Where did I ever state that the Cambrian period lasted less than many millions of years? Ah, I didn't. Thanks.
Good, so presumably you also accept that the "explosion" actually occurred over millions of generations, so no suggestion of radical change from one generation to the next...
BTW The species without apparent precursors are usually microscopic. Their precursors were probably too small to have a significant chance of leaving a fossil that is detectable today.