Crinoids

Crinoids, commonly known as ‘sea lilies’, are marine echinoderms that first appear in the fossil record in marine sediments deposited approximately 530 million years ago during the Cambrian Period and were very abundant during the Palaeozoic Era. Paleozoic seas were dominated by crinoid echinoderms. The Permian extinction, 244 million years ago, devastated the marine biota. Tabulate and rugose corals, blastoid echinoderms, graptolites, and most crinoids died out, as did the last of the trilobites.

Articulate brachiopods and one lineage of crinoids survived, but never again dominated the marine environment. All modern crinoids have evolved from this lineage. Crinoid bodies consist of three main parts: A calyx or aboral cup, the arms and the stem. The calyx contains the vital organs of the animal. It is small when compared to the total mass, most of which is devoted to food collection. A simple digestive system is located within the upper body. The arms are composed of an articulated series of ossicles that are used in suspension feeding and respiration.

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Reproductive organs are also located in the arms as fertilisation takes place in open water during mass spawnings. The stem supports the animal and together with the roots and cirri serve as a means of attachment to the sea bed. However, the function of these skeletal parts may vary for different species of crinoids. For example, some fossil crinoids have been found anchored to logs that presumably floated as the animal dangled in the water. Crinoids possess an endoskeleton composed of calcareous plates and covered by a thin epidermis.

Living, shallow water forms are extensively pigmented. Each plate is a single, very porous calcite crystal. Unfused plates are held together with ligaments or muscles. Most modern crinoids have more flexible arms than the fossil species and do not have stalks (at least as adults), but are free to swim or crawl over the sea floor. These types of crinoids are known as ‘feather stars’ and are related to sea stars, sea cucumbers and sea urchins. Crinoids are marine animals. They are generally gregarious, sometimes living in large accumulations known as ‘crinoid gardens’.

Feather stars swim though the water or crawl along the ocean floor in search of food. They often use rocks, corals or sponges to raise themselves above the bottom during their nocturnal feeding and hide in nooks and crannies during the day. Feather stars are found in shallow and deep ocean waters but are most diverse in tropical reef environments. Sea lilies live permanently attached to the substrate or ocean floor and today are restricted to depths greater than 100 meters. In Paleozoic times stemmed crinoids inhabited continental shelves and shallow inland seas.

When the animals died, the ocean currents often broke up the remains and rolled them together in vast amounts to form thick deposits of limestone today known as crinoidal limestone. Many Paleozoic limestones are made up largely of crinoid skeletal fragments. Comparitively wide spread in the fossil record, excellent crinoid fossils are associated with the Ordovician in Morocco, the Mississippian of the central U. S. and Canada, the Jurassic – Triassic of Germany, the Permian of Australia and Timor, and the Tertiary of Oregon.

Gascoyne Junction of Western Australia is well known for it’s crinoid fossils. Crinoids provide evidence of organic evolution and continuing changes to the geography, environments and ecosystems of our planet. As fossils, they help document the drifting of the continents as well as the creation and obliteration of ancient seas. The Class Crinoidea survived several mass extinctions over the past 530 million years. Crinoids are, thus, one of the most successful forms of life on earth.

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