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 fossil shown above is a crinoid stem. 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). Note the pentaradiate lumen at the center (= the star-shaped center).

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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