I think there was some sort of chicken-like creature born with a very thin shell, that was practically a membrane, but you could still call it a shell. The chicken before it had more of a membrane around it, so you wouldn't call it a shell. But when that occurred would be subjective, and the differences between the membrane-shell and the shell-membrane would be so slight over the generations, that it's unlikely any two people would choose the same exact chicken/egg combination to answer the riddle. Sorry I couldn't be of more help.
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