Classes of the Phylum Mollusca (currently in Atlas)

What is a mollusk?
Mollusks are a diverse group of invertebrates which make up a large part of the modern fauna. Included within this category are gastropods (snails), cephalopods (octopi, squids, and nautili) and bivalves (clams and mussels). These organisms inhabit marine, freshwater and terrestrial habitats, and are frequently characterized by having aragonitic, not calcitic, shells.

Physical attributes
Mollusks have unsegmented bodies covered by a fleshy mantle, which commonly produces calcareous shells. Mollusks are characterized by a fleshy foot that is modified to perform different functions in each class: in bivalves, the foot is modified for digging; in gastropods, it is used for locomotion on the substrate; in cephalopods, the foot is modified into tentacles for swimming. Mollusks are bilaterally symmetrical, but some groups are secondarily asymmetrical. During development, gastropods and cephalopods  undergo the process of torsion, where their body has rotated 180 degrees and the anus is positioned at the top of the shell.

How are mollusks preserved in Cincinnatian strata?
In general, most mollusk fossils occur as molds. With gastropods and cephalopods, this tends to be an internal mold, whereas with bivalves it tends to be external. However, it should be noted that this is not always the case.  Fossils of certain taxa, such as Cyclonema and Caritodens, often preserve skeletal material. On rare occasions, instances of replacement and recrystallization of other mollusks result in incredibly detailed preservation.

Which mollusks are present in the Cincinnatian?
Gastropods, cephalopods, and bivalves are all common in the Cincinnatian. Mollusks appear throughout all sequences; however, they tend to be more concentrated in the C4 sequence, due to the fact that they flourish when environmental conditions are too harsh for other organisms, such as brachiopods, to survive. Therefore, a high abundance of mollusks in the C4 sequence shows a change, and deterioration, of the environmental conditions during this time frame.