Newsweek has published another fascinating excerpt from Richard Dawkins' latest book, The Greatest Show on Earth: The Evidence for Evolution. Definitely worth a read. Here was perhaps the most brilliant paragraph in the excerpt:
I have dealt with the Cambrian Explosion at length before. Here I'll add just one new point, illustrated by the flatworms, Platyhelminthes. This great phylum of worms includes the parasitic flukes and tapeworms, which are of great medical importance. My favorites, however, are the free-living turbellarian worms, of which there are more than 4,000 species: that's about as numerous as all the mammal species put together. They are common, both in water and on land, and presumably have been common for a very long time. You'd expect, therefore, to see a rich fossil history. Unfortunately, there is almost nothing. Apart from a handful of ambiguous trace fossils, not a single fossil flatworm has ever been found. The Platyhelminthes, to a worm, are "already in an advanced state of evolution, the very first time they appear. It is as though they were just planted there, without any evolutionary history." But in this case, "the very first time they appear" is not the Cambrian but today. 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 millions, the flatworms must have been living happily alongside them, but without leaving the slightest 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 is 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 and finally destroys the creationist case that the Precambrian gap in the fossil record can be taken as evidence against evolution.
I've done a little research on this, and seems that there might actually be one flatworm fossil, which you can read about here. However, an article in American Scientist states that "no fossil flatworms are definitely identified in the fossil record" so perhaps that finding is controversial, or perhaps it is the lonely (or near lonely) representation of flatworms in the fossil record. Either way Dawkins' point still stands.