Wenlock creatures: Crinoids

Gissocrinus

The crinoid in the image, Gissocrinus, would have attached itself to the coral reef.

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Example of modern crinoid

This image shows a living crinoid in deep water.

What are crinoids?

Crinoids are often known as sea lilies, but they are not plants. They are most closely related to starfish and sea urchins, and belong to a group of animals called echinoderms.

All echinoderms are characterised by their unusual body plan- they have five-fold radial symmetry, which means that their body features, such as the feeding arms on crinoids, are all in multiples of five.

Crinoids live only in sea water and do not like fresh or even brackish water, so fossil crinoids are good indicators of salinity in the geological past.

Crinoids live on or near to the sea floor. Many fossil crinoids have a long cylindrical 'stem' which fixed them to the sea bed, but far enough above it to lift them above the feeding level of other filter feeders. Modern crinoids are often stemless and can move around, using their 'arms' to help them to crawl over the seafloor.

DietCrinoids are filter feeders, using their feathery ‘arms’ to trap tiny food particles from the surrounding water

Key facts about Wenlock crinoids

Reef builderCrinoids were common reef dwellers on the Wenlock Reef. Their calcium carbonate skeletons were made of many segments, known as ossicles. Once a crinoid dies, the ossicles break up as the soft parts holding them together rot. Crinoid ossicles make up a large proportion of the sediment around the Wenlock reef, and some parts of the Wenlock Limestone are almost entirely made of ossicles.

Abundance through geological time

AbundanceCrinoids are first found in Ordovician rocks, and are still found living today in deep water.

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