“What we found is that, as a group, tyrannosaurs got big twice in their history: once early on in in the Cretaceous, in the form of Yutyrannus, and another later episode starting around 110 million years ago in the lead up to T. rex and its kin,” Bell said in an interview. “The weird thing is that they did it differently. “Yutryannus did it with feathers, and T. rex and his lot dropped the feathers in favor of scales. “Why? It’s difficult to say.”
and, of course, TRex didn't have lips either.