Then God said, “Let the waters abound with an abundance of living creatures, and let birds fly above the earth across the face of the firmament of the heavens.”(Genesis 1:20 NKJV
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.
Our description of a supposed period of time sounds suspiciously like the fauna of a marine habitat. Just sayin’ We have a list of marine organisms. The physical conditions described sound profoundly similar to the Cambrian because it’s a description of the same event: Noah’s flood. The difference being that the list of animals aren’t bottom dwellers, they’re swimmers.
These aren’t the first vertebrates in the narrative of evolution, they’re the first vertebrates to be buried in sediment because they were swimming in the ocean close to the sea floor.