Bivalve preservation (casts and molds)
Many bivalve fossils are not the actual bivalve shell. Living bivalve shells (and most other mollusks with shells) are composed of the mineral aragonite. Aragonite is a carbonate mineral that is stable at the earth’s surface, but unstable during burial and fossilization. In some cases, the aragonite is preserved. More often, aragonite is recrystallized or replaced by other minerals (such as quartz) during burial. In other cases, the aragonite shells dissolve. When shells dissolve, the void left behind or any sediment that was trapped between the valves when the bivalve died is what is left to be fossilized. The void can form a cast and-mold type of fossil. External molds may preserve external ornamentation of the shell, such as growth lines, ribs, and costae (fine ribs). Many bivalve fossils, however, are casts and molds of the internal surface of the shells, so they preserve the shape of the inside surface of the shell, which is usually smooth and lacks growth lines and ornamentation. It is more difficult to determine the genus and species of fossil internal molds, than external molds.