This week's Nature reports a new tyrannosauroid sporting a rather impressive crest.
This animal appears to be the most primitve member of the tyrannosaur lineage. Tyrannosaurs are but one of a larger group of meat-eating dinosaurs (theropods), and are along a major branch that eventually led to birds. The thought of T. rex as a relative of birds is a rather striking conclusion when you think of it. But these early, primitive tyrannosaurs help us reconstruct what the ancestors of the whole lineage might have been like. Indeed, the ancestors of much larger tyrannosaurs were quite a bit smaller.
The important thing to note is that this new tyrannosauroid, Guanlong wucaii is not an ancestor of animals like T. rex. At least, it isn't interpreted that way. It is its own unique branch along the base of that family's tree. Certainly, it's rather extravagant crest sets it apart as something quite unique on its own. But it doesn't preclude it from special relationships to other groups. It can be thought of, in a sense, as a side branch. When we have a number of such primitive 'side branches', we can use features common to all of them, but unique to the group, to reconstruct attributes that may have belonged to the anestor and were subsequently inherited by all (or most) of its descendents.
Scientists can see that Guanlong belongs to the tyrannosaur linage because it has a number of features that are unique to tyrannosaurs: the very front teeth in the upper jaw are "D"-shaped in cross-section with serrations along the "corners" of the D; and the way the bones along the top of the snout , the nasal bones and the frontal bones, fuse to each other -- just like T. rex and all its closest kin, but quite unlike all other theropods. However, Guanlong is what we might call "primitive" in other respects of its anatomy. That is, it is lacking a number of other characters unique to tyrannosaurs. For instance, you might have heard the scornful tone of an annoyed six-year-old who sees an ill-informed reconstruction of T. rex with three fingers. Of course, most dino-buffs or otherwise observant folks will know that tyrannosaurs normally only have two fingers. For most theropods, three fingers is the norm and two is the unique property of tyrannosaurs. Guanlong was quite clearly still has hands doing things the 'old fashioned way'. And thus, it was not descended from the common ancestor of those two-fingered tyrannosaurs.
Check out Carl Zimmer's post on this subject -- especially if I just confused you!
Xu, X. et al. 2006. A basal tyrannosauroid dinosaur from the Late Jurassic of China. Nature 439: 715-718.