Aphelecrinus

The most common Echinoderm you will find are the abundant crinoid stem pieces and crown fragments. In the Chester section of the upper Mississippian Period the blastoid Pentremites are common. Complete crinoid crowns and other types of Echinoderms are challenging to find. Many amateurs, collectors and paleontologist collect echinoderms, especially crinoids, because they are a very good index fossils and are uniquely attractive complex fossils.

Crinoids are commonly called sea lilies or feather stars and are represented by one surviving Subclass with about 600 colorful living species. Sea lilies are stalked crinoids that live at great depths in the ocean today and resemble ancient Paleozoic crinoids of the past. Feather stars, comatulids crinoids, generally inhabit shallow reef ecosystems and lack much of a stem. There are numerous cirri at the base of the cup to grapple objects and hold the crinoid in place to feed in the currents. Crinoids are passive filter feeders and need the ocean currents to provide them with food and respiration.

A Crinoid is composed of two major sections, the Crown and the Column or Stem. Many types of crinoids have a modified stems or have gotten rid of the stem during an early stage of life. The crown is composed of a cup shaped body made of many plates, and like most echinoderms has five rays. Each ray of the cup usually has an appendages attached called an arm. The arm is usually long, flexible, branched and commonly featherlike that radiates from the cup in all directions for feeding. Their common name is derived from the feather like arm structure, and the flower like appearance of a crown.

The arms that radiate from the cup gather food for the animal and usually emit pinnules to increase its feeding area. The arms have taken on a great variety of forms and structure during its long history too. Each arm and pinnule has an ambulacral groove on the inside and the ambulacral grove is lined on each side with tube feet which is part of its water vascular system. Food from the water column is captured by the tube feet using mucous and is transported down the arm toward the mouth.

The stem and cirri are used by the crinoid to hold itself up off sea bottom in the ocean currents to feed. The stem and cirri are composed of disc shaped plates, with a central nerve canal, that usually change structure and function throughout its length. The stem usually emits cirri along its length. Throughout the crinoids long extensive history the stem and its termination segment has taken on many forms, looks, functions, and modifications. The stem may terminate in loose coil, a root looking hold fast, an encrusting holdfast, a discoid type holdfast, a grapnel looking holdfast, a bulbous holdfast, or the stem just ends and the cirri are able to cling and release. Some crinoid genera loose their stem before maturity and live free of a stem or cirri.

Crinoids appear in the fossil record during the Cambrian Period and by the Ordovician Period diversified, with many Subclasses appearing, and became a common and important part of the Paleozoic marine community. Crinoids became so abundant in the Paleozoic and Mesozoic Periods that their calcite plates are the principal component in many of the vast limestones of the Tennessee Valley and world. Primarily crinoids with corals, brachiopods and bryozoans were the limestone builders of the Paleozoic. Limestone is one of the most important materials for building and modern industry.

There are four crinoid Subclasses Articulata, Cladida, Camerata, and Flexibilia. Camerates were the dominate Subclass in the lower Paleozoic. Then Cladida crinoids became the dominate Subclass in the upper Mississippian when a major fauna change occurred towards the end of the middle Mississippian Period. Cladida crinoids dominated crinoid populations till the end of the Paleozoic Era. Articulate crinoids appear in the fossils record at the end of the middle Mississippian and are the only Class of crinoids that survived the Permian mass extinction event and again diversified and radiated during the Mesozoic Era to build limestones and become an important part of the marine ecosystem again.