In Noah's flood the sea floor was buried first, that's called the Cambrian. The things which were swimming just above the sea floor got buried next, that's called the Ordovician.
First Vertebrates and a Great Variety of Marine Invertebrates
August 5th
The Ordovician is supposedly period of time but the description that we have sounds suspiciously like the fauna of a marine habitat. The Cambrian is the sea floor, the Ordovician is the open ocean immediately above it.