Crinoid
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| Crinoids Fossil range: Ordovician - Recent |
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Articulata (540 species) |
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 6,000 meters.[citation needed] Crinoids are characterized by a mouth on the top surface that is surrounded by feeding arms. They have a U-shaped gut, and their anus is located next to the mouth. 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.
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[edit] Morphology
Crinoids comprise three basic sections; the stem, the calyx, and the arms. The stem is composed of highly porous ossicles which are filled with muscular tissue. The Calyx contains the crinoid's digestive and reproductive organs, and the mouth is located at the top of the dorsal cup, while the anus is located peripheral to it. The brachials(arms) display pentameral symmetry and comprise smaller ossicles than the stem and are equipped with cirri which facilitate feeding by moving the the organic media down the arm and into the mouth.
[edit] Ecology
[edit] Nutrition
Crinoids feed by filtering small particles of food from the sea water with their feather like arms.
[edit] Reproduction
Crinoids reproduce sexually by the males releasing their sperm and the females releasing their eggs into the current where they will develop into a bottom-dwelling non-feeding larval stage and then eventually grow a stalk (in the stalked crinoids), and within 10 to 16 months will be able to reproduce. In some cases the female of the species has been known to temporarly brood the larva.
[edit] Motility
Most modern crinoids are free-swimming and lack a stem. Examples of free-swimming crinoid fossils include Marsupitsa, Saccocoma and Uintacrinus.
In 2005, a stalked crinoid was recorded pulling itself along the sea floor off the Grand Bahama Island. While it has been known that stalked crinoids move, prior to this recording the fastest motion of a crinoid was 0.6 meters/hour (2 ft/h). The 2005 recording showed a crinoid moving at 140 meters/hour (460 ft/h).[1]
[edit] Evolution
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.
The crinoids underwent two periods of abrupt adaptive radiation; the first during the Ordovician, the other after they underwent a selective mass extinction at the end of the Permian period.[2] This Triassic radiation resulted in forms possessing flexible arms becoming widespread; motility, predominantly a response to predation pressure, also became far more prevalent.[3] After the end-Permian extinction, crinoids never regained the morphological disparity they enjoyed in the Paleozoic; they occupied a different region of morphospace, employing different ecological strategies to those that had proven so successful in the Paleozoic.[2]
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. However, none of these others survived the crisis at the end of the Permian period.
[edit] Fossils of passing interest
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. The largest fossil crinoid on record had a stem 40 m (130 ft) in length.[4][verification needed]
[edit] Crinoids in pop culture
- Fossilised crinoid columnals extracted from limestone quarried on Lindisfarne, or found washed up along the foreshore, which were threaded into necklaces or rosaries, became known as St Cuthbert's beads.
- Crinoids are the state fossil of Missouri[citation needed]
[edit] References
- ^ Baumiller, Tomasz K. and Messing, Charles G. (6 October 2005). "Crawling In Stalked Crinoids: In Situ Observations, Functional Morphology, and Implications for Paleozoic Taxa". Geological Society of America Abstracts with Programs, Vol. 37, No. 7: 62.
- ^ a b Foote, M. (1999). "Morphological diversity in the evolutionary radiation of Paleozoic and post-Paleozoic crinoids". Paleobiology 25 (sp1): 1-116.
- ^ Baumiller, Tomasz K. (2008). "Crinoid Ecological Morphology". Annual Review of Earth and Planetary Sciences 36: 221. doi:.
- ^ Ponsonby, Dr. David; Prof. George Dussart (2005). The Anatomy of the Sea. Vancouver: Raincoast Books, 129. ISBN 0-8118-4633-4.
[edit] External links
- Tree of Life (source of taxonomic information on this page)
- Crinoids of the North Sea
- Article and video of crinoid crawling
- On the Crinoid Genus Scyphocrinus and its Bulbous Root Camarocrinus by Frank Springer. 3 p., 74 p. illus., IX pl. (2 fold.) 33 cm. Washington: Smithsonian Institution, 1917
- American Silurian Crinoids by Frank Springer. iv, 239 p. 33 pl., 2 port. 32 cm. Washington: The Smithsonian Institution, 1926
- Introduction to the Crinoidea: Sea lilies and feather stars... ucmp.berkeley.edu. Retrieved on February 7, 2007.
