The Cambrian is the first geological period of the Paleozoic Era. The Cambrian is unique in its high proportion of lagerstatten. Lagerstatten are geographic sites with large amounts of fossilized organisms. These large deposits of fossilized organisms provide us with a better understanding of the Cambrian era than many others.
"The Cambrian Period marked a profound change in life on Earth. Before the Cambrian, life was on the whole small and simple. Complex organisms became gradually more common in the millions of years immediately preceding the Cambrian, but it wasn't until this period that mineralised — hence readily fossilised — organisms became common."
So if the organisms prior to the Cambrian Era were not mineralized they would not have fossilized.