Encrinite from the Mississippian of Kentucky, USA. (field of view ~17.4 cm across)

The Ft. Payne Formation of southern Kentucky is richly fossiliferous and is dominated by crinoids. 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. Individual pieces of a crinoid 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).

The limestone shown above is almost entirely composed of crinoid columnals. Such crinoidal limestones are called "encrinites".

Classification: Animalia, Echinodermata, Crinoidea

Stratigraphy: Ft. Payne Formation, Osagean Stage, upper Lower Mississippian