Oh, and one more thing, they are called mammals because they have mammary glands, not because of their bone structure.
It might not be a "necessary" characteristic if one was to design a mammal from scratch, but in the real world the two are invariably, with no exceptions, found together. Mammals could just as easily have been called "animals with three inner ear-bones" or "animals with a single-boned jaw". Those characteristics are just as specific to mammals as mammary glands. Mammary glands are just a little more obvious, that's all. They are no more characteristic of the mammalian group than the earbones or the jaw.
It is a statistical one only and such statistical ones such as the statement still made by a museum that all mammals have live birth shows that such statistical characteristics are not evidence as the platypus showed. Further, as I pointed out, since it is not a necessary characteristic and since evolution supposedly works in a gradual manner, it cannot be said that any non-extant species had mammary glands or how such an function ever developed in a gradual evolutionary manner.
When it comes to extinct species, there are no mathematical proofs. You can use the evidence available to support your conclusion. The bones say they're mammals. There is nothing in the bones that goes against this hypothesis, so it stands. Assuming that evolution explains how the various groups of mammals came to exist, then lactation arose before the three modern groups diverged. And even if I didn't find evolution convincing and believed that God actively created each individual species over hundreds of millions of years, I would still be persuaded by the skeletons that these animals were mammals. The alternative is that some other, unknown, non-mammalian group may have just happened to have the exact same types of skeletons. The evidence says mammal and there is absolutely nothing to suggest that they were anything else, evolution or no evolution.
As I already pointed out, one of the characteristics of mammals was thought to be live birth (and as I also pointed out some think it still is). The only reason it has been taken out of the definition is that we found a LIVING BREATHING EXAMPLE of a mammal which did not give live birth. So just because there are certain not necessary bone formations which all present mammals share it does not mean that all mammals have had those traits all the time. This is particularly important if one was to try to trace the gradual evolution of mammals. These organic traits are the most important, that is why call them mammals and such traits cannot be found in the fossil record. The problems go both ways also, there are sharks that give live birth for example and we would not know that if we did not have LIVING EXAMPLES of such sharks. What this shows very well is the inability of the fossil record to: show us anything we do not already know about species, show us anything which may be unique about them, and to verify that evolution has occurred. Further, and even evolutionists admit this, similar bone structures do not mean similar functioning. The bat is a great example of this. What all this means is that you cannot prove evolution through fossils.