Explanations for the Cambrian Explosion

One difficulty with cambrian and precambrian animals is the rarity of fossiliferous rocks from this age. A great deal of them have been destroyed or metamorphosed.

Here is a chart taken from this article written in 98 about the Cambrian explosion. The chart is a little old since it originated in 95. Image caption:
The earliest appearance of body fossils of living phyla in geologic time. All
well-skeletonized phyla except Bryozoa are known from the Early Cambrian. First
records of soft-bodied forms are scattered through time from localities where
unusually favorable preservation has occurred. Modified from Valentine (1995).Edited by dokukaeru, : Thumbnail