This is the kind of nerdy thing I spend time reading about.
Silicon compounds are not very soluble and the Earth is full of silicon (silicon makes up about 28% of Earth’s crust). As a result, the shells of sea creatures made of silicon compounds don’t dissolve.
Diatoms are eukaryotic algae which make their cell walls out of silica. Their shells are literally grown rock. They are sometimes called “living jewels” because of their crystalline shells. When ocean diatoms die at ripe old ages of 8 days or so, they precipitate to the sea floor.
Seafloor spreading happens at the mid ocean ridges. Ridges are where lava upwells as part of the magma cycle. Sea water cools the lava and it forms new seafloor pushing outwards. The seafloor near the mid ocean ridge in the Atlantic is hundreds of millions of years younger than the seafloor off the coast of the Americas, Europe and Africa.
As a result, we have a nice geological progression of diatom oozes(this is what happens to the diatoms that precipitate to the ocean floor: they form a layer of dead algae). Because of their hard shells, diatoms easily fossilize. Recent diatom fossils are found near the mid ocean. Closer to the continents, there are layers of diatom oozes with the most ancient diatom fossils at the bottom.
Diatoms have the best fossil record of any known life form. From 200 million years ago to today, we have been able to track every morphological change that has appeared. It is a fossil record without gaps. I wish I could find pictures of some of the progressions for you on the internet, but I was unable to. So here is a picture of a rock containing fossilized diatoms:
I would tell you about “rock snot” (no, I am not making that up), but that is a freshwater diatom phenomenon so it doesn’t fit with the theme of oceans.