Crinoid

Crinoid

Crinoids are interesting marine invertebrates that belong to the class Crinoidea. They are part of a larger group called echinoderms. This group also includes starfish, brittle stars, sea urchins, and sea cucumbers. Crinoids can live in shallow water or very deep oceans, even deeper than 9,000 meters!

There are two main types of crinoids. Sea lilies stay attached to the sea floor with a stalk their whole lives. The other type is called feather stars or comatulids. Feather stars lose their stalk when they grow up and can move around. Feather stars are the largest group of crinoids today.

Crinoids may look complicated, but they eat in a simple way. Their mouths are on the top of their bodies, surrounded by special arms with feathery parts. These arms catch tiny bits of food in the water, and the crinoids eat them. Many crinoids start life attached to the ocean floor but can move around when they grow up.

Today, there are about 700 different kinds of crinoids in the oceans. In the past, they were much more common. Some thick layers of limestone from long ago are made mostly from the remains of ancient crinoids.

Etymology

The name "Crinoidea" comes from an Ancient Greek word, krínon, which means "a lily". The ending –oid means "like". This name matches the flower-like look of many crinoids.

Morphology

Crinoids are special sea animals with a unique body shape. They have a stem in some types, a cup-like main body called the theca, and five arms that are usually branched and look feathery. The mouth and anus are on the top of the theca, which is different from other sea animals like starfish and sea urchins. Their bodies are made of small, bony plates with a little soft tissue.

Sea lilies have a stem made of porous pieces connected by tissue, and they stick to the ocean floor with a holdfast or root-like structures. Feather stars lose their stem as they grow but keep a few root-like pieces. Most living crinoids can move freely and have only a tiny stalk. Their arms can curl up and have tiny branches called pinnules that make them look feathery. These arms help them catch food.

Locomotion

Most modern crinoids, called feather stars, can move around on their own. They crawl using their feather-like arms and can even swim for short bursts. When they swim, they move their arms together to go up and then sideways, moving about 7 cm per second.

In 2005, scientists saw a stalked crinoid called Neocrinus decorus moving faster than they thought—up to 4 to 5 cm per second. This showed that even stalked crinoids can move quickly along the sea floor.

Evolution

See also: List of echinodermata orders

Reconstructions of different kinds of crinoids show they had many shapes. The first crinoids appeared about 480 million years ago during a time called the Ordovician. Scientists think crinoids may have come from other sea creatures called blastozoans or edrioasteroids.

Crinoids were very common in the seas long ago and changed a lot over time. After many sea creatures died at the end of the Permian period, crinoids looked different and lived in new ways. Later, around 230 million years ago, crinoids grew flexible arms to help them move. Some crinoids lived on floating wood, which sometimes sank and became fossils on the ocean floor. Today, we can find many crinoid fossils in certain rocks.

Taxonomy

Crinoidea is a group of animals called echinoderms. People have known about them since 1821. There are about 540 kinds of crinoids alive today.

The living crinoids belong to four orders: Comatulida, Cyrtocrinida, Hyocrinida, and Isocrinida. These four orders are part of a group called Articulata.

Crinoidea also includes many groups that are no longer alive. One of these groups is Pentacrinoidea. Scientists are still learning more about how crinoids are classified.

In culture

Fossilized pieces of crinoids have been used as beautiful decorations for many years. In the Middle Ages, people in places like Lindisfarne made necklaces and rosaries from these fossils, calling them St. Cuthbert's beads. In the Midwestern United States, similar fossils are sometimes called Indian beads. One type of crinoid, Eperisocrinus missouriensis, is the official state fossil of Missouri. Also, the design of the aliens in the movie Alien was inspired by crinoids.

Fossil crinoid gallery

Here are some interesting crinoid fossils from different times and places. We see crushed stems from the Jurassic period in Iran, and fossils from Germany showing stems, calyxes, and arms. There are also 330 million year old crinoid fossils from Iowa. Other fossils include crinoid holdfasts and bryozoans from the Upper Ordovician in northern Kentucky, and Seirocrinus subangularis from the Early Jurassic Posidonia Shale in Holzmaden, Germany. We also find crinoid columnals (Isocrinus nicoleti) from the Middle Jurassic Carmel Formation in Utah, and root-like holdfasts from the Upper Ordovician in southern Ohio. There are also internal molds of crinoid stems from the Lower Carboniferous in Ohio, and fossils of Seirocrinus subsingularis from the Jurassic Holzmaden Black Shale Formation in Germany. Some extinct crinoids, like Monstrocrinus from the Early to Middle Devonian, had complex spines attached to their calyx plates.