Know Your Dinosaurs

One of these dinosaurs is a Tyrannosaur and the other is not.  Can you see tell which is and which is not and say why?  Answer is below the fold.

 

The top dinosaur is a Tyrannosaurid (a member of the Tyrannosaur familly).  Tyrannosaurs have only two fingers on their forelimbs.  If it has three fingers, it's something else, maybe an Allosaurus.  Of course, they could just be imaginary...