Nanotyrannus ("dwarf tyrant") is a genus of tyrannosaurid dinosaur, known only from two juvenile specimens, which may in fact represent juvenile specimens of the contemporary species Tyrannosaurus rex.
Description: Differences from the T-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.
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.
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