One of the earliest tyrannosaurs yet to be discovered, Guanlong (the name, "crown dragon," alludes to this meat-eater's prominent crest) roamed eastern Asia during the late Jurassic period. Like other early theropods, Guanlong was nothing special in terms of size, only a fraction as large as T-rex. This points to a common theme in evolution, the development of plus-sized animals from small progenitors.
How do paleontologists know that Guanlong was a tyrannosaur? Clearly, this dinosaur's crest--not to mention its fairly long arms and (possibly) its coat of feathers--make it an ill-fitting match with the classic tyrannosaurs of the late Cretaceous period. The giveaway is the characteristic shape of Guanlong's teeth and pelvis, which point to its being a "basal" (i.e., early) member of the tyrannosaur family. Guanlong itself appears to have descended from earlier, smaller theropods known as coelurosaurs, the most prominent genus of which was Coelurus.