Ever since Gould's Wonderful Life: The Burgess Shale and the Nature of History, the popular view has been that the Cambrian was an "explosion" of living forms, and for some, usually but not always creationists, this has been touted as contrary to "Darwinism" (whateverthehell that is) or even evolutionary theory. PvM at Panda's Thumb has a nice post about this and recent work. And I'm not just saying that because he links to one of my articles on the web. One point I would make, that he doesn't mention, is that figures derived from "genera" or "classes" in the fossil record are weak signals about actual diversity at best, because both are incommensurable across the tree of life and are purely conventional artifacts. So it may be even better than PvM says, because the metrics used to identify diversity at that time are flawed.
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Re-Frame it as The Cambrian Slow-Cooker...
While it may seem sudden, many trace fossils suggest a longer history before fossils of large conspicuous organisms become obvious. The oldest macrofossil I have heard of is a small seaweed, perhaps of red algal affinities, but it's dated to 2.1 gya, which greatly predates the Cambrian explosion (Han & Runnegar, 1992, Science, 257:232). So organisms were playing around with being larger than plankton way before the Cambrian. Oh, but wait, this isn't an animal, so maybe it doesn't count.