Crinoid stem in calcareous shale (Lower Mercer Shale, Middle Pennsylvanian; Rock Cut railroad cut, south-southeast of Dresden, Ohio, USA) (33303675816)

Description:

Description: Fossil crinoid stem from the Pennsylvanian of Ohio, USA. 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 gray, fossiliferous, calcareous shale from the Lower Mercer Shale, a marine unit that caps the Lower Mercer Limestone in the Pottsville Group. The fossil at left is a compressed crinoid stem. 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 stem is composed of numerous stacked columnals, which are somewhat like small poker chips. Each columnal is a single crystal of calcite. 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. Stratigraphy: Lower Mercer Shale (just above the Lower Mercer Limestone), Pottsville Group, lower Atokan Stage, lower Middle Pennsylvanian Locality: Rock Cut railroad cut - 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) See info. at: en.wikipedia.org/wiki/Crinoid. Date: 27 February 2017, 16:24. Source: Crinoid stem in calcareous shale (Lower Mercer Shale, Middle Pennsylvanian; Rock Cut railroad cut, south-southeast of Dresden, Ohio, USA). Author: James St. John.