This infrequent wetting of shell material causes calcite and aragonite to dissolve and then precipitate as microcrystalline calcite cement upon drying. These processes that cement modern shell material are essentially the same processes that helped to preserve shells and organic remains as fossils in the geologic past. Several additional important geologic processes related to fossils should also be noted. When exposed to seawater minerals precipitate within pores and ion substitution replaces some of the spaces within the aragonite mineral lattice. Another important process is the formation of concretions.
Concretions are nodules that commonly form around dead organisms or shell material.
Concretions can preserve delicate features, including structures that do not consist of shell or bone, such as crab carapaces. Many different geochemical processes can lead to the formation of concretions. One way this happens is as an organism slowly decays it releases ammonia which reacts with sea water in the sediment to precipitate minerals. Definitively older ages are noted where possible. The companion Modern Sea Shore Creatures of the New York Bight web page provides many images of invertebrates that have fossil relatives or close analogs.