|Scientific classification |
Crinoids, also known as "sea lilies" or "feather-stars", are marine animals that make up the class Crinoidea of the echinoderms (phylum Echinodermata). They live both in shallow water and in depths as great as 6000 meters. Crinoids are characterized by a mouth on the top surface that is surrounded by feeding arms. Although the basic echinoderm pattern of five-fold symmetry can be recognized, most crinoids have many more than five arms. Crinoids usually have a stem used to attach themselves to a substrate, but many live attached only as juveniles and become free-swimming as adults. There are only a few hundred known modern forms, but crinoids were much more numerous both in species and numbers in the past. Some thick limestone beds dating to the mid- to late-Paleozoic are entirely made up of disarticulated crinoid fragments.
The earliest known crinoids come from the Ordovician. They are thought to have evolved from primitive echinoderms known as Eocystoids. Confusingly, another early group of echinoderms were also the Eocrinoids, but that group is currently thought to be an ancestor of blastoids rather than of crinoids.
Some fossil crinoids, such as Pentacrinites, seem to have lived attached to floating driftwood and complete colonies are often found. Sometimes this driftwood would become waterlogged and sink 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.
Most modern crinoids are free-swimming and lack a stem. Examples of free-swimming crinoid fossils include Marsupitsa, Saccocoma and Uintacrinus. Many fossils of free-swimming crinoids (such as Pterocoma) are found in the Jurassic-dated Solnhofen limestone of Solnhofen in Germany, and the Cretaceous-dated Niobrara chalk of Kansas contains large numbers of Uintacrinus.
The crinoids have had an eventful geologic history. Once evolved, 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 in certain horizons. They were evidently competing with the crinoids on an equal basis. However, none of these others survived the crisis at the end of the Permian period.
An abundance of (stemmed) crinoids occurs in the rocks of the Silurian period in 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.