exclusively marine animals, a variety of echinoderms, “Spiny Skinned”, which
appeared and developed during Paleozoic era. Crinoids are considered to be the
dominated at that time echinoderm. Commonly, people call them "sea
lilies" because they resemble a plant. Mississippian is considered to be
the age of crinoids, however they appeared first in the fossil records about
530 million years ago. During the carboniferous period, the crinoids showed big
morphological diversity. Large number of Paleozoic crinoids started to die off
during Mesozoic era generally after Permo-Triassic
extinction. However, during Mesozoic era, there was another great development
of the animals with more flexible arms. Their population also was reduced
during Cenozoic era, and they mostly existed in the deep ocean. Crinoids are
considered to be among the most abundant and important fossils. Crinoids were
major carbonate producing organisms during the Paleozoic and Mesozoic. In fact,
entire carbonate shelves were made of parts of crinoidal
In general, crinoids lived in groups of several thousands, permanently attached to the ocean floor. Some crinoids, though, were ustalked. Their environments graded from very shallow to quite deep waters, 13,000 feet and more. Being filter feeders, crinoids eat different microorganisms in the passing seawater. It is amazing, that some crinoids came into a symbiosis with gastropods, which would seat on the crinoid and eat the waste product rich on nutrients.
Some crinoids have lived on into modern times. There are about several hundred types of stalked and unstalked crinoids that live in the modern ocean in comparison with about five thousands species in the past. Unstalked crinoids are much more prolific nowadays than stalked. There is about 540 species of them, which makes 85% of all extant crinoids. Unstact crinoids called Comatulids, feather stars, which are the most abandoned. Feather stars do not have a stem and float in water or crawl on the ocean floor looking for food. It reminds of a starfish, however its mouth faces upward, and uses its special cirri-structures for crawling. They appeared in Mesozoic Era living not deeper than 200 meters in the ocean.
The North Eaten Pacific feather star, Florometra Serratissima
There are approximately only 80 species of stalked crinoids existing nowadays. They are deep-water creatures and mainly survive in the depth greater than 200 meters and not shallower than 100 meters. They are considered to be the last alive example of the attached to the bottom suspension-feeding echinoderms.
A sea lily or stalked crinoid collected by a research submersible from a depth of 300m off of the Bahamas
The three main parts of the crinoid are the stem, the calyx, and the arms. The ventral or oral surface of the animal is the part of the calyx above the base of the arms and is called Tegmen. The dorsal surface, cup, is situated below the arm base. The calyx and the arms together are called Crown. The stem, also called stalk or column, together with the roots, or base of attachment is known as pelma.
It is amazing what incredible amount of variations of the crinoid roots, stalks, calyx, arms, and spines exists! Many crinoids have rich roots that grow into mud. Sometimes, however, the column resolves into a plate that is actually cemented to a hard object. Lichenocrinus is a plate-covered base of crinoids with tiny calyxes. Each base must have covered a pad of flash.
Ancyrocrinus is another base, which has four prongs that are used as anchor. It snagged the hook-like holdfast on seaweed or other firmly attached crinoids. The roots of Scyphocrinates evolved into bulb-looking base, which serves as a weight to keep the animal upward being sometimes covered with mud.
Crinoid’s stalks are also
diversified. In general, stalks are made of a lot of mostly circular circular plates, or columnals,
made of calcite with the muscle going through the holes in the center of the
plates joining them together. When crinoids die, the muscle decomposes and the
plates broke covering the ocean floor. The long stem supports the animal and
together with the roots and branchlets, or cirri,
attaches the crinoid to the hard ocean floor, to the
soft sediments or to the logs and other materials. A lot of ancient crinoids
were attached to the bottom of the ocean by its somewhat flexible stem. The
stem is composed of many ossicles, which were curcular or sometimes five-rayed in sross-section, and
penetrated by a central opening in which nervous organs are situated. It is
amazing that the stem, looking so flexible from the first site, allow just very
limited freedom of movement, because its columnals
put quite close to each. One of the spices,
Platycrinite to improve its ability to move,
developed twisted columnals making the stem bend to
any side. A lot of stems either had a root-like system “groing”
inside of the sediments, or were actually cemented to the ocean floor. There is
an incredible difference between the length of the stems: from about an inch to
twenty meters. Such type of crinoids as Pentacrinites,
which fossils showed that it lived in Jurassic, seams to attach itself to a
piece of floting wood by its cirri. Sometimes the
piece sank and pulled the Pentacrinites along to the
depth. So their stems could be several meters long. Most stems are used for
support and are “installed” at the base. Myelodactis,
a strange animal, does not have any base and attaches itself to different objects
with its cirri. The lower part of the stem is usually thick, the top part is
pretty thin, and approximately in the middle it curves so that reminds exactly
of a question mark. Some the species do not have any stalk at all. Antedon, which lives in our times, first makes a stem, then
abandons it and sticks to surrounding objects with a claw-like cirri.
Sometimes, the animal releases the object and swim freely in water waving its arms. Another creature, Halysiocrinus, had its stem laying on the bottom or attached to it with its calyx turned upward, perpendicular to the stem. It was, probably, was an adjustment against some strong currents.
