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.

Actually, iron rusts away so quickly that it's hard to recover them from the ground after only a millennium or two. Bronze tends to fare much better in that regard.

Wood can last quite a while if it's buried in an environment that inhibits microbial degradation, such as a highly acidic bog, where iron doesn't stand a chance.