Level 2 — The inference of a character that leaves a signature on the skeleton of only one of the extant sister groups. For example, saying that "Tyrannosaurus rex" had air sacs running through its skeleton is a level 2 inference as birds are the only extant sister group to "Tyrannosaurus rex" to show such air sacs. However the underlying pneumatic fossae, air sacs, in the bones of extant birds are remarkably similar to the cavities seen in the fossil vertebrae of "Tyrannosaurus rex". The high degree of similarity between the pneumatic fossae in "Tyrannosaurus rex" and extant birds makes this a fairly strong inference, yet not as strong as a level 1 inference.