Sequence stratigraphy has been narrowing down the possibilities for us. The fossils were formed by rapid, complete, and deep burial in genetically related ocean sediments–especially, sand (to become sandstone), clay (to become shale), and calcium carbonate (to become limestone).
In fact, the ordering of these layers even tell us whether the ocean flows were “transgressive”–a marine incursion across the continents (in which case, there is a “thinning up” of sediments)-- or “regressive,” where ocean flows were retreating off the continents (in which case, there is a “coarsening up” of sediments).
Distinct “packages” of these facies are bound, above and below, by erosion layers called “unconformities.”
“Genetically related” means that the stacked sedimentary rock layers lay adjacent to one another in the ocean environment.