English: Megalomoidea canadensis (Hall, 1852) - fossil bivalve from the Silurian of Ohio, USA, preserved as an internal mold with encrusting red sphalerite crystals.

Orientation: anterior to the right; posterior to the left; dorsal (hinge line) at top; ventral at bottom; right valve is facing the viewer

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.

Shown here is a famous fossil clam from the Silurian of Ohio. These fossils were frequently found by quarrymen working Silurian dolostones - they called them "beefhearts" because they are remarkably similar in size and appearance to the hearts of cows.

"Beefhearts" are really internal molds of large fossil bivalves - the original aragonite shells have dissolved away. All that's left is impressions of the inside surfaces of the shells. The internal mold itself is composed of the same material as the host rocks - dolostone. Dolostone is a finely-crystalline textured, chemical sedimentary rock composed of the mineral dolomite (CaMg(CO3)2 - calcium magnesium carbonate).

The small, deep red crystals on this specimen are sphalerite (ZnS - zinc sulfide).

Classification: Animalia, Mollusca, Bivalvia, Heterodonta, Hippuritoida, Megalodontidae (this fossil clam was formerly known as Megalomus canadensis)

Stratigraphy: unrecorded/undisclosed (but probably from the Lockport Dolomite, upper Niagaran Series, (upper Wenlockian), upper Middle Silurian)

Locality: unrecorded/undisclosed quarry in Ohio, USA