File:Cucullaea vulgaris fossil bivalve (Coon Creek Formation, Upper Cretaceous; Coon Creek, Tennessee, USA) 1 (31971664065).jpg

Cucullaea vulgaris Morton, 1830 - right valve of a fossil bivalve from the Cretaceous of Tennessee, USA. (4.9 cm across at its widest)

Orientation: anterior to the right; posterior at left; dorsal at top; ventral at bottom

Bivalves are bilaterally symmetrical molluscs having two calcareous, asymmetrical shells (valves) - they include the clams, oysters, and scallops. In most bivalves, the two shells are mirror images of each other (the major exception is the oysters). They occur in marine, estuarine, and freshwater environments. Bivalves are also known as pelecypods and lamellibranchiates.

Bivalves are sessile, benthic organisms - they occur on or below substrates. Most of them are filter-feeders, using siphons to bring in water, filter the water for tiny particles of food, then expel the used water. The majority of bivalves are infaunal - they burrow into unlithified sediments. In hard substrate environments, some forms make borings, in which the bivalve lives. Some groups are hard substrate encrusters, using a mineral cement to attach to rocks, shells, or wood.

The fossil record of bivalves is Cambrian to Recent. They are especially common in the post-Paleozoic fossil record.

This well preserved fossil bivalve is a false ark clam. It comes from a famous Cretaceous-aged marine deposit in western Tennessee called Coon Creek. During the Cretaceous, the lower and middle Mississippi River Valley region was below sea level - ocean extended from Louisiana and Mississippi up north to Arkansas, western Tennessee, and the Illinois-Missouri-Kentucky tristate area.

Classification: Animalia, Mollusca, Bivalvia, Pteriomorphia, Arcoida, Cucullaeidae

Stratigraphy: Coon Creek Formation (a.k.a. Coon Creek "Tongue" of the Ripley Formation), lower Maastrichtian Stage, upper Upper Cretaceous