What would a transitional fossil look like?

I did spell out some time ago the differences that have to be navigated to get from the reptilian ear to the mammalian ear, which is a much more limited project than getting from a cow to a fish but I may try that one two. Remove legs from genome. Add fins and fish tail and fish breathing apparatus. This is silly. Just to make those two changes would require millions of years of mutations and still you wouldn't get them.

It really doesn't matter what you want to descend from what. It's impossible unless the genetic stuff is already present for the completely new creature. For instance getting a human being from a chimpanzee is equally impossible. Each has its own genome that makes the given creature and nothing else. To get from the chimpanzee to the human being requires a whole slew of changes into something altogether different from the chimpanzee, things that don't exist in the chimpanzee genome and that makes any such transformation impossible because mutation has to invent something completely new rather than simply changing something that already exists into another version of it which is already potential in the genome anyway.

Randomn variation plus selection gets you a version of the same creature, it's nothing but microevolution or standard variation. You can't get macroevolution out of any such events. The reason it doesn't work is that macroevolution requires something completely new to be formed. All those mechanisms do is rearrange what already exists.