The Lower Mercer Limestone is a Middle Pennsylvanian-aged, laterally persistent, marine fossiliferous limestone unit in the Pottsville Group of eastern Ohio, USA. It is richly fossiliferous, principally dominated by brachiopods and crinoid stems. The fossils shown above are crinoid stems. Individual pieces of the stem are called columnals - they are usually somewhat 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: Lower Mercer Limestone, Pottsville Group, lower Atokan Stage, lower Middle Pennsylvanian

Locality: roadcut along the northern side Rt. 16, southern margin of Irish Ridge, west of the Rt. 16-Rt. 60 intersection, northwest of the town of Trinway, northwestern Muskingum County, eastern Ohio, USA (40° 09’ 12.95” North latitude, 82° 02’ 43.27” West longitude)

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