In such a case, we would simply need to look backward in history until we find an ancestor that doesn’t meet the chicken criteria. Fowl as a clade were separated from other bird clades before the K-T Extinction Event, and many such species before the event had teeth, which means they weren’t chickens.
No, we don’t. It doesn’t matter when that is, because you and I both agree that it’s out there somewhere, and that at the point in time referenced, a non-chicken laid an egg and a chicken hatched out of it. That’s all we need out of that point, and neither of us are disputing that part of it.
Agreed. I, personally, use the broader egg definition you reference in the last paragraph, but a definition of “chicken egg” would put the whole thing to rest, and I propose this: Not every chicken egg contains a viable chicken. We all agree that these eggs are still chicken eggs when we buy them at the supermarket, though, so my proposed definition is that a chicken egg is laid by a chicken. Otherwise, we end up with unclassified eggs in our omelettes, and we can’t have that.