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.
c/196 has 1 rule: if you arrive, you have to post something before you leave. People have taken to putting the word “rule” in their titles to show their compliance, but it’s not strictly necessary.