly suggesting a distinction between Nanotyrannus and T. rex. This could be accounted for by changes in the skull shape as T. rex grows.
The Land Before Time
Nanotyrannus appeared as an enemy in The Land Before Time for Gameboy Advance. They appear in the second area of the game, Smelly Swamp, and the fourth area of the game, Dark Cavern. They will walk back and forth until they see the player, with it then trying to charge and bite the player. However, it can be defeated by jumping on its back twice. It gives the player 400 points when defeated, the highest of any enemy by this point in the game.