I bought some antique maple flooring (incredible stuff - you can hit it with a hammer and it won't dent) and by looking at it the guy used a circular saw or grinder disk and cut the nails between the floor board and joist. ( The joists are secondary). If you pry it up too much the nails can break off one side of the grooves or tongue - whichever they origianlly nailed into. And it was always over the joists - subfloor is a modern thing.