Since the Burgess Shale discoveries about 1880-90, no intermediates worthy of the name have been found. The layers of the Earth are not missing for those Geologic periods. Bad luck or no fossils? We don't know and either possibility still exists.-Now we know a bit. Very elementary Eukaryote forms have been found about 500 million years before the Cambrian Explosion according to an article in Nature just published.-http://www.nature.com/nature/journal/v473/n7348/full/nature09943.html