Crinoids are marine organisms which have a skeleton made up of calcareous plates. They first appeared in the early in the middle of the Cambrian period and survive up until the present day, becoming abundant and important fossils from the Ordovician period onwards. Modern crinoids have a wide distribution, and the fossil record shows a worldwide distribution during the geological past.

Crinoids were so numerous in the Palaeozoic era that their remains often form vast thicknesses of limestone. These great 'forests' of crinoids probably lived in fairly shallow water. Modern crinoids are also gregarious, tending to live in groups often of immense numbers, although today they are less common in shallow water than they were during the Palaeozoic and Mesozoic eras. Crinoids seem to prefer relatively clear waters.

The popular names for crinoids are "sea lilies" (for fixed crinoids) and "feather stars" (for free-swimming crinoids). Crinoids (class crinoidea) are members of a larger group of animals called the echinoderms (phylum echinodermata) which also includes echinoids ("sea urchins") and starfish.

Most fossil crinoids lived attached to the sea-floor by a flexible stem, which may be either cemented to the sea-floor by the crinoid, or may have a root-like system buried into the sediment. Fossil crinoids have stems ranging in size from a few centimetres in length to as much as 21 metres (70 feet). The stem is either circular or pentagonal (five-sided) in section and composed of numerous disc-like plates, each one usually measuring around 1 centimetre (0.4 inches) across. At the top of this stem is the main part of the animal, a cup-shaped head (called the calyx) to which arms are attached. The calyx and the arms together are sometimes referred to as the crown. There are usually five arms (or more often a multiple of five), and the arms may be repeatedly branched. The arms carry fine pinnules which are used to filter small food particles from the water. Grooves in the arms channel food to the mouth, which is located in the centre of the calyx. Normally soon after death the entire skeleton of the crinoid disintegrates into the small separate plates (called ossicles). These ossicles are composed of a form of calcium carbonate known as calcite.

Detailed classification of the crinoids is based primarily upon the arrangement of the plates within the calyx, the number and branching pattern of the arms, and the external surface detail of the calyx. Internal features of crinoid anatomy, such as the nervous and respiratory systems, are also important in their classification.

          The calyx of the crinoid Pentacrinites.

Some fossil crinoids, such as Pentacrinites shown in the above illustration, seem to have lived attached to floating driftwood and complete colonies are often found. Sometimes the driftwood to which they were attached became waterlogged and sank to the bottom, taking the attached crinoids with it. The stem of Pentacrinites can be several metres long. Modern relatives of Pentacrinites live in gentle currents attached to rocks by the end of their stem, which is fairly short.

However, most modern crinoids are free-swimming and lack a stem. Examples of free-swimming crinoid fossils include Marsupites, Saccocoma and Uintacrinus. Many fossils of free-swimming crinoids (such as Pterocoma) are found in the Jurassic period lithographic limestone of Solnhofen in Germany, and the Cretaceous period Niobrara chalk of Kansas contains large numbers of Uintacrinus.

The crinoids have had an eventful geological history. Once established they soon spread to a variety of marine habitats. The group as a whole suffered a major crisis during the Permian period when most of the crinoid forms of the Palaeozoic era died out, with a few surviving into the Triassic period.
During the Mesozoic era there was another great radiation of the crinoids, with more modern forms possessing flexible arms becoming widespread.

The long and varied geological history of the crinoids demonstrates how well the echinoderms have adapted to filter-feeding. The fossils of other stalked filter-feeding echinoderms, such as blastoids, are also found in the rocks of the Palaeozoic era. These extinct groups can exceed the crinoids in both numbers and variety at certain horizons, and evidently they were competing with the crinoids on an equal basis. However, none of them survived the crisis at the end of the Permian period.

An abundance of (stemmed) crinoids occurs in the rocks of the Silurian period of the United Kingdom and the eastern United States, the Devonian period of Kentucky, Michigan, New York state and the Eifel region of Germany, the Carboniferous period of the United Kingdom, Belgium and Russia, the Mississippian period of Iowa and Indiana, the Pennsylvanian period of the mid-continental United States, the Permian period of the island of Timor, and the Triassic period of Germany.

Often only the stem ossicles of crinoids are found as fossils. Complete, well preserved specimens are rare and beautiful fossils, looking more like exotic plants or flowers than animals (hence the popular name "sea lilies"). Sometimes bumps or swellings with one or more holes in their centre are found on the stems of fossil crinoids. These were caused by parasitic worms (whose remains are not preserved).