The humerus, ulna, radius, carpals, metacarpals, and phalanges are the shared derived characteristics of all tetrapods. None of them are specific to either birds or bats. Teats, fur, three middle ear bones, cusped cheek teeth, and single lower jaw bone are the classic mammalian traits that are not shared with other groups. Feathers and flow through lungs are classic bird features that are not shared with other groups. Do you see how this works?
Added in edit:
For those looking for a primer on phylogenetics, I would strongly recommend Understanding Evolution’s website which does a great job of introducing the concepts:
