Fossil bivalve in limestone in the Ordovician of Ohio, USA.

This outcrop is in the famous Cincinnatian Series of the tristate area of Ohio-Kentucky-Indiana. Rocks in the Cincinnatian were deposited in relatively shallow marine facies during the Late Ordovician. The Cincinnatian succession is mostly interbedded fossiliferous limestones and shales. Most of the limestones are event beds (= tempestites), deposited during ancient storms. The exposure seen here consists of interbedded shales and nodular limestones of the upper Arnheim Formation.

The fossil at center is a very weathered bivalve - a bilaterally symmetrical mollusc having two calcareous, asymmetrical shells (valves). Bivalves 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.

Classification: Animalia, Mollusca, Bivalvia

Stratigraphy: upper Arnheim Formation, lower Richmondian Stage, upper Cincinnatian Series, upper Upper Ordovician