A single mutation causes the presence or absence of a tail in larvaceans.
Likewise, a single mutation makes wings leglike and vice versa in birds.
Also, there is no need for wholesale new parts to appear all at once. Fish
can get some extra oxygen by gasping at the surface. Even a rather
rudimentary lung would be useful to them. For that matter, the first
tetrapods were apparently still aquatic, so the development of lungs and
feet were not entirely in sync for an invasion of land. Heart development
has some transitions evident within modern animals, as the crocodiles have
rudimentary division of the 3rd chamber into two , whereas most reptiles
have no such division and birds are fully four-chambered. Monotremes may
also have the heart division less well-developed than more advanced
mammals, but I do not remember for certain.
Lungs and hearts also have a lousy fossil record. Transitions in skeletal
features are much better documented.
Irreducible complexity can be disproved, but I cannot think of how it can
be proven.
David C.