Apart from a handful of ambiguous trace fossils, not a single fossil flatworm has ever been found… Do you see what this means, or at least ought to mean, for creationists? Creationists believe that flatworms were created in the same week as all other creatures. They have therefore had exactly the same time in which to fossilize as all other animals. During all the centuries when all those bony or shelly animals were depositing their fossils by the thousands, the flatworms must have been living happily alongside them, but without leaving any significant trace of their presence in the rocks. What, then, is so special about gaps in the record of those animals that do fossilize, given that the past history of the flatworms amounts to one big gap; even though the flatworms, by the creationists’ own account, have been living for the same length of time?
If the gap before the Cambrian Explosion is used as evidence that most animals suddenly sprang into existence in the Cambrian, exactly the same ‘logic’ should be used to prove that the flatworms sprang into existence yesterday. Yet this contradicts the creationist’s belief that flatworms were created during the same creative week as everything else. You cannot have it both ways. This argument, at a stroke, completely destroys the creationist case that the Precambrian gap in the fossil record weakens the evidence for evolution.
— Richard Dawkins, The Greatest Shown on Earth