The calyx, a cup-shaped head, is attached to the stem. It is made up of many regular-shaped plates, which have five-fold symmetry. It contains the vital organs of the chrinoid. This strange animal has also a mouth and an anus, which are situated in the center of the calyx in the oral its side and connected by a small gut. The weight of the organs is quit light, but it gets much heavier when it is filled with food. The upper part of the cup, tegmen, is covered by a lot of small, irregulary arranged plates. The calyx includes two parts: the dorsal cup, and tegmen. Some tegmens are made of leather, others - of thick plates. Some of them cover just the cup, others rise in a tall anal tube, others form a solid roof which covers the mouth and the food grooves. Some crinoids have small cups with two circlets of plates named basals and radials. Radial plates make up the top ring of the calyx. Calyx also has basal plates, which are attached right below the radial plates. Some types of crinoids have also infrabasal plates, which go right below the basal ones. Crinoids, which have recognizable infrabasal plates, are called dicyclic. Other type of crinoids has only basal plates, which sometimes difficult to notice. This type crinoids is called monocyclic. Calyx of some crinoids is modified by adding an additional, anal plate. The anal plate is between two radials and above radianal or aniradianal plates. Therefore, small cups have about twelve plates. Other creatures added infrabasal plates next to the anal, increaced the number of arm plates placing them into the cup and, also, developed interradial plates not connected with the anal. Small crinoids, such as Uperocrinus, developed seventy seven plates. Others had even more, hundreds of plates. Cretacious Torinocrinus had a short stalk attached to the base. Its calyx was facing sidewise at an angle with its columnals and plates firmly attached to one another. The arms were projecting from its solid mass. This crinoid also was designed for living in water with strong currents.
Typical crinoid has five long and branching arms, composed of an
articulated series of ossicles that are used in
suspension feeding and respiration. Sometimes the arms develop pinnules, looking like barb of a feather. The arms have
organ extensions in them and they are attached to the radial plates of the
calyx: one arm per one plate. The pinnules and arms
have a special food grove lined by cilia, small, hair-like organelles, which
moved small particles of the food to the mouth. Water enters the body through a
perforated plate, goes through a ring and radial canals, and feels tube feet
that extend through pores in the arms and are used for breathing. Mouth is
situated on the upper surface of the mouth. The animal catches food with the
arms and pinnule, and delivers it to its mouth using
tube feet located in them. The reproductive glands are also located in the
arms, and fertilisation happens in open water during
mass spawning. Arms of different crinoids are also very different. Carabocrinus has tiny arms. Petalocrinus’
arms became broad plates radiating from its calyx. Some crinoids maintained a
single row of brachials but made them long. Those are
called uniserial arms and usually are quite rigid.
Other arms are biserial with uniserial
bases. Both types of arms could develop huge and complicated branches, or pinnules, which are largely used by Paleozoic and later
animals. Other crinoids, such as Cornucrinus, are
A lot of crinoids have spines. Ancyrocrinus has basal spines that play the role of prongs of an anchor. Scyphocrinite has even more spines, which grow from the rough plates of the calyx and from lower brachials.
Scientists classify crinoids due to the amount and branching pattern of their arms, to the arrangement of the plates within the calyx, to their pentameral radial symmetry, and the exterior of the calyx. Internal organs of crinoids, such as nervous and respiratory systems, could also help a scientist to place them in different types. There are four subclasses of chrinoids distinguished: Inadunata - from early Ordovician to late Permian; Camerata, which existed from middle Ordovician to Permian; Flecibilia – from middle Ordovician to late Permian; and Articulata, the only subclass that has existed from Triassic to recent.
Subclass Inadunata must have been the oldest one, and, probably, the most simple one. Calyx’s plates firmly tied together, the tegmen covers its lower food grooves and the mouth. Its interradials grow just on one side of the cup. Its arms, uniserial and biserial, pinulate and non-pinulate and usually dicyclic, grow freely above the radials. The animal does not have any webbing between its arms. Sometimes these anymals could look very strange. They were especially active from Ordovician to the Triassic and had about 480 genera.
Subclass Camerata usually has mouth and food grooves, which are covered by a large and rigid tegmen. Its plates usually thick and firmly attached to each other. The lower, basal, parts of the arms are installed in the calyx as well, making it hard to recognize the plates, and therefore, name the type of crinoid. They have calyxes that are box shaped, that can be monocyclic or dicyclic. Camerata has biserial and pinulate arms. It has an anal tube. They lived from Ordovician to the Permian, especially flourished during Mississippian, and had about 210 genera.
Subclass Flexibilia has calyx plates very different from those of Inadunata. Flexibilia’s plates are not joined together firmly, but allow some degree of movement. The creature’s tegmen covers mouth and the food grooves. They always dicyclic and never have cirri. Their uniserial and non-punnulate arms are usually free above the radial plates. The arms could be curled at the tips. Its stem might have thin stems, which are called proxistele. This subclass must have evolved from Indianata, and lived from the Ordovician to the Permian consisting of about 60 genera.
The last subclass, Articulata, has small monocyclic or dicyclic cup and pinnulate arms. The cup plating is usually reduced, and commonly attached by slightly flexible joints. and these creatures do not have anus. It has the tegmen which usually is unplated, leathery, and flashy covering the mouth and food grooves. Some crinoids of this type abandond their stem after the early larval stage and swam freely in water. A centrodorsal plate supported their infrabasals. Some of articulate crinoids had arms more than four feet long. They are considered to be post-Paleozoic animals, and 255 genera of them lived from Triassic to the Holocene. It is the only subclass of crinoids, which exists nowadays. There are about 165 genera of them left.
Parts of a crinoid
Another diagram of crinoid morphology
Casanova Richard. An Illustrated Guide to Fossil Collection. Revised ed. Naturegraph,
Garden City, N.Y., 1958.
Kirkaldy J.F. Fossils in
Color. Blandford Colour
Series, Blandford Press,
Introduction to Crinodea.
Kazlev Alan M. Class Crinodea.
Crinoids. Description and Characteristics. Cravenand Pendle Geological Society.