Crinoid

The name "Crinoidea" comes from the Ancient Greek word κρίνον (krínon), "a lily", with the suffix –oid meaning "like". ==Morphology==

The basic body form of a crinoid is a stem (not present in adult feather stars) and a crown consisting of a cup-like central body known as the theca, and a set of five rays or arms, usually branched and feathery. The mouth and anus are both located on the upper side of the theca, making the dorsal (upper) surface the oral surface, unlike in the other echinoderm groups such as the sea urchins, starfish, and brittle stars, where the mouth is on the underside. Numerous calcareous plates make up the bulk of the crinoid, with only a small percentage of soft tissue. These ossicles fossilize well and some beds of limestone date from the Lower Carboniferous around Clitheroe, England, formed almost exclusively from a diverse fauna of crinoid fossils. The stem of sea lilies is composed of a column of highly porous ossicles, which are connected by ligamentary tissue. It attaches to the substrate with a flattened holdfast or with whorls of jointed, root-like structures known as cirri. Further cirri may occur higher up the stem. In crinoids that attach to hard surfaces, the cirri may be robust and curved, resembling birds' feet, but when crinoids live on soft sediment, the cirri may be slender and rod-like. Juvenile feather stars have a stem, but this is later lost, with many species retaining a few cirri at the base of the crown. Most living crinoids are free-swimming and have only a vestigial stalk. In those deep-sea species that still retain a stalk, it may reach up to in length (although usually much smaller), and fossil species are known with stems. Various crinoid fossils hint at possible prehistoric predators. Coprolites of both fish and cephalopods have been found containing ossicles of various crinoids, such as the pelagic crinoid Saccocoma, from the Jurassic lagerstatten Solnhofen, while damaged crinoid stems with bite marks matching the toothplates of coccosteid placoderms have been found in Late Devonian Poland. The calyxes of several Devonian to Carboniferous-aged crinoids have the shells of a snail, Platyceras, intimately associated with them. Some have the snail situated over the anus, suggesting that Platyceras was a coprophagous commensal, while others have the animal directly situated over a borehole, suggesting a more pernicious relationship. Water vascular system Like other echinoderms, crinoids possess a water vascular system that maintains hydraulic pressure in the tube feet. This is not connected to external sea water via a madreporite, as in other echinoderms, but only connected through a large number of pores to the coelom (body cavity). The main fluid reservoir is the muscular-walled ring canal that is connected to the coelom by stone canals lined with calcareous material. The coelom is divided into a number of interconnecting spaces by mesenteries. It surrounds the viscera in the disc and has branches within the stalk and arms, with smaller branches extending into the pinnules. It is the contraction of the ring canal that extends the tube feet. Three narrow branches of the coelom enter each arm, two on the oral side and one aborally, and pinnules. The action of cilia cause a slow flow of fluid (1 mm per second) in these canals, outward in the oral branches and inward in the aboral ones, and this is the main means of transport of nutrients and waste products. They have no heart and separate circulatory system, but a large blood vessel known as the axial organ at the base of the disc contains some slender, blind-ended tubes of unknown function, which extend into the stalk. The larva's free-swimming period lasts for only a few days before it settles on the bottom and attaches itself to the underlying surface using an adhesive gland on its underside. The larva then undergoes an extended period of metamorphoses into a stalked juvenile, becoming radially symmetric in the process. Even the free-swimming feather stars go through this stage, with the adult eventually breaking away from the stalk. Crinoids have been able to regenerate parts since Paleozoic times. These regenerative abilities may be vital in surviving attacks by predatory fish. ==Locomotion==

Most modern crinoids, i.e., the feather stars, are free-moving and lack a stem as adults. Examples of fossil crinoids that have been interpreted as free-swimming include Marsupites, Saccocoma, and Uintacrinus. In general, crinoids move to new locations by crawling, using the feather-like arms to heft the body. Such a movement may be induced in relation to a change in current direction, the need to climb to an elevated perch to feed, or because of an agonistic behaviour by an encountered individual. Crinoids can also swim. They do this by co-ordinated, repeated, sequential movements of the arms in three groups. At first, the direction of travel is upward, but soon becomes horizontal, travelling around per second with the oral surface in front. Swimming usually takes place as short bursts of activity lasting up to half a minute, and in the comatulid Florometra serratissima at least, only takes place after mechanical stimulation or as an escape response evoked by a predator. ==Evolution==

Origins If one ignores the enigmatic Echmatocrinus of the Burgess Shale, the earliest known unequivocal crinoid groups date back to the Ordovician, 480 million years ago. Two competing hypotheses pertain to the origin of the group; the traditional viewpoint holds that crinoids evolved from within the blastozoans (the eocrinoids and their derived descendants, the blastoids and the cystoids), whereas the most popular alternative suggests that the crinoids split early from among the edrioasteroids. The debate is difficult to settle, in part because all three candidate ancestors share many characteristics, including radial symmetry, calcareous plates, and stalked or direct attachment to the substrate. However, other phylogenies are ambiguous regarding whether it is closer to the crinoids, eocrinoids, or eleutherozoans. Diversity Echinoderms with mineralized skeletons entered the fossil record in the early Cambrian (540 Mya), and during the next 100 million years, the crinoids and blastoids (also stalked filter-feeders) were dominant. At that time, the Echinodermata included 20 taxa of class rank, only five of which survived the mass extinction events that followed. The long and varied geological history of the crinoids demonstrates how well the echinoderms had adapted to filter-feeding. After the end-Permian extinction, crinoids never regained the morphological diversity and dominant position they had enjoyed in the Paleozoic, instead adopting a different suite of ecological strategies from those that had proven so successful previously. This radiation occurred somewhat earlier than the Mesozoic marine revolution, possibly because it was mainly driven by increases in benthic predation, particularly from echinoids. As predation by sea urchins intensified around 225 Mya, stalked crinoids gradually retreated to deeper waters, while motile forms retained stalks only during their early life stages. Fossils '', a fossil crinoid from the Carboniferous of Indiana (Callovian) Apiocrinites crinoid pluricolumnals from the Matmor Formation in southern Israel 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. In 2012, three geologists reported they had isolated complex organic molecules from 340-million-year-old (Mississippian) fossils of multiple species of crinoids. Identified as "resembl[ing ...] aromatic or polyaromatic quinones", these are the oldest molecules to be definitively associated with particular individual fossils, as they are believed to have been sealed inside ossicle pores by precipitated calcite during the fossilization process. Crinoid fossils, and in particular disarticulated crinoid columnals, can be so abundant that they at times serve as the primary supporting clasts in sedimentary rocks. Rocks of this nature are called encrinites. == Taxonomy ==

, 2018 Crinoidea has been accepted as a distinct clade of echinoderms since the definition of the group by Miller in 1821. It includes many extinct orders as well as four closely related living orders (Comatulida, Cyrtocrinida, Hyocrinida, and Isocrinida), which are part of the subgroup Articulata. Living articulates comprise around 540 species. • Class Crinoidea • †Protocrinoidea (incertae sedis) • Subclass †Camerata • Order †Diplobathrida • Order †Monobathrida • Subclass Pentacrinoidea • Parvclass †Disparida • Order †Eustenocrinida • Order †Maennilicrinida • Order †Tetragonocrinida • Order †Calceocrinida • Parvclass Cladida • Superorder †Porocrinoidea • Order †Hybocrinida • Order †Porocrinida • Superorder †Flexibilia • Order †Sagenocrinida • Order †Taxocrinida • Magnorder Eucladida • †Ampelocrinida (incertae sedis) • Superorder †Cyathoformes • Superorder Articulata • Order †Encrinida • Order †Holocrinida • Order †Millericrinida • Order †Roveacrinida • Order †Uintacrinida • Order Comatulida • Order Cyrtocrinida • Order Hyocrinida • Order Isocrinida Phylogeny The phylogeny, geologic history, and classification of the Crinoidea was discussed by Wright et al. (2017). These authors presented new phylogeny-based and rank-based classifications based on results of recent phylogenetic analyses. Their rank-based classification of crinoid higher taxa (down to Order), not fully resolved and with numerous groups incertae sedis (of uncertain placement), is illustrated in the cladogram. }} == In culture ==

Fossilised crinoid columnal segments extracted from limestone quarried on Lindisfarne, or found washed up along the foreshore, were threaded into necklaces or rosaries, and became known as St. Cuthbert's beads in the Middle Ages. Similarly, in the Midwestern United States, fossilized segments of the columns of crinoids are sometimes known as Indian beads. A species of crinoid, Eperisocrinus missouriensis, is the state fossil of Missouri. The aliens in the movie franchise Alien were inspired by crinoids. == Fossil crinoid gallery ==

File:Crushed crinoid stems from the Jurassic, Iran.jpg|Crushed crinoid stems from Shamshak Formation, Jurassic, Iran File:Fossile-seelilie.jpg|Fossil from Germany showing the stem, calyx, and arms with pinnules File:Crinoids iowa 330m.jpg|330 million year old crinoid fossils from Iowa File:OrdCrinoidHoldfasts.jpg|Crinoid holdfasts and bryozoans on an Upper Ordovician cobble from northern Kentucky File:Seirocrinus subangularis (fossil Crinoid).jpg|Seirocrinus subangularis from the Early Jurassic Posidonia Shale at Holzmaden, Germany File:Isocrinus nicoleti Encrinite Mt Carmel.jpg|Crinoid columnals (Isocrinus nicoleti) from the Middle Jurassic Carmel Formation at Mount Carmel Junction, Utah File:CrinoidHoldfastRoots.JPG|Root-like crinoid holdfast from the Upper Ordovician, southern Ohio File:Crinoid internal mold lumen.jpg|Internal mold of crinoid stem lumen (and external mold of stem) from Lower Carboniferous, Ohio File:Seirocrinus subsingularis, view 2, Jurassic, Hlzmaden Black Shale Formation, Holzmaden, Germany.JPG|Fossils of Seirocrinus subsingularis from the Jurassic Holzmaden Black Shale Formation, Germany File:Crinoid-MCG 1219-P4150562-black.jpg|Crinoid File:Digitally cropped NHMMZ PWL 2021-6145-LS Monstrocrinus granosus.png|Some extinct crinoids, like Monstrocrinus from the Early to Middle Devonian, could bear arrangements of complex spines attached to calyx plates ==See also==