According to the principle of parsimony when applied to the heart as a trait, birds and mammals should indeed be placed in the same clade. This classification would provide the simplest explanation.
However, although similar, there are differences between the avian and mammal heart. For example, the former has smoother interior walls for the atria and ventricles as well as simpler valves.
Furthermore, other traits as well as the fossil record do not support this theory. When a clade is determined, more than one trait is taken into account. One such divergent trait is the temporal fenestra, large holes in the side of the skull.
The avians and their ancestors, the diapsids, have two temporal fenestrae behind the orbit. However, the synapsids and their mammalian descendants have only one, behind the eye.
