I'm still hung up on creatures evolving from the sea to the land but then evolving back into the sea. That would seem odd until we found evidence of it, but then it must be accepted because it is observed.
If we ever FOUND a headless vertebrate, we would have to re-think our assumptions, I guess, not that I think we'll ever find such a creature. I've never met a creature who used to have a head and now doesn't but is still alive.
Of course, I've never met a creature with a half-formed head either, but I presume we must believe they existed at some point since we obviously didn't evolve an entire head with a single mutation (if we did, then one could postulate a single mutation that would reverse the process).
Oddly, we evolved a tail, and then managed to evolve it away, so it's not like we don't have evolutionary examples of body parts coming and going.
But not, I repeat again, a head. Because as I said a hundred posts ago, asking about the missing heads was a way of focusing on an issue by using an obvious example, and was in no way construable as saying that the birds in the example didn't have heads.
But it was much easier to argue that they must have had heads and it was stupid to suggest otherwise (now THAT was a strawman argument) than it was to discuss the issue of seeing what you expected to see.