Acanthopecten carboniferous (Stevens, 1858) - fossil scallop in fossiliferous marine calcareous shale in the Pennsylvanian of Ohio, USA.

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.

Scallops are common marine bivalves in much of the fossil record and in the modern oceans. Individual bivalve shells, unlike brachiopod shells, are asymmetrical. Scallop shells sometimes approach bilateral symmetry, but the subtriangular, wing-like auricles along the hingeline will still display asymmetry. One of the auricles has a basal notch. A notched auricle on the right side of a shell indicates a right valve. A notched auricle on the left side of a shell indicates a left valve.

The fossil scallop shown above is in a talus block of Lower Mercer Shale. The scallop is Acanthopecten carboniferous, which is characterized by having spines along the ventral margin of the shell. This is an external mold of a left valve (anterior is to the right; posterior is to the left).

The Lower Mercer Shale is a fossiliferous marine calcareous shale unit that occurs just above the Lower Mercer Limestone in the Pottsville Group of eastern Ohio, USA.

A detailed description of Acanthopecten carboniferous is on pp. 40-41 of Hoare et al. (1979).

Classification: Animalia, Mollusca, Bivalvia, Pteriomorphia, Pterioida, Pectinoidea, Aviculopectinidae

Stratigraphy: Lower Mercer Shale (= marine fossiliferous shale capping the Lower Mercer Limestone), Pottsville Group, lower Middle Pennsylvanian

Locality: talus block in Berry Run headwaters, western side of Brownsville Road, a little south of Flint Ridge Road, southern flanks of Flint Ridge, Hopewell Township, southeastern Licking County, east-central Ohio, USA (~39° 59' 09.11" North latitude, ~82° 15' 52.67" West longitude)

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