Yeah, I guess it would have to have either been long enough ago that it's been erased by geology (which seems somewhat infeasibly long ago wrt the evolutionary

Yeah, I guess it would have to have either been long enough ago that it's been erased by geology (which seems somewhat infeasibly long ago wrt the evolutionary timeline) or recent enough that we would likely have found a trace of it by now. I guess the only chance is for some missing link far in the evolutionary past that was erased by e.g., one of the past mass extinctions.