File:Fossiliferous flint (Upper Mercer Limestone, Middle Pennsylvanian; Rock Cut railroad cut, south-southeast of Dresden, Ohio, USA) 4 (33188549452).jpg

The Pottsville Group is a Pennsylvanian-aged cyclothemic succession containing 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 Upper Mercer Limestone is a moderately laterally persistent, chertified limestone horizon in the Pottsville Group. It is often composed of black-colored chert/flint 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 light-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.

Non-chertified limestone is frequently present in the Upper Mercer horizon, 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.

The whitish fossil just right of center in the photo is a crinoid stem. Individual pieces of the stem are called columnals - they are usually shaped like poker chips. Each columnal is composed of a single crystal of calcite (CaCO3 - calcium carbonate). Crinoids ("sea lilies") are sessile, benthic, filter-feeding, marine invertebrates that were abundant in Paleozoic oceans. The group nearly went extinct at the Permian-Triassic mass extinction 251 million years ago. Crinoids are not common in modern oceans - they are usually deep-water forms now, but some shallow-water forms also exist today. A crinoid is essentially a starfish on a stick. The stick, or stem, lifts the organism to a moderately high tier above the seafloor, which is conducive to non-competitive filter feeding. The flower-like "head" of the crinoid consists of numerous cemented calcite plates that surround the digestive system and other soft parts. The arms are feather-like and are the structures that engage in filter-feeding. In the fossil record, crinoid stems are common, whereas crinoid heads are uncommon to rare, because they disaggregate quickly after death.

Classification: Animalia, Echinodermata, Crinoidea

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

Locality: Rock Cut railroad cut - large, loose talus block derived from outcrop along the southern side of Ohio Central Railroad tracks (west of milepost 134), ~southwest of Copeland Island & south-southeast of the town of Dresden, northern Muskingum County, eastern Ohio, USA (~vicinity of 40° 04’ 24.41” North latitude, ~81° 59’ 11.25” West longitude)

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