Fossil bivalve in tripoli in the Pennsylvanian of Ohio, USA.

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

The Pottsville Group is a Pennsylvanian-aged cyclothemic succession in eastern Ohio that contains nonmarine shales, marine shales, siltstones, sandstones, coals, marine limestones, and chert ("flint"). The lower Pottsville dates to the late Early Pennsylvanian. The upper part dates to the early Middle Pennsylvanian. The Lower-Middle Pennsylvanian boundary is apparently somewhere near the Boggs Member (?).

The rock shown above is a very weathered fossiliferous flint surface. A small area of unweathered flint is present in the top-right part of the picture (= dark bluish). The fossil at center is a bivalve, preserved as an internal mold.

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.

The shell of this fossil clam has not been preserved. Many fossils of shelly organisms (e.g., bivalves, gastropods, brachiopods, cephalopods) have had the original hard parts dissolved away. In such cases of dissolution, the fossil itself is an impression of the outside of the shell (external mold) or an impression of the inside of the shell (internal mold). This example is an internal mold.

The host rock for this fossil is the Upper Mercer Flint, a moderately laterally persistent chert horizon in the Pennsylvanian of eastern Ohio, USA. It is often black-colored but can be dark bluish to bluish-black colored as well (the latter colors are referred to as "Nellie Blue Flint"). Upper Mercer Flint has whitish-colored fossils and fossil fragments that include fusulinid foraminifera, crinoid ossicles, and other Late Paleozoic normal marine fossils. Apparent phylloidal algae can also be present as squiggly lines.

This chert horizon is actually a chertified fossiliferous limestone. Non-chertified limestone is frequently present, although minor in volume. Limestone usually occurs along the outside portions of chert masses, but also in relatively small patches within the chert.

In places, the Upper Mercer Flint/Limestone horizon is missing, usually removed by paleoerosion.

American Indians sometimes used Upper Mercer Flint to make arrowheads and spear points and knife blades. "Flint Ridge Flint" (= Vanport Flint) was the most desirable source rock for these objects, but other chert horizons also attracted attention.

Very weathered flint is a mottled brownish, rough-textured, often-friable rock called tripoli, or rottenstone. Tripoli often occurs as a weathering rind on Upper Mercer Flint masses. Sometimes, entire or near-entire masses of flint have been converted to tripoli.

Classification: Animalia, Mollusca, Bivalvia

Stratigraphy: Upper Mercer Flint, Pottsville Group, lower Atokan Stage, lower Middle Pennsylvanian