Bivalve lifestyles and ecology
Modern bivalves live in a variety of depositional settings in fresh, brackish, and marine water. Fossil bivalves inhabited the same habitats as modern bivalves. Marine, brackish, and freshwater origins for fossil bivalves are usually determined by their associations with other fossils more diagnostic of marine, brackish or freshwater. For example, fossil bivalves found in the same rock layer as fossil corals, are likely marine bivalves because corals only live in marine conditions. Sedimentology of the rock strata encasing bivalve fossils is also used to interpret the type of environment (e.g., river, lake, estuary, marine shelf, etc.) in which the original sediment was deposited. Almost all of the bivalve fossils found in the Paleozoic rocks of Kentucky were marine (or brackish) bivalves.
Modern bivalves can be free-swimming, live on or attached to another organism or a substrate (epifaunal), or live in the substrate (infaunal). Some infaunal and epifauntal bivalves attach to the substrate or other objects by strong, thread-like features called byssus. A small gap, called a byssal gape may occur along the commissure of both valves in bivalves with byssus (Cox and others, 1969; Carter and others, 2012). Neither the foot or byssus are preserved as fossils, but the gaps in shells may be preserved as indicators of their position when the bivalve was alive.
In some cases, fossil bivalves are found in life position, especially those that had infaunal lifestyles. Infaunal bivalves are already partly buried, so are already part way to fossilization. In most cases, however, fossil bivalve shells have been transported to where they were deposited and their original lifestyle or orientation in life must be interpreted. Details of well-preserved fossil shells can be used to determine if fossil bivalves had byssus, a foot, siphons, and other soft parts, which are partly related to certain lifestyles. Studies of modern bivalves also show that many shell shapes are adapted to free-swimming, epifaunal, semi-infaunal, or infaunal lifestyles (Stanley, 1970, 1972). Hence, modern shell shapes (profiles, outlines) can sometimes be used to infer the lifestyles of fossil bivalves with similar shapes (e.g., Pojeta, 1971; Hoare and others, 1979).