If you take a jar full of pebbles of various sizes and you shake it for a long time, smaller pebbles will likely be found on the bottom of the jar and larger ones near the top. Some scientific creationists (people who hold that there are good scientific reasons for rejecting evolution in favour of a view that all animals were created at the same time) think that the reason we see smaller, simpler animals in lower levels of the fossil record is that there was some such 'hydrodynamic sorting' in a great flood event. That is, the animals we see in the lower levels of the fossil record did not live earlier than the ones we find further up, they just settled to the bottom because of their size and shape. The fact that this theory is clearly inspired by the Biblical story of the flood counts as evidence against the theory.