Christians: Does this disprove creationism?

Creationism says that God made everything in a few days. Thus, all animals were supposedly created in from what I believe to be three days overall (considering every animal). That would mean that every animal would be in the same layer of fossils. But we are yet to find mammals in certain layers of fossils. The deeper you go, it seems that the diversity of life continues to mildly decrease. If everything was together at the same time, wouldn't we find everything fossilized in the same layer of fossils? Explain.