Of course you're running into problems reconciling everything: it's all a classification issue. There aren't any mammals, or reptiles, or bacteria, just stuff on a spectrum grouped for convenience. You can say that a mammal is a being with all these various characterstics, and you can convincingly lie to yourself about that most of the time, but if you go backwards in their evolution you'd run across creatures so close to mammals it'd be hard to tell the difference. At that point, all that separates this from that is a wish and a whim. Too many so-called scientists forget that.