This beastie lived during the late Cretaceous period.
When I was little, I remember watching a TV program about when the skull of this dinosaur was put into a CT scanner to determine whether this was an adult or juvenile; if the plates were fused, it would indicate that it was an adult; if they were not fused, it would indicate that it was a still-growing juvenile when it died. At the time this program aired (around 1988), the scan found that it was an adult, and then it was classed as Nanotyrannus, meaning “dwarf giant”.
But in 2001, a more complete skeleton of a juvenile “regular” T. rex was uncovered (nicknamed “Jane”), may shed more information on whether it simply represents a juvenile T. rex, or whether it is a new species of a previously identified genus of tyrannosaur.
http://en.wikipedia.org/wiki/Nanotyrannus
