In Bemidbar 19:16, the Torah mentions impurity conveyed by a person's bone. From this, the gemara learns out that those parts of a corpse that convey impurity are those parts that are like a bone in two respects: a person is born with them, and if removed they don't grow back (Niddah 55a).
One of the body parts thus excluded is teeth, on the grounds that a person is not born with them. But a person is born with them, they just haven't yet extended through the gums. My question, therefore, is in two parts:
1) Were the rabbis mistaken, thinking that because the teeth aren't yet visible they therefore don't exist?
2) By suggesting that the teeth don't exist yet did the sages mean that they're just not visible?
In the case of 1), does that mean that there is a valid reason to consider teeth as conveyers of impurity, now that we know that a baby is born with them? And in the case of 2), does that mean that other body parts that are not yet visually apparent (such as testes, in the case of boys) also therefore don't convey impurity in a corpse